The Caucasian squirrel lives in the hollows of centuries-old olive trees. The number of Caucasian squirrels in the North Aegean region is decreasing every year, noticeably.

Like most red squirrels, spotting a Caucasian Squirrel is definitely becoming a rare sight. Climate change affects nature differently in every corner of the world.

 

The Caucasian squirrel - Sciurus anomalus ; The Caucasian squirrel or Persian squirrel, is a tree squirrel in the genus Sciurus found in temperate broadleaf and mixed forests in south-western Asia.

The species is usually said to have first been described in 1778 by Johann Friedrich Gmelin in the 13th edition of Systema Naturae,and named Sciurus anomalus. However, some authors argue that this work was actually published in 1788, and that the true first description was made by Johann Anton Güldenstädt in 1785.

Description -

Caucasian squirrels are small tree squirrels, with a total length of 32 to 36 cm (13 to 14 in), including the 13 to 18 cm (5.1 to 7.1 in) tail, and weighing 250 to 410 g (8.8 to 14.5 oz). The color of the upper body fur ranges from greyish brown to pale grey, depending on the subspecies, while that of the underparts is rusty brown to yellowish, and that of the tail, yellow brown to deep red. The claws are relatively short, compared with those of other tree squirrels, and females have either eight or ten teats.

Samuel Griswold Goodrich described the Caucasian squirrel in 1885 as "Its color is grayish-brown above, and yellowish-brown below".

 

Physical Description -

Caucasian squirrels have a dental formula of incisors 1/1, canines 0/0, premolars 1/1, and molars 3/3, totaling 20. They have four fingered fore feet and five fingered hind feet. Sex differences in body length or mass are not evident.

Distribution and habitat -

 

Caucasian squirrels are native to south-western Asia, where they are found from Turkey, and the islands of Gökçeada and Lesbos in the west, Iran in the southeast, and as far as Israel and Jordan in the south.It is one of only two species of the genus Sciurus to be found on Mediterranean islands,and, although Eurasian red squirrels have been recently introduced to some areas, is the only species of Sciurus native to the wider region.

The species mainly lives in forested areas dominated by oak, pine, and pistachio, up to altitudes of 2,000 metres (6,600 ft).

 

Biology and behavior -

The squirrels are diurnal, and solitary, although temporary groups may forage where food is plentiful. Their diet includes nuts, seeds, tree shoots, and buds,with the seeds of oak and pine being particularly favored. Like many other squirrels, they cache their food within tree cavities or loose soil, with some larders containing up to 6 kg (13 lb) of seeds. They live in trees, where they make their dens, but frequently forage on the ground, and are considered less arboreal than Eurasian red squirrels. They commonly nest in tree hollows lined with moss and leaves, and located 5 to 14 m (16 to 46 ft) above the ground, but nests are also sometimes found under rocks or tree roots. Their alarm call is high-pitched, and said to resemble the call of the European green woodpecker, and they mark their territories with urine and dung.

Breeding occurs throughout the year, but is more common in spring or autumn. Litters range from two to seven, with three or four being typical, and the young are fully mature by five or six months of age.

 

Conservation -

A survey in 2008 found that the species remained abundant within Turkey, however declines are noted in population within the Levant region. The guides for a survey in 1993 in Israel stated that they considered the species to be nearly extinct within the area studied. Whilst the Caucasian squirrel is threatened by poaching and deforestation, the declines recorded are not sufficient to qualify them as anything other than "Least Concern" by the International Union for Conservation of Nature.[1] Hunting of the species is banned by the Central Hunting Commission, and the Caucasian squirrel is protected by the Bern Convention and the EU Habitats Directive.

 

This information is sourced from "Wikipedia".

  

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Giraffes are fairly social animals and get together in herds from time to time. However, there is no group bonding. Because of their very long necks, Giraffes are able to feed on the foliage of trees that is not accessible to other herbivores. The long prehensile tongue is used to pull pods and leaves into the mouth which are then stripped from the stems with the spatulate incisor teeth. Their skin colour is tan with light brown patches on females and dark brown patches on males. Both males and females have short horns covered in skin.

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

 

Although a great many fossil fishes have been found and described, they represent a tiny portion of the long and complex evolution of fishes, and knowledge of fish evolution remains relatively fragmentary. In the classification presented in this article, fishlike vertebrates are divided into seven categories, the members of each having a different basic structural organization and different physical and physiological adaptations for the problems presented by the environment. The broad basic pattern has been one of successive replacement of older groups by newer, better-adapted groups. One or a few members of a group evolved a basically more efficient means of feeding, breathing, or swimming or several better ways of living. These better-adapted groups then forced the extinction of members of the older group with which they competed for available food, breeding places, or other necessities of life. As the new fishes became well established, some of them evolved further and adapted to other habitats, where they continued to replace members of the old group already there. The process was repeated until all or almost all members of the old group in a variety of habitats had been replaced by members of the newer evolutionary line.

 

The earliest vertebrate fossils of certain relationships are fragments of dermal armour of jawless fishes (superclass Agnatha, order Heterostraci) from the Upper Ordovician Period in North America, about 450 million years in age. Early Ordovician toothlike fragments from the former Soviet Union are less certainly remains of agnathans. It is uncertain whether the North American jawless fishes inhabited shallow coastal marine waters, where their remains became fossilized, or were freshwater vertebrates washed into coastal deposits by stream action.

 

Jawless fishes probably arose from ancient, small, soft-bodied filter-feeding organisms much like and probably also ancestral to the modern sand-dwelling filter feeders, the Cephalochordata (Amphioxus and its relatives). The body in the ancestral animals was probably stiffened by a notochord. Although a vertebrate origin in fresh water is much debated by paleontologists, it is possible that mobility of the body and protection provided by dermal armour arose in response to streamflow in the freshwater environment and to the need to escape from and resist the clawed invertebrate eurypterids that lived in the same waters. Because of the marine distribution of the surviving primitive chordates, however, many paleontologists doubt that the vertebrates arose in fresh water.

 

Heterostracan remains are next found in what appear to be delta deposits in two North American localities of Silurian age. By the close of the Silurian, about 416 million years ago, European heterostracan remains are found in what appear to be delta or coastal deposits. In the Late Silurian of the Baltic area, lagoon or freshwater deposits yield jawless fishes of the order Osteostraci. Somewhat later in the Silurian from the same region, layers contain fragments of jawed acanthodians, the earliest group of jawed vertebrates, and of jawless fishes. These layers lie between marine beds but appear to be washed out from fresh waters of a coastal region.

 

It is evident, therefore, that by the end of the Silurian both jawed and jawless vertebrates were well established and already must have had a long history of development. Yet paleontologists have remains only of specialized forms that cannot have been the ancestors of the placoderms and bony fishes that appear in the next period, the Devonian. No fossils are known of the more primitive ancestors of the agnathans and acanthodians. The extensive marine beds of the Silurian and those of the Ordovician are essentially void of vertebrate history. It is believed that the ancestors of fishlike vertebrates evolved in upland fresh waters, where whatever few and relatively small fossil beds were made probably have been long since eroded away. Remains of the earliest vertebrates may never be found.

 

By the close of the Silurian, all known orders of jawless vertebrates had evolved, except perhaps the modern cyclostomes, which are without the hard parts that ordinarily are preserved as fossils. Cyclostomes were unknown as fossils until 1968, when a lamprey of modern body structure was reported from the Middle Pennsylvanian of Illinois, in deposits more than 300 million years old. Fossil evidence of the four orders of armoured jawless vertebrates is absent from deposits later than the Devonian. Presumably, these vertebrates became extinct at that time, being replaced by the more efficient and probably more aggressive placoderms, acanthodians, selachians (sharks and relatives), and by early bony fishes. Cyclostomes survived probably because early on they evolved from anaspid agnathans and developed a rasping tonguelike structure and a sucking mouth, enabling them to prey on other fishes. With this way of life they apparently had no competition from other fish groups. Cyclostomes, the hagfishes and lampreys, were once thought to be closely related because of the similarity in their suctorial mouths, but it is now understood that the hagfishes, order Myxiniformes, are the most primitive living chordates, and they are classified separately from the lampreys, order Petromyzontiformes.

 

Early jawless vertebrates probably fed on tiny organisms by filter feeding, as do the larvae of their descendants, the modern lampreys. The gill cavity of the early agnathans was large. It is thought that small organisms taken from the bottom by a nibbling action of the mouth, or more certainly by a sucking action through the mouth, were passed into the gill cavity along with water for breathing. Small organisms then were strained out by the gill apparatus and directed to the food canal. The gill apparatus thus evolved as a feeding, as well as a breathing, structure. The head and gills in the agnathans were protected by a heavy dermal armour; the tail region was free, allowing motion for swimming.

 

Most important for the evolution of fishes and vertebrates in general was the early appearance of bone, cartilage, and enamel-like substance. These materials became modified in later fishes, enabling them to adapt to many aquatic environments and finally even to land. Other basic organs and tissues of the vertebrates—such as the central nervous system, heart, liver, digestive tract, kidney, and circulatory system— undoubtedly were present in the ancestors of the agnathans. In many ways, bone, both external and internal, was the key to vertebrate evolution.

 

The next class of fishes to appear was the Acanthodii, containing the earliest known jawed vertebrates, which arose in the Late Silurian, more than 416 million years ago. The acanthodians declined after the Devonian but lasted into the Early Permian, a little less than 280 million years ago. The first complete specimens appear in Lower Devonian freshwater deposits, but later in the Devonian and Permian some members appear to have been marine. Most were small fishes, not more than 75 cm (approximately 30 inches) in length.

 

We know nothing of the ancestors of the acanthodians. They must have arisen from some jawless vertebrate, probably in fresh water. They appear to have been active swimmers with almost no head armour but with large eyes, indicating that they depended heavily on vision. Perhaps they preyed on invertebrates. The rows of spines and spinelike fins between the pectoral and pelvic fins give some credence to the idea that paired fins arose from “fin folds” along the body sides.

 

The relationships of the acanthodians to other jawed vertebrates are obscure. They possess features found in both sharks and bony fishes. They are like early bony fishes in possessing ganoidlike scales and a partially ossified internal skeleton. Certain aspects of the jaw appear to be more like those of bony fishes than sharks, but the bony fin spines and certain aspects of the gill apparatus would seem to favour relationships with early sharks. Acanthodians do not seem particularly close to the Placodermi, although, like the placoderms, they apparently possessed less efficient tooth replacement and tooth structure than the sharks and the bony fishes, possibly one reason for their subsequent extinction.

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

copyright © Mim Eisenberg/mimbrava studio. All rights reserved.

 

Zoe Bear had two of her bottom incisors removed yesterday because they were loose. While under, she also had her teeth cleaned. Aren't they nice and white now? She came out of anesthesia really well and was home in less than three hours. This shot was taken shortly after she got home. She is doing mostly fine except for a honking cough caused by irritation from the breathing tube. I'm hoping it'll go away soon.

 

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I invite you to stroll through My Galleries.

Ethiopian government is taking over and inciting girls to give up tribal customs such as scarification and lip plates. Without labret, a girl will be sold less for wedding (25 cows + 1 kalaschnikov)

Surma woman with her giant lip plate, a sign of beauty in Surma tribe, like in Mursi one. When they are ready to marry, they start to make a hole in the lip with a wood stick.

It will be kept for one night , and is removed to put a bigger one. This is very painful at this time... Few months after, the lip plate has its full size, and the girl is seen as beautiful by the men.

The lip plate made of wood or terracotta, and they have to remove the lower incisors to let some space for the disc. it's amazing to see them speak without any trouble, put it and remove it as a classic jewel.

Sometimes the lip is broken by the pressure of the lip plate. This is a very big problem for the girl cos men will consider her as ugly, she won't be able to marry anyone in the tribe apart the old men or the sick people...

The women are shaved, like the men, cos they hate hairiness!

 

© Eric Lafforgue

www.ericlafforgue.com

The Pokhot live in the Baringo and Western Pokot districts of Kenya and in Uganda.

There are two main sub-groups depending of their location and way of life. The first group consist of the Hill Pokot who live in the rainy highlands in the west and in the central south, and are mainly farmers and pastoralists. The second group is made up of the Plains Pokot who live in dry and infertile plains, with their cattles. A homestead is composed of one or more buildings for a man, his wife and children; eventual co-wives live in separate houses. The role of the community in teaching children ethical rules. Most of the Pokot are nomadic and thus have interacted with different peoples, incorporating their social customs.The Pokot are very proud of their culture. The Songs, storytelling, and decorative arts, especially bodily decoration, are very appreciated. They adorn the body with beads, hairstyling, scarification, and the removal of the lower central incisors. Pokot girls wear a beaded necklace made of the stems of an asparagus tree. Most Pokot have some knowledge of herbal medicine, so they often use these treatments along with those of the hospitals. They belong to the Kenya's Nilotic-speaking peoples. .

For the Pokot, the universe has two realms: the above is the realm of the most powerful deities—Tororot, Asis (sun), and llat (rain); and the below is the one where live humans, animals, and plants. Humans are responsible for the realm that they inhabit, but they rely upon divinities to achieve and maintain peace and prosperity. They worship many deities like the sun, moon and believe in the spirit of death.The Pokot communicate with their deities through prayer and sacrifice. They perform it during ethnic festivals and dances. Oracles are responsible for maintaining the spiritual balance within the community. They are superstitious and believe in sorcery, so sometimes they call on shielding lucky sorcery. They have prophets, either male or female, who foresee advise, usually by the means of animal sacrifices. His or her ability is considered as a divine gift. Clan histories recount the changes of location, through poetry and song, emphasizing the vulnerability of humans and the importance of supernatural powers to help them overcome hunger, thirst, and even death. Ceremonies mark the transitions in the people's social lives. Among these are: the cleansing of a couple expecting their first child; the cleansing of newborn infants and their mothers; the cleansing of twins and other children who are born under unusual circumstances; male and female initiation; marriage; sapana, a coming-of-age ceremony for men; and summer-solstice, harvest, and healing ceremonies. The most important rite of passage for most Pokot is circumcision for boys and clitoridectomy for girls. These rites consist of a series of neighborhood-based ceremonies, emphasizing the importance of having a good behavior. When boys are circumcised, they acquire membership in one of eight age sets. Women do not have age-sets. After excisions, for several months, girls have a white painting on their face and wear a hood made of blackened leather with charcoal and oil. This means they are untouchable until the lepan ceremony, that marks the passage to womanhood. Unlike other tribes, the Pokot keep the affiliation to their clan throughout their lives, there is no disruption with marriage. Surprisingly, the agreement before marriage is made by gift giving, from the groom and his family to the bride and her family, often over a period of years (and not the contrary). It often implies the gift of a combination of livestock, goods, and cash to the bride's family, and the allotment of milk cows and rights to land to the bride. The bond between a husband and wife lasts for 3 generations, after what marriages can take place again between the two groups. Polygamy exists but is not prevalent among men before 40. The spirits of the elder anticipate reincarnation in their living descendants: when a child is said to resemble the elder, the same name is given. Disputes are resolved in neighborhood councils and in government courts. Some of the sanctions include shaming, cursing, and bewitching.

 

© Eric Lafforgue

www.ericlafforgue.com

 

The Mursi (also called Murzu) is the most popular tribe in the southwestern Ethiopia lower Omo Valley, 100 km north of Kenyan. They are estimated to 10 000 people and live in the Mago National Park, established in 1979. Due to the climate, they move twice a year between the winter and summer months. They herd cattle and grow crops along the banks of the Omo River. The Mursi are sedentary rather than nomadic. Their language belongs to the Nilo-Saharan linguistic family.Very few Mursi people speak Amharic, the official Ethiopian language. Although a small percentage of the Mursi tribe are Christians, most still practice animism. Mursi women wear giant lip plate, a sign of beauty, like in Suri tribe, and also a prime attraction for tourists which help to sustain a view of them, in guidebooks and travel articles, as an untouched people, living in one of the last wildernesses of Africa. When they are ready to marry, teenagers start to make a hole in the lower lip with a wood stick.

It will be kept for one night, and is removed to put a bigger one. This is very painful at this time. Few months after, the lip plate has its full size, and the girl is seen as beautiful by the men. The lip plate is made of wood or terracotta. They have to remove the lower incisors to let some space for the disc. Sometimes the lip is broken by the pressure of the plate. This is a big problem for the girl because men will consider her as ugly, she won't be able to marry anyone in the tribe apart the old men or the sick people. Women and men are shaved because they hate hairiness. Both like to make scarifications on their bodies. Women as a beauty sign, men after killing animals or ennemies as competition for grazing land has led to tribal conflicts.

The Mursi men have a reputation for being aggressive and are famous for their stick fighting ceremony called donga. The winner of the donga will be able to select the girl of his choice to have relations with if she agrees. Similar to the Surma tribe, the Mursi tribe commonly drink a mixture of blood and milk. Over the past few decades they and their neighbours have faced growing threats to their livelihoods cause the Ethiopian government officials have been actively evicting Mursi people from the Omo National Park, without any compensation to rent their land to foreign investors. Drought has made it difficult for many families to feed themselves by means of their traditional mix of subsistence activities. The establishment of hunting concessions has added to the pressure on scarce ressources.

 

© Eric Lafforgue

www.ericlafforgue.com

    

BIG 5. Elephant. Hluhluwe–Imfolozi Park. South Africa. Dec/2019

 

Elephant

Elephants are large mammals of the family Elephantidae and the order Proboscidea. Three species are currently recognised: the African bush elephant (Loxodonta africana), the African forest elephant (L. cyclotis), and the Asian elephant (Elephas maximus). Elephants are scattered throughout sub-Saharan Africa, South Asia, and Southeast Asia. Elephantidae is the only surviving family of the order Proboscidea; other, now extinct, members of the order include deinotheres, gomphotheres, mammoths, and mastodons.

All elephants have several distinctive features, the most notable of which is a long trunk (also called a proboscis), used for many purposes, particularly breathing, lifting water, and grasping objects. Their incisors grow into tusks, which can serve as weapons and as tools for moving objects and digging. Elephants' large ear flaps help to control their body temperature. Their pillar-like legs can carry their great weight. African elephants have larger ears and concave backs while Asian elephants have smaller ears and convex or level backs.

Elephants are herbivorous and can be found in different habitats including savannahs, forests, deserts, and marshes. They prefer to stay near water. They are considered to be a keystone species due to their impact on their environments. Other animals tend to keep their distance from elephants while predators, such as lions, tigers, hyenas, and any wild dogs, usually target only young elephants (or "calves"). Elephants have a fission–fusion society in which multiple family groups come together to socialise. Females ("cows") tend to live in family groups, which can consist of one female with her calves or several related females with offspring. The groups are led by an individual known as the matriarch, often the oldest cow.

Males ("bulls") leave their family groups when they reach puberty and may live alone or with other males. Adult bulls mostly interact with family groups when looking for a mate and enter a state of increased testosterone and aggression known as musth, which helps them gain dominance and reproductive success. Calves are the centre of attention in their family groups and rely on their mothers for as long as three years. Elephants can live up to 70 years in the wild. They communicate by touch, sight, smell, and sound; elephants use infrasound, and seismic communication over long distances. Elephant intelligence has been compared with that of primates and cetaceans. They appear to have self-awareness and show empathyfor dying or dead individuals of their kind.

Source: Wikipedia

Elefante

Os elefantes são animais herbívoros, alimentando-se de ervas, gramíneas, frutas e folhas de árvores. Dado o seu tamanho, um elefante adulto pode ingerir entre 70 a 150 kg de alimentos por dia. As fêmeas vivem em manadas de 10 a 15 animais, lideradas por uma matriarca, compostas por várias reprodutoras e crias de variadas idades. O período de gestação das fêmeas é longo (20 a 22 meses), assim como o desenvolvimento do animal que leva anos a atingir a idade adulta. Os filhotes podem nascer com 90 kg. Os machos adolescentes tendem a viver em pequenos bandos e os machos adultos isolados, encontrando-se com as fêmeas apenas no período reprodutivo.

Devido ao seu porte, os elefantes têm poucos predadores. Exercem uma forte influência sobre as savanas, pois mantêm árvores e arbustos sob controle, permitindo que pastagens dominem o ambiente. Eles vivem cerca de 60 anos e morrem quando seus molares caem, impedindo que se alimentem de plantas.

Os elefantes-africanos são maiores que as variedades asiáticas e têm orelhas mais desenvolvidas, uma adaptação que permite libertar calor em condições de altas temperaturas. Outra diferença importante é a ausência de presas de marfim nas fêmeas dos elefantes asiáticos.

Durante a época de acasalamento, o aumento da produção de testosterona deixa os elefantes extremamente agressivos, fazendo-os atacar até humanos. Acidentes com elefantes utilizados em rituais geralmente são causados por esse motivo. Cerca de 400 humanos são mortos por elefantes a cada ano.

Elefante é o termo genérico e popular pelo qual são denominados os membros da família Elephantidae, um grupo de mamíferos proboscídeoselefantídeos, de grande porte, do qual há três espécies no mundo atual, duas africanas (Loxodonta sp.) e uma asiática (Elephas sp.). Há ainda os mamutes (Mammuthus sp.), hoje extintos. Até recentemente, acreditava-se que havia apenas duas espécies vivas de elefantes, o elefante-africano e o elefante-asiático, uma espécie menor. Entretanto, estudos recentes de DNA sugerem que havia, na verdade, duas espécies de elefante-africano: Loxodonta africana, da savana, e Loxodonta cyclotis, que vive nas florestas. Os elefantes são os maiores animais terrestres da actualidade, com a massa entre 4 a 6 toneladas e medindo em média quatro metros de altura, podem levantar até 10.000 kg. As suas características mais distintivas são as presas de marfim

Fonte: Wikipedia

  

Hluhluwe–Imfolozi Park

Hluhluwe–Imfolozi Park, formerly Hluhluwe–Umfolozi Game Reserve, is the oldest proclaimed nature reserve in Africa. It consists of 960 km² (96,000 ha) of hilly topography 280 kilometres (170 mi) north of Durban in central KwaZulu-Natal, South Africa and is known for its rich wildlife and conservation efforts. The park is the only state-run park in KwaZulu-Natal where each of the big five game animals can be found

Due to conservation efforts, the park in 2008 had the largest population of white rhino in the world

 

Umfolozi

This area is situated between the two Umfolozi Rivers where they divide into the Mfolozi emnyama ('Black Umfolozi') to the north and the Mfolozi emhlophe ('White Umfolozi') to the south. This area is to the south of the park and is generally hot in summer, and mild to cool in winter, although cold spells do occur. The topography in the Umfolozi section ranges from the lowlands of the Umfolozi River beds to steep hilly country, which includes some wide and deep valleys. Habitats in this area are primarily grasslands, which extend into acacia savannah and woodlands.

Hluhluwe

The Hluhluwe region has hilly topography where altitudes range from 80 to 540 metres (260 to 1,770 ft) above sea level. The high ridges support coastal scarp forests in a well-watered region with valley bushveld at lower levels. The north of the park is more rugged and mountainous with forests and grasslands and is known as the Hluhluwe area,[3] while the Umfolozi area is found to the south near the Black and White Umfolozi rivers where there is open savannah.

Source: Wikipedia

Parque Hluhluwe–Imfolozi

O Parque Hluhluwe – Imfolozi, anteriormente Reserva de Caça Hluhluwe – Umfolozi, é a mais antiga reserva natural proclamada da África. Consiste em 960 km² (96.000 ha) de topografia montanhosa a 280 quilômetros (170 milhas) ao norte de Durban, no centro de KwaZulu-Natal, África do Sul e é conhecida por seus ricos esforços de vida selvagem e conservação. O parque é o único parque estatal em KwaZulu-Natal, onde cada um dos cinco grandes animais de caça pode ser encontrado.

Devido aos esforços de conservação, o parque em 2008 teve a maior população de rinocerontes brancos do mundo

Umfolozi

Essa área está situada entre os dois rios Umfolozi, onde se dividem no Mfolozi emnyama ('Black Umfolozi') ao norte e o Mfolozi emhlophe ('White Umfolozi') ao sul. Essa área fica ao sul do parque e geralmente é quente no verão, e temperatura amena no inverno, embora ocorram períodos de frio. A topografia na seção de Umfolozi varia desde as planícies do leito do rio Umfolozi até a região montanhosa íngreme, que inclui alguns vales largos e profundos. Os habitats nesta área são principalmente pradarias, que se estendem até a savana de acácias e bosques.

Hluhluwe

A região de Hluhluwe possui topografia montanhosa, onde as altitudes variam de 80 a 540 metros (260 a 1.770 pés) acima do nível do mar. As altas cordilheiras sustentam florestas costeiras escarpadas em uma região bem regada, com vales em níveis mais baixos. O norte do parque é mais acidentado e montanhoso, com florestas e campos e é conhecido como a área de Hluhluwe, enquanto a área de Umfolozi fica ao sul, perto dos rios Umfolozi, onde há savanas abertas.

Fonte: Wikipedia (tradução livre)

 

Woodchucks, also known as groundhogs and whistle pigs, are the largest members of the squirrel family in their geographic range. They have ever-growing incisors that must be worn down by chewing to prevent serious health issues. They are true hibernators, which means they enter the burrow in the late fall and sleep until spring. Their burrows have multiple entrances, may have 45 feet of tunnels and go as deep as five feet.

Antoon van Dyck (Antwerp, 1599 - London, 1641) - Jacobus Neefs (engraver) - Martin Rychart - Van Dyck exhibition Court Painter - Turin, Royal Museums - Palatine Hall of the Sabauda Gallery

This animal lives at Banham Zoo in Norfolk.

 

These primates are called gelada baboons, however, they aren't baboons, instead, geladas form a separate genus of their own.

Geladas, which spend up to 99% of their time on the ground, are easily recognizable due to the hairless patches of skin on their chests. During the mating season, these chest patches acquire bright crimson coloration in females. Geladas have buff to dark brown hair with a dark face and pale eyelids. Males average 41 lb. (18.5 kg) and females average 24 lb. (11 kg) in weight. Their head-body length is 20 to 30 in. (50 to 75 cm) with a tail of 12 to 20 in. (30 to 50 cm).

They are the last surviving member of a grass-grazing primate group, members of which were abundant and widespread in the past. They have a seasonal diet generally feed upon grasses, blades, seeds and bulbs to which they are well adapted. Their small, powerful fingers are designed for pulling grass, while small incisors allow them to chew it. When eating, geladas move around with characteristic shuffle gait. When walking, they use all of their four limbs and slide their feet without changing the body posture, so that the bright red patch on their chest is conspicuous, whereas the rump remains hidden.

The natural range of this species is restricted to Ethiopia, where these animals mainly occur in in the Semien Mountains National Park. During the night, they typically sleep on rocky cliffs and outcrops. In the morning, geladas typically look for food in nearby grasslands, at heights of 6,550 to 16,400 ft. (2,000 to 5,000 m) above sea level.

Geladas are highly social animals, forming so called 'one male units' (OMU). These are female led groups that consist of a single male and multiple females with their young. When a male from the outside challenges the male of the OMU in order to displace it, females of the group may support or oppose both of them, accepting the winning male and fiercely driving away the defeated one. Various OMU's occasionally share the same area, thus forming larger units called bands. As these animals are non-territorial, they may be observed grazing in separate bands in areas with abundant food without any conflicts. Males and females can often be observed grooming each other. In general, all members of the community participate in grooming, which enhances social bonds within the OMU.

Geladas are polygynous, meaning that one male gets an exclusive right to mating with multiple females. Although geladas can mate at any time of the year, births appear to peak during the rainy season. Gestation period lasts for 5 to 6 months, yielding a single baby, which feeds upon its mother's milk for 1 to 1.5 years. The infant is mainly cared for by its mother, who will carry, groom, nurse and protect the baby, until it reaches the age of independence. Meanwhile, the father will take little part in rearing its offspring. Average life span in the wild is between 14 to 20 years.

Predators of geladas are leopards, hyenas, feral dogs, jackals, foxes, servals and bearded vultures.

One of the biggest threats to the population of this species is habitat reduction due to development of agriculture. Additionally, geladas are considered pests and thus shot because of their destruction of crops. According to the IUCN Red List, the total population of Geladas is around 200,000 animals. Currently, this species conservation status is classified as 'Least Concern' but its numbers are decreasing.

It's so much fun to watch these busy little beavers! This one is sitting on its lodge gnawing away at the twigs on top.

 

Thank you, my kind Flickr friends, for visiting my site and taking the time to leave a comment. Truly appreciated!

 

A beaver is a large rodent, or gnawing animal. Like all other rodents, the beaver has four chisel-shaped front teeth called "incisors". It is with these teeth that it cuts trees and bushes for food and for building dams. The beaver lives on wood, branches, saplings, and the roots of water plants.

 

Why do beavers build dams? The beaver lives in the water and it remains active all winter. Therefore, it needs a pool of water deep enough not to freeze quite to the bottom during the winter. So it builds a dam to raise the water level of the pond or stream in which it lives!

 

To build a dam, beavers place willow, alder, or other brush on the bottom of the stream. This is held in place with mud and stones. As the dam grows in height, sticks and branches may be placed in any position. Often the twigs take root and wind together and help to make the dam strong.

 

To cut a tree, the beaver gnaws two notches, one above the other. It pries out the wood between the notches with its teeth. Only one or two bites are needed to cut a stick 2 centimeters thick. Trees about 25 centimeters thick are used. A tree this size may be cut in one night. Generally trees with soft wood are used, such as the poplar, cotton wood, alder, willow, or birch.

 

Since beavers eat only the inner bark of trees and bushes, they may use the peeled sticks and logs to strengthen the dam. A dam is usually not more than two meters high, but it may be extremely long.The home of the beaver is called a "lodge." It may be a stick-covered shelter in the stream bank or a house of sticks and logs built in a shallow part of the pond. The floor of the room is just above the water line and is covered with weeds or shredded wood. The entrances are all under water.

 

In late summer and autumn, the beaver collects food for the winter. So brush, branches, and logs are cut and stored underwater near the lodge. These food supplies are sometimes over one meter high and contain hundreds of branches and saplings.

   

Sagui

Callitrichinae (também chamada Hapalinae) é uma subfamília de Macacos do Novo Mundo, da família Cebidae. Popularmente, são conhecidos por saguis, soim ou sauim, apesar de que para o gênero Leontopithecus, é mais comum o termo mico-leão.

 

Sagui-de-tufos-pretos

 

Um sagui[1][2] (do tupi sauín), soim ou mico são as designações comuns dadas a várias espécies de pequenos macacos pertencentes à família Callitrichidae. A palavra sagui tem origem no tupi e sua pronúncia é feita observando-se o som da vogal "u".

 

Estes primatas são representados por várias espécies em território brasileiro. Todos os quais possuem o dedo polegar da mão muito curto e não oponível, as unhas em forma de garras, e dentes molares de fórmula 2/2. São espécies de pequeno porte e de cauda longa.

 

São os menores símios do mundo, estão dispersos por toda a América do Sul e vivem geralmente em bandos que se hospedam em árvores, como os esquilos. Travessos e ágeis, movem-se em saltos bruscos, emitindo guinchos e assobios que são ouvidos de longe.

 

The black-tufted marmoset (Callithrix penicillata), also known as Mico-estrela in Portuguese, is a species of New World monkey that lives primarily in the Neo-tropical gallery forests of the Brazilian Central Plateau. It ranges from Bahia to Paraná,[3] and as far inland as Goiás, between 14 and 25 degrees south of the equator. This marmoset typically resides in rainforests, living an arboreal life high in the trees, but below the canopy. They are only rarely spotted near the ground.

 

Physical description:

 

The black-tufted marmoset is characterized by black tufts of hair around their ears. It typically has some sparse white hairs on its face. It usually has a brown or black head and its limbs and upper body are gray, as well as its abdomen, while its rump and underside are usually black. Its tail is ringed with black and white and is not prehensile, but is used for balance. It does not have an opposable thumb and its nails tend to have a claw-like appearance. The black-tufted marmoset reaches a size of 19 to 22 cm and weighs up to 350 g.

 

Behavior:

 

Diurnal and arboreal, the black-tufted marmoset has a lifestyle very similar to other marmosets. It typically lives in family groups of 2 to 14. The groups usually consist of a reproductive couple and their offspring. Twins are very common among this species and the males, as well as juvenile offspring, often assist the female in the raising of the young.

 

Though the black-tufted marmoset lives in small family groups, it is believed that they share their food source, sap trees, with other marmoset groups. Scent marking does occur within these groups, but it is believed that the marking is to deter other species rather than other black-tufted marmoset groups, because other groups typically ignore these markings. They also appear to be migratory, often moving in relation to the wet or dry seasons, however, the extent of their migration is unknown.

 

Though communication between black-tufted marmosets has not been studied thoroughly, it is believed that it communicates through vocalizations. It has known predator-specific cries and appears to vocalize frequently outside of predator cries.

 

Food and predation:

 

The Black-tufted Marmoset diet consists primarily of tree sap which it gets by nibbling the bark with its long lower incisors. In periods of drought, it will also include fruit and insects in its diet. In periods of serious drought it has also been known to eat small arthropods, molluscs, bird eggs, baby birds and small vertebrates.

 

Large birds of prey are the greatest threat to the black-tufted marmoset, however, snakes and wild cats also pose a danger to them. Predator-specific vocalizations and visual scanning are its only anti-predation techniques.

 

Reproduction:

 

The black-tufted marmoset is monogamous and lives in family groups. It reproduces twice a year, producing 1 to 4 offspring, though most often just twins. Its gestation period is 150 days and offspring are weaned after 8 weeks. There is considerable parental investment by this species, with both parents, as well as older juveniles, helping to raise the young. The offspring are extremely dependent on their parents and though they are sexually mature at 18 months, they typically do not mate until much later, staying with their family group until they do.

 

Ecosystem roles and conservation status:

 

The black-tufted marmoset is a mutualist with many species of fruit trees because it distributes the seeds from the fruit it consumes throughout the forests. However, it is a parasite on other species of trees because it creates sores in trees in order to extract sap, while offering no apparent benefit to the trees. Though this marmoset is not a main food source to any specific species, it is a food source to a number of different species, specifically large birds of prey, wild cats, and snakes.

 

While there are no known negative effects of marmosets towards humans, it carries specific positive effects by being a highly valuable exotic pet. It is also used in zoo exhibits and scientific research.

 

The black-tufted marmoset is listed as having no special status on the IUCN Red List or the United States Endangered Species Act List. It is listed in Appendix II of CITES and is not currently considered an endangered or threatened species.

 

Herewith is an intimate look at beavers placidly wintering in the inner harbour of Kingston, Ontario on 22 February to 3 March, 2021.

Contrary to widespread belief, beavers do not hibernate. Although they may be less active, they continue to eat and if necessary build throughout the winter.

Here, Johnny and his partner are cautiously emerging from their urban-style lodge, which they share with muskrats in a well-documented cohabitation behavior. Beavers will not build damns if they do not have to, such as is the case for those featured herein.

Beavers groom frequently to remove debris from their coat and to waterproof it with oil from glands, a live-or-die ritual especially in the cold winter months.

Beavers have poor eyesight. They rely on their acute sense of smell and hearing to investigate their surroundings for signs of danger prior to venturing away from the safety of their den.

Beavers go on frequent excursions into nearby woodlands walking through thick and thin to reach their food sources. They are strictly herbivores. They consume fallen wood sticks and freshly cut fine twigs, often on-site.

Using their ever-growing and sharpened incisors aided by a powerful jaws, beavers can cut shrubs and tree members with amazing ease. Harvesting large cuts may at times yield comical results. Such harvests are often dragged to and/or inside the den for immediate consumption or stowing.

The beaver’s scaly tail is his most recognized feature. It is used as a rudder when swimming and to sound alarms to other beavers when sensing danger. The tail stores fat for the winter months. Contrary to fictitious belief, they do not use their tails to carry mud or pack it down.

Beavers are semiaquatic rodents with large webbed hind feet that are ideal for swimming. During the winter, they will dive under the ice to dig up aquatic plants and waterlogged wood and bring the materials up onto the ice surface or to shore for consumption.

Beavers hand-like front paws allow them to manipulate objects with great dexterity. They seem to have methods to eating twigs and the cambium (soft new growth close to the surface) of larger pieces of wood. Their chewing sounds often give away their location.

Natives of America greatly respected beavers, calling them “Little People” as they too have the ability to greatly alter their habitats to suit their own needs. Beavers have been ecosystem engineers for more than 10 millions years and it may be in our best interest to let them create and maintain wetlands that sustain biodiversity.

Video footage included in this video were by and large captured with long lenses with the outmost respect for the beavers’ well-being and safety. The close proximity shots were from the beavers voluntarily approaching my position which at times required mine backing off in order to maintain lens focus on the subjects.

 

Video Footage & Post-Production by Réjean Lemay

©2021 Réjean Lemay, all rights reserved (except for music listed below)

 

All Music from YouTube Library (in chronological order)

When Johnny Comes Marching Home by Air Force Band of Liberty

Whispering Stream by E's Jammy Jams

Dark Fog by Kevin MacLeod licensed under a Creative Commons Attribution 4.0 license. creativecommons.org/licenses/by/4.0/ Source: incompetech.com/music/royalty-free/index.html?isrc=USUAN1... Artist: incompetech.com/, Kevin MacLeod

Fresh Fallen Snow by Chris Haugen

 

Surma woman with her giant lip plate, a sign of beauty in Surma tribe, like in Mursi one. When they are ready to marry, they start to make a hole in the lip with a wood stick.

It will be kept for one night , and is removed to put a bigger one. This is very painful at this time... Few months after, the lip plate has its full size, and the girl is seen as beautiful by the men.

The lip plate made of wood or terracotta, and they have to remove the lower incisors to let some space for the disc. it's amazing to see them speak without any trouble, put it and remove it as a classic jewel.

Sometimes the lip is broken by the pressure of the lip plate. This is a very big problem for the girl cos men will consider her as ugly, she won't be able to marry anyone in the tribe apart the old men or the sick people...

The women are shaved, like the men, cos they hate hairiness!

 

© Eric Lafforgue

www.ericlafforgue.com

The collared mangabey (Cercocebus torquatus), also known as the red-capped mangabey, or the white-collared mangabey[3] (leading to easy confusion with Cercocebus atys lunulatus), is a species of primate in the Cercopithecidae family of Old World monkeys. It formerly included the sooty mangabey as a subspecies. As presently defined, the collared mangabey is monotypic.

 

The collared mangabey has grey fur covering its body, but its common names refer to the colours on its head and neck. Its prominent chestnut-red cap gives it the name red-capped, and its white collar gives it the names collared and white-collared. Its ears are black and it has striking white eyelids, which is why some refer to it as the "four-eyed monkey". It has a dark grey tail that exceeds the length of the body and is often held with the white tip over its head. It has long molars and very large incisors. The average body mass for captive individuals ranges from 9 to 10 kg (20 to 22 lb) for males and 7.5 to 8.6 kg (17 to 19 lb) for females. Head-body length is 47–67 cm (19–26 in) in males and 45–60 cm (18–24 in) in females.

 

The collared mangabey is found in coastal, swamp, mangrove, and valley forests, from western Nigeria, east and south into Cameroon, and throughout Equatorial Guinea, and Gabon, and on the Gabon-Congo border by the Atlantic shore.

 

The collared mangabey lives in large groups of 10 to 35 individuals including several males. Vocal communication in the form of cackles and barks are used to keep the group in contact and signal their position to other groups. It has a diet of fruits and seeds, but also eats leaves, foliage, flowers, invertebrates, mushrooms, dung, and gum. The collared mangabey has no defined breeding season, it reaches sexual maturity at five to seven years, and has an average gestation period of 170 days.

 

These mangabeys have a throat sac which they use to make loud calls that can be heard over long distances.

 

Like their fellow subspecies, Alaska moose are remarkable herbivores that thrive on a diet rich in terrestrial vegetation, particularly forbs and new shoots from willow and birch trees. What truly sets them apart is their astonishing daily caloric intake, which can reach an impressive 9,776.5 calories! Despite the absence of upper front teeth, these majestic creatures adapt beautifully, employing eight sharp incisors on their lower jaw, a tough tongue, resilient gums, and flexible lips to chew through even the most challenging woody vegetation expertly. This incredible adaptability highlights the strength and resourcefulness of the Alaska moose in their natural habitat.

© Eric Lafforgue

www.ericlafforgue.com

 

It takes 2 days driving in an all wheel drive from Nairobi to arrive in Loiyangalani on the Turkana lake shores… you have never heard about this place? And yet it’s here that they filmed « The Constant Gardener » with Ralph Fiennes.

The Lake Turkana region presents a lunar landscape, somewhat desert, covered in black volcanic rocks. It’s an extremely inhospitable environment for humans and their livestock. There is no potable water and limited pastures. The rainfall averages is less than 6 inches a year. During the day the high temperatures (up to 45°C) are come with strong winds (up to 11 meters per second), pushing dust. But it’s just a magical place on earth !

No human should be able to live in these conditions and yet 250,000 Turkana people are living here. Their territory extends to northern Kenya around Lake Turkana, and on the boundaries with south Sudan and Ethiopia. In 1975, the lake (400 km long, 60 large) was named after them.

 

Herders Above All Else : The importance of livestock

They are a traditionally pastoralist tribe, moving their livestock (goats, sheep, camels, cattle, and donkeys) and their homes to search water for their animals. Turkana have not been affected by western civilization yet and live in a very traditional way. The number of animals and the diversity of the herd are closely linked to a family’s status in the community. The herds are their bank account.

They depend on the rain to provide grazing for their animals, and on their animals for milk and meat. Because water is so hard to find in the area, they often fight with other tribes like Dassanech. Their main concerns are land and how to win it or to keep it!

The Turkana place such a high value on cattle that they often raid other tribes to steal animals. These razzias have become more dangerous as they now use guns. As the Turkana are one of the most courageous groups of warriors in Africa, fights are serious!

After a raid, the robbers ask some friends from neighboring villages to keep some cows. Their herd is scattered between several places to reduce the risk of being stolen the whole.

 

The Turkana choose their good friends as neightbors more so than people they share kinship ties with. The clans (ekitela), 28 in number, no longer have a social function. Each clan owns water wells dug in the dried river beds. Unless an explicit request is made, the community can deny water to those passing by.

Even today, the Turkana never kill their livestock to sell their meat. They only kill for celebrations. The Turkana need their animals since they use them as currency in marriage or various social transactions. If a man loses his livestock to drought, he is not only impoverished but shamed. In these cases, NGOs often help get him back on his feet but he can’t reclaim his pride until he has reestablished his herd.

The animals are given very poetic names which the owners often take on as well. It’s common to call a good friend the name of his favorite bull. The Turkana even write songs for their favorite animals. Once a young man has selected his favorite bull, he shapes its horns into bizarre forms to make it stand out. Many tribes use to do this in the area.

 

The Fish is Taboo for the Herdsmen

 

Turkana people traditionally do not fish and do not eat fish. But during the droughts, Turkana people are encouraged to fish to get some food. Fishing has been regarded as something of a taboo, a practice reserved for the very poorest in Turkana society.

 

Social Structure

The Turkana are organized into generational classes. All males go through three life stages (child, warrior, and elder).

To become a man, the turkana teen must go through a ceremony where he will have to kill an animal with a spear, but he must kill it in one throw! Once done, the old men will open the stomach of the animal and put the content on the body of the new adult. It is the way they bless him.

For women, the process is different. They become adult when they reach puberty. Unlike many other tribes in Kenya, the Turkana do not practice FGM and circumcision.

The Turkana live in small households. Inside live of a man, his wives !as he can marry more than one), their children and sometimes some dependent old people. The house is called « awi ». It is built with wood, animal skin, and doum palm leaves. Only the women build the houses!

Herding is a family affair. The father assigns various tasks to his children depending on their age. It’s common to see kids walking long distances with the cattle. Later they will take care of sheep and goats. The girls carry water and collect wood.

Newborns receive their names in a unique way. They take the name of a parent who has huge prestige and add the name of the most beautiful animal in the herd.

Parents learn very early to the kids the taboos: you must not lie, be coward, steal, neglect elders…

Turkana have their own justice and the revenge system is working well: if a crime is committed, the family of the victim will try to kill the murderer or someone from its close family. They also can steal to the suspect a large amount of cattle. Usually, the elders try to make a reconciliation ceremony. It is an never ending story as the family will also want to make a vandetta of the vendetta !

If the homicide was an accident, it can be solved by giving a daughter in marriage.

 

Marriage

When a man wants to marry a girl, he must ask his own parents if they agree. His mother will have to check if the girl he wants is a good worker! The blood relationship between the families is forbidden, so the elders will check the family links before any agreement.

The man must pay the bride parents (30 cattle, 30 camels and 100 small stock minimum, sometimes a gun is added). It means that a man cannot marry until he has inherited livestock from his dead father. It also means that he collects livestock from relatives and friends. This strengthens social ties.

Daily life

Cattle dungs are used as fuel to cook the food, the urine is used as soap for washing when chemical soap is not available. I saw people using the urine to wash the milk containers, so I always refused to drink milk!

Camels are used for transportation of goods and are well adapted to the very arid climate of Turkana and the lack of water. They are also used in transactions for weddings, or economics deals.

Donkeys have a special status in Turkana tribe: the people do not drink its milk. They use them to carry their houses when they move or weak people with a special wood saddle. But even if donkeys are very useful, they are mocked by the turkana people. Donkey meat is eaten only in the Turkana, where it is savored as a delicacy while others tribe hate it!

They like chewing tobacco and often walk around with a chewed up ball of it on their ear. They also like snorting powdered tobacco.

Danses and songs are important in the social life. Dances allow the people to meet and to flirt. Circle dances are are performed by group of young unmarried girls. The men and young girls join hands and the circles move around. The men may then jump into the centre of the circle raising their arms to imitate the cow horns.

Spirituality, Superstitions, Beliefs

In 1960, a famine started in Turkana area, and so the « Africa Inland Mission » established a food-distribution centre in Lokori, bringing also christianity. But conversion did not meet a huge success (5 % may be converted) as Turkana are nomadics and still have strong believes in their own god. Some Turkana elders even told me :

« I wear a christian cross around my neck and go to the church to get an access to the help provided by the the missionaries for food and clothes! »

The majority of the Turkana still follow their traditional religion. There's one supreme God called Akuj, who is associated with the sky. If God is happy, he will give rain. But if he is angry with the people, he will punish them. In the old believings, giraffes were supposed to tickle the clouds with their high heads, and make the rain come !

Four million years ago, the Lake Turkana bassin may have been the cradle of mankind. You can spot some very nice engraving sites showing a mixture of giraffes and geometrics patterns made around 2000 years ago close to the lake.

Deviners, called the « emuron » are able to interpret or predict Akuj's plans through their dreams, or through sacrificed animal's intestines, tobacco, and through the tossing of …sandals ! Sandals are very important for the oracle. He blesses the sandals by spitting on them. He throws them up into the air and gives a meaning to the patterns they create when they fall on the ground.

When someone dies, the Turkana only hold funerals and burry the body. In the old times, people were were not given a burial, but were abandoned to hyenas.

 

As I was taking pictures of an old Turkana lady, after 3 pictures, she asked me to stop, and started to shout : « You’re sucking my blood, you make me feel weak » and she left. I was explained by a young boy that the old people believe that pictures are taking their blood away.

 

Medecine

Scarifications on the belly are made by traditional doctors to cure ill people: it is a way to put out the illness from the body. Scarification is practiced for aesthetic reasons too. Scars are a sign of beauty or to show how many people he has killed, if he is a man.

The skin is cut with an acacia or a sharp razor blade that may be shared by the people and bring diseases.

 

Turkana believe that a person who experienced illness and recovered from it can treat someone else who’s suffering from the same illness. This means that everybody can be a doctor ! If this does not work, they say that the animal slaughtered was the wrong one.

A good Turkana tip : if you suffer from a severe headache, you just have to take out the brain from a living animal, like a goat, and put it on your head !

Or, another solution : to lift a sheep over the patient, to cut the throat so that the blood strickles on the patient’s head.

 

The Turkana have the highest instance in the world of echinoccocus (7%) due to their proximity with dogs, who live and defecate everywhere. The dogs lick up blood and vomit and the women use the dog’s excrement as a lubricant for the necklaces that touch their neck.

This parasite has three hosts : sheep, dogs, and humans. In Turkana, these three species live very close, surrounded by little else in the vast desert, ideal conditions for the proliferation of the parasite. The diease causes huge cysts that can be removed by surgery. The locals believe that this "disease of the large belly" is due to a spell cast by the neighboring enemy tribe: the Toposa.

 

Beauty

Turkana girls and women love to adorn themselves with a lot of necklaces. Beads can be made of glass, seeds, cowry shells, or iron. They never remove them! This can only happen when they are ill or during a mourning time. It means they sleep with those huge necklaces… A married Turkana woman will also wear a plain metal ring around the neck. This is a kind of wedding ring (alagama). A Turkana man will do all he can to make sure that his women folk are dressed in beads of class. Even if some are not able to take their girls to school, they will still ensure that they have beads. By the quantity and style of jewelry a woman wears, you can guess her social status.

 

Beads colors have specific meaning. Yellow and red beads are given to girl by a man when they are fiancé. If a woman wears only white beads, it means she is a widow. Little girls wear few beads, usually given to them by their mothers, but the older ladies and women wear many, which are in sets rows.

A woman who cannot move her neck is envied! The big necklaces are heavy, like 5 kilos.

 

A woman without beads is bad, men will ignore her. « You look like an animal without beads! »

Young children only wear a simple strand of pearls. Adolescents wear small articles of clothing to cover their sex. These articles are often decorated with mulitcolored pearls or ostrich egg shells. They wear more and longer clothing as they approach puberty.

 

NakaparaparaI are the famous ear ornaments. They are made by the men of the tribe in aluminium most of the time and look like a leaf.

 

Men love to make an elaborate mudpack coiffures called emedot. It is a kind of chignon: the hairstyle takes the shape of a large bun of hair at the back of the head. They decorate it with ostrich feathers to show they are elders or warriors. 2 ostrich feathers costs 1 goat.

 

Men use a wood pillow (ekicolong) to sleep on it and protect the bun. It can last 2 months and must be rebuild after.

 

Tattooing is also common and usually has special meaning. Men are tattooed on the shoulders and upper arm each time they kill an enemy — the right shoulder for killing a man, the left for a women.

Lower incisors are removed in childhood, with a tool called « corogat », a finger hook. The origin of this practice was against tetanus, as people are lock-jawed, so they can feed them with milk through the hole. It is also a way to force the teeth at the top to stand out and not interfere with the labret many put on the lower lip. The is useful to spit through the gap of the teeth, without even opening the mouth. The Turkana enjoyed to have labrets, but nowadays, only the elders can be seen with on. They used to put an ivory lip plug, then a wood one, and for some years, they use a lip plug made of copper or even with plaited electric wires.The hole between the lower lip and chin is pierced using a thorn.

The finger hook is also used as a weapon, for gouging out an ennemy’s eye !

Hygiene

Since water is so rare, it’s used only for drinking, never for washing. The Turkana clean themselves by rubbing fat all over their skin.

Turkana women put grease paint on their bodies which is made from mixing animal fat with red ochre and the leaves of a tree to have nice perfume. They say it is good for the skin and it protects from the insects.

Women also put animal fat all around their neck and also on their huge necklaces to prevent from skin irritation.

They also use dog shit as a medicine and lubrificant for their neck.

 

Both men and women use the branch of a tree called esekon to clean their teeth. You can see them using it all day long…The Turkana people have the cleanest bill of dental health in the country.

For long, Turkana people did not use latrines because it is a taboo for men and women to share same facilities like a latrine. Campaigns have now been initiated to sensitize people on the importance of using latrines for hygiene.

 

Animal fat is considered to have medicinal qualities, and the fat-tailed sheep is often referred to as "the pharmacy for the Turkana. »... when they do not grill it to eat it!

 

Futur

Recently, oil has been found on their territory… many fear Turkanas people may loose their traditions, but the Turkana succeeded in maintaining their way of life for centuries. Against all odds they manage to raise livestock in the confines of the desert. Their knowledge allows them to live where most humans could not.

The recent discovery of massive groundwater reserves in the ground (3 billion cubic meters, nearly three times the water use in New York City) could allow them to keep their traditions for a long time.

 

Civica Raccolta d'Arte e Raccolta degli incisori marchigiani

Sassoferrato (AN)

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

A tribute to the men who did this little masterpiece...

 

Omaggi dal cuore:

- a mio padre per la creazione delle miscele di vetro blu per incamiciare i bicchieri (non sono tinti!)

- al soffiatore di cui non ricordo il nome (sono soffiati a bocca!)

- a Gino l'incisore (sono lavorati a mano!)

Tutto ormai 40 anni fa ...

(Vetreria S.I.V.I.S. - Vado Ligure)

 

Do not use any of my images on websites, blogs or other media without my explicit written permission.

All rights reserved - Copyright © fotomie2009 - Nora Caracci

www.russinitalia.it/dettaglio.php?id=191

 

Fedor Brenson

 

Luogo e data di nascita: Riga, 1893

Luogo e data di morte: Peterborough (New Hampshire), 21 settembre 1959

Professione: pittore, incisore, illustratore

 

Avviato allo studio dell’arte nelle scuole di Riga, Pietrogrado e Mosca, Brenson arriva in Italia nel 1924 e si stabilisce a Roma, dove entra a far parte del Gruppo Romano Incisori Artisti e prende contatti con altri russi, tra cui Vjačeslav Ivanov e Maksim Gor’kij. La fama di Brenson giunge presto a Milano. Qui, nel gennaio 1927, presso la Galleria Pesaro, si tiene la sua prima mostra personale; nell'introduzione al catalogo il suo talento viene celebrato da Pavel Muratov.

Durante la sua permanenza in Italia, Brenson dipinge numerosi paesaggi italiani: tra questi, Roma, Firenze, Napoli, Venezia ed alcune località della Calabria, che vengono poi riunite nel volume Visioni di Calabria, edito da Vallecchi nel 1929.

Nel 1931 Brenson è già residente a Parigi, ma mantiene saldi i contatti con il mondo artistico italiano: nell’archivio di Pietro Maria Bardi, storico proprietario dell’omonima galleria milanese e dal 1929 direttore della Galleria di Roma, è conservata una lettera del 14 marzo 1931 da Parigi, nella quale Brenson riferisce di aver parlato a Monsieur Louis Godefroy, proprietario di una galleria di Parigi, a proposito di un’esposizione di incisori contemporanei che lui e Bardi progettavano di organizzare a Parigi. Nel novembre dello stesso anno, presso la Galleria Milano di Milano Brenson partecipa ad una mostra collettiva intitolata "Peintres graveurs contemporaines" e dedicata ai maestri francesi del bianco e nero. È invece dell’aprile 1933 una mostra personale, allestita presso la Galleria Tre Arti di Milano.

Traendo spunto da questa mostra, Vincenzo Bucci coglie l’occasione per tracciare sul "Corriere della Sera" un bel ritratto di Brenson e del suo amore per l’Italia:

 

Prima di darsi all’incisione e alla pittura, Brenson studiò architettura, e dell’architetto è rimasta qualche traccia nel vedutista, amante delle belle prospettive. In Italia venne, com’egli ha detto, a cercare "la forma": grandi linee e grandi masse, in una classica luce senza nebbia, per le esigenze di un’arte plastica e costruttiva. E preso nel fascino di Roma, […] cominciò col ritrarre, in due gruppi d’acqueforti, prima i ruderi solenni del Ponte Rotto, delle Terme di Caracalla, del Palatino, poi la cupola, il colonnato, l’architettonica maestà di San Pietro. Il suo amore dell’Italia crebbe coi viaggi. Pellegrino appassionato, visitò la Puglia riportandone le punte secche che illustrano Trani, il Volture, Castel del Monte: dell’Umbria fermò a sanguina alcuni aspetti d’Assisi; e fino dal treno la sua passione d’annotatore si sfogò a cogliere di volo tra Firenze e Bologna, in rapidi disegni, i profili caratteristici dell’Appennino pistoiese. Anche l’intensa vita operosa del porto di Genova, e Genova stessa con le sue prospettive tutte sorprese, spazzatura, dislivelli, gli diedero qualche bello spunto, e nel ’27, da un attento pellegrinaggio in Calabria riportò cinquanta disegni, che furono raccolti in volume e lodati pel modo come v’era resa e sentita la grandiosità del paesaggio calabrese (V. Bucci, Notizie d'arte, «Corriere della Sera», 25 aprile 1933).

Dal 1941 Brenson risiede con la moglie Vera e il figlio Michael negli Stati Uniti, dove è protagonista di numerose mostre. È professore di arte in diversi college della costa Est degli Stati Uniti.

 

Pubblicazioni

A. De Stefani, I giocattoli con disegni di Loris Riccio, Ugo Ugoletti, Carlo Staffetti, Teodoro Brenson, «Il 1919. Rassegna mensile illustrata della vecchia guardia fascista (1926-1927)», Milano, Arti grafiche Pizzi e Pizio, 1927, n. 2.

Theodor Brenson, Voyage à travers la Pouille: notes d'un peintre, Milano - Roma, Bestetti & Tumminelli, 1928.

Visioni di Calabria. Cinquantadue disegni di Teodoro Brenson con una introduzione storico-artistica di Luigi Parpagliola, Firenze, Vallecchi, 1929, Collezione meridionale diretta da Umberto Zanotti-Bianco; serie III : il Mezzogiorno artistico.

 

Bibliografia

P. Frank, Theodor Brensons graphiche Dichtungen, "Die Woche in Bild und Wort", n. 24, Riga, oktober 1925, pp. 727-730.

Pavel Muratov, Teodoro Brenson, Collana de L’odierna arte del bianco e nero, Milano, Edizione della Galleria Pesaro, 1927.

Mostre milanesi, «Le Arti Plastiche», 16 novembre 1931.

Vincenzo Bucci, Notizie d’arte, «Corriere della Sera», 25 aprile 1933.

Carlo Carrà, Mostre milanesi, «L’Ambrosiano», 24 maggio 1933.

Obituaries: Theodore Brenson 1893-1959, «College Art Journal» 1959, vol. 19, n. 1, p. 74.

Lidija Ivanova, Vospominanija, Moskva, Rik Kul'tura, 1992, p. 153.

 

Memorie sul soggiorno in Italia

Come ricorda Lidija Ivanova, Brenson fece un'incisione bizzarra del ritratto di Vjačeslav Ivanov, seduto in mezzo a Piazza del Popolo, imbacuccato in un enorme impermeabile nero, mentre sopra di lui, nel cielo, vola un dirigibile.

 

Nota

Nelle fonti italiane si incontra come Teodoro o Theodore Brenson.

 

Fonti archivistiche

Biblioteca Trivulziana, Archivio Storico Civico di Milano, Fondo Pietro Maria Bardi .

 

Nell'immagine schizzo di Tropea di Teodoro Brenson, tratto da "Visioni di Calabria", 1929

www.tropeamagazine.it/helenareis/index.html

 

Patrizia Deotto e Raffaella Vassena

 

Bell Apartments

 

The Bell Apartments, also known as the Austin A. Bell Building is a historic building located at 2326 1st Avenue in the Belltown neighborhood of Seattle Washington. The building was named for Austin Americus Bell, son of one of Seattle's earliest pioneers, but built under the supervision of his wife Eva following Bell's unexpected suicide in 1889 soon after proposing the building.[2]

 

It was designed with a mix of Richardsonian, Gothic and Italianate design elements by notable northwest architect, Elmer Fisher, who designed many of Seattle's commercial buildings following the Great Seattle fire.

 

The Bell Building, along with the adjacent Barnes and Hull Buildings, form the nucleus of a development attempt in Belltown in the 1890s that never materialized. Early on, the building earned the moniker of Bell's Folly for being built so far away from the central business district in the then underdeveloped and economically depressed Belltown neighborhood, named for Bell's father, William Nathaniel Bell, once landowner of the entire north end of Seattle.[2] The area today is considered the heart of Belltown and the Bell building remains one of Belltown's most historic landmarks.

 

The building fell into disrepair throughout most of the 20th century, eventually losing its massive cornice to a fire in 1913. The building was first surveyed in June 1969 and included on the Municipal Art Commission List of Historic Buildings, at which time it was nominated for inclusion on the National Register. It was finally listed on the National Register of Historic Places on July 12, 1974.[3] It also became a Seattle City Landmark in 1978.[4] The upper floors stayed vacant until the 1990s, sustaining much weather damage in the meantime and later being destroyed by fire. Most of the building was rebuilt behind the main facade in 1997–1998 and now houses condominiums with a Starbucks Coffee on the first level.

 

The Austin A. Bell Building is a four-story structure of brick with Terra cotta ornamentation. Both brick and Terra cotta are of a distinct reddish-orange hue and the mortar joints are narrow. Its architect, Elmer H. Fisher would later design many of the buildings in the Pioneer Square district of Seattle. While Fisher's designs were predominantly Richardsonian, the Bell Building is less so, with several Gothic elements thrown into the mix.

 

The ground floor served as a commercial space for two businesses and the windows are large for display purposes. Slender cast iron columns frame the recessed doorway. Wooden decorative panels appear above the transom of the double doors. A single door in an arched entry way at the south end of the storefront provides access to the upper floors. cast iron columns support the small arch and a decorative wooden panel also appears above the doorway.

 

Rock faced stone columns on either side of the doorway and storefront continue for the first story only and then are extended by brick pilasters to the full height of the building. The brick pilasters above the doorway continue above the fourth story to form a small tower. The tower is flat-roofed now but was originally capped by a shallow-pitched roof.

Detail of the central pediment displaying Bell's name and scars of the lost cornice.

 

Window openings are deeply recessed. Windows occur in sets of three above the commercial section of the ground floor in pairs on either side of bordering pilasters. Single windows are placed on each floor above the south doorway. All windows on the second floor are tall and rectangular and the central group of three windows are separated by cast iron columns. The same pattern is repeated on the third and fourth floors although flanking openings on the third floor have segmental pointing arches and the narrow fourth-floor windows are fully arched. Terra cotta decorations appear above window openings on the second and third floors.

 

A high parapet extends above the fourth floor and it is composed of a more coarse brick than is the rest of the facade. This is the scar remaining from a large cornice, either made of pressed tin or Terra cotta, that was removed from the building after a 1913 fire, harming its architectural integrity greatly.[5] The pilasters continue through the parapet and the central pilasters and recessed within a brick equilateral arch is a Terra cotta wheel window design flanked at the base by Terra cotta fillets filling out the base of the arch.

 

The interior once featured a central courtyard that spanned the top three floors. It was lighted by a set of three large skylights. The interior finishing and moldings were typical for the time period and much of it was salvaged during the building's reconstruction in 1997.[6]

 

Austin Americus Bell was the only son of William N. Bell, one of Seattle's founders and a member of the Denny Party that landed at Alki Point on November 13, 1851 on the schooner Exact. His land claim, adjacent to those of the Denny and Boren families, developed into the community of Belltown, which would later be absorbed by Seattle. Austin was born on January 9, 1854 in a log cabin across from the present site of the Bell Building, the second male white child born in the city. The cabin was destroyed in 1856 during The Battle of Seattle. Fed up with the instability of the area, the Bell family removed to California, where they remained for several years.[7]

 

Following the death of William Bell's wife in 1870, the family moved back to Seattle, now much more built up then when they had left. Austin however remained in San Francisco where he had built up a career in real estate. The value of Bell's property had skyrocketed in the previous years and he proceeded to make improvements to the land. In 1877, he constructed a two-story wooden building for the Odd Fellows on the site where his son's building would eventually stand.[8] In late 1883, the senior Bell completed his last great project, the four story wooden Bell Hotel at the corner of First and Battery Streets, adjacent to the Odd Fellows Hall. Suffering from mental illness in his later years, William Bell died September 8, 1887 at the age of 70. He left to Austin a quarter his estate valued at $400,000, which continued to increase in value. Bell soon became one of Seattle's wealthiest citizens.[7]

 

During his father's illness and subsequent death, Austin began to see signs of similar mental illness in himself, now thought to be Alzheimer's disease. Austin was living and working in Seattle as a printer at the Puget Sound Dispatch when he was severely injured in a fireworks explosion on July 4, 1872. The explosion severely lacerated the lower part of his face, removing all the incisors from his lower jawbone.

 

He had spent most of his life in San Francisco and many years traveling to seek medical treatment to restore his health, while retaining a real estate office in Seattle. Upon returning to Seattle in 1889, he made plans to erect a large brick building on the property he inherited just south of his father's hotel, since renamed the Bellevue House.

 

On April 23, 1889, he took his nephew, William M. Coffman, on a buggy ride to share his plans.[9] The next morning, his demeanor had changed and he was suffering from indigestion. He went to his realty office as usual, locked the door behind him and shot himself in the head. A neighboring merchant heard the shot and rushed across the street to tell a pharmacist that a murder must have occurred. After summoning a doctor, several men broke down Bell's door to find him dead by his own hand. He was 35. A suicide note to Eva was discovered stating that he did not consider life with poor health worth living and expressed sorrow that he must take this way out.[7]

The Bell Building

 

Eva carried out her husband's plans, drawn by local architect Elmer Fisher, and dedicated the building in her husband's name. Completed in 1890, the building adorned in stone, pressed red brick shipped in from San Francisco and red Terra cotta cost $50,000 and was hailed as one of the showiest buildings in the city. The building contained two store rooms on the first floor and 63 apartments above, dubbed The Belle Apartments; the owners having parlayed Bell's name into the French word for fine or pretty.[10] The first commercial tenants included the Woodhouse – Longuet & Co. Hardware store and Rafael Sartori's liquor store. Newspapers later noted the building as having the "dour, brooding aspect of the unhappy man that built it."[7]

 

During the Alaska Gold Rush of 1897, the Bell served as a hotel and dance hall. Pool tables were manufactured in the building in the early 1900s. In the early morning of May 12, 1913, a fire from a nearby building destroyed the roof and upper floor of the Bell Building, leading to the loss of most of the architectural elements above the roof line. At the time of the fire the top floor held a fully occupied boarding lodge known as The Sioux, but no lives were lost in the fire.[5] In 1937, for $9,800, the building was added to bargain real estate tycoon (and later Samis Foundation founder) Sam Israel's stock of low-maintenance properties. Like most of Israel's property, he kept the roof fixed, rented the retail space for cheap, and left the upper floors vacant.[11] While Belltown began to flourish as an artists' community in the 1970s, the Bell building continued to deteriorate.[12] Despite the shape it was in, it was listed on the National Register of Historic Places in 1974 and became a City of Seattle Landmark on October 23, 1978.[4]

The Bell Building today

 

In 1982, the upper floors were ravaged by a fire, destroying much of what was left of the interior.[13] Developers Cassimar US Inc. and Murray Franklyn Co., working jointly as Austin A. Bell Associates L.L.C., bought the gutted and boarded up building and an adjoining parking area from the Samis Foundation in 1997 for $1 million.[6][14] Under the supervision of architect Chris Snell, all but the facade and the south wall of the building was demolished and a new 52-unit condominium structure with underground parking and 6,600 square feet (610 m2) of retail space was built in its shell while a new connecting building sympathetic in design to its neighbors was built on the parking lot.[15] The 6 million dollar project was completed by the Spring of 1998.[16][17] One of the first retail tenants was the fifth installment of Starbucks Coffee's short-lived concept restaurant, Cafe Starbucks. A regular Starbucks Coffee store currently occupies the entire first floor retail space.[18]

en.wikipedia.org/wiki/Bell_Apartments

======================

Belltown is the most densely populated neighborhood in Seattle, Washington, United States,[1][2] located on the city's downtown waterfront on land that was artificially flattened as part of a regrading project. Formerly a low-rent, semi-industrial arts district, in recent decades it has transformed into a neighborhood of trendy restaurants, boutiques, nightclubs, and residential towers as well as warehouses and art galleries. The area is named after William Nathaniel Bell,[3] on whose land claim the neighborhood was built.

 

In 2007, CNNMoney named Belltown the best place to retire in the Seattle metro area, calling it "a walkable neighborhood with everything you need."[4]

 

Belltown is home to Antioch University, Argosy University, City University of Seattle, and the Seattle School of Theology & Psychology. It lies directly west of the Denny Triangle neighborhood, where online retailer Amazon is constructing three office towers to house its downtown headquarters, and where the Cornish College of the Arts is located.

en.wikipedia.org/wiki/Belltown,_Seattle

The hippopotamus, hippopotamuses or hippopotami, also shortened to hippo, hippos or Hippopotamus amphibius, further qualified as the common hippopotamus, Nile hippopotamus, or river hippopotamus, is a large semiaquatic mammal native to sub-Saharan Africa. It is one of only two extant species in the family Hippopotamidae, the other being the pygmy hippopotamus (Choeropsis liberiensis or Hexaprotodon liberiensis). Its name comes from the ancient Greek for "river horse" (ἱπποπόταμος).

 

After elephants and rhinoceros, the hippopotamus is the next largest land mammal. It is also the largest extant land artiodactyl. Despite their physical resemblance to pigs and other terrestrial even-toed ungulates, the closest living relatives of the hippopotamids are cetaceans (whales, dolphins, porpoises, etc.), from which they diverged about 55 million years ago. Hippos are recognisable for their barrel-shaped torsos, wide-opening mouths with large canine tusks, nearly hairless bodies, pillar-like legs, and large size: adults average 1,500 kg (3,300 lb) for bulls (males) and 1,300 kg (2,900 lb) for cows (females). Despite its stocky shape and short legs, it is capable of running 30 km/h (19 mph) over short distances.

 

Hippos inhabit rivers, lakes, and mangrove swamps. Territorial bulls each preside over a stretch of water and a group of five to thirty cows and calves. Mating and birth both occur in the water. During the day, hippos remain cool by staying in water or mud, emerging at dusk to graze on grasses. While hippos rest near each other in the water, grazing is a solitary activity and hippos typically do not display territorial behaviour on land. Hippos are among the most dangerous animals in the world due to their aggressive and unpredictable nature. They are threatened by habitat loss and poaching for their meat and ivory (canine teeth).

 

Etymology

The Latin word hippopotamus is derived from the ancient Greek ἱπποπόταμος (hippopótamos), from ἵππος (híppos) 'horse' and ποταμός (potamós) 'river', together meaning 'horse of the river'. In English, the plural is "hippopotamuses", but "hippopotami" is also used.

 

Taxonomy and origins

Classification

The modern hippopotamus and the pygmy hippopotamus are the only living members of the family Hippopotamidae. Some taxonomists place hippos and anthracotheres in the superfamily Anthracotheroidea. Hippopotamidae are classified along with other even-toed ungulates in the order Artiodactyla.

  

Detail of the head

Five subspecies of hippos have been described based on morphological differences in their skulls as well as differences in geographical range:

 

H. a. amphibius – (the nominate subspecies) ranges from Gambia east to Ethiopia and then south to Mozambique and historically ranged as far north as Egypt; its skull is distinguished by a moderately reduced preorbital region, a bulging dorsal surface, elongated mandibular symphysis and larger chewing teeth.

H. a. kiboko – found in Kenya and Somalia; was noted to be smaller and more lightly coloured than other hippos with wider nostrils, somewhat longer snout and more rounded and relatively raised orbits with the space between them being incurved.

H. a. capensis – found in Zambia and South Africa; distinguished by wider orbits.

H. a. tschadensis – ranges between Chad and Niger; featured a slightly shorter but broader face, and pronounced, forward-facing orbits.

H. a. constrictus – ranged from the southern Democratic Republic of Congo to Angola and Namibia; skull characterised by a thicker preorbital region, shorter snout, flatter dorsal surface, reduced mandibular symphysis and smaller chewing teeth.

The suggested subspecies above were never widely used or validated by field biologists; the described morphological differences were small enough that they could have resulted from simple variation in nonrepresentative samples. A study examining mitochondrial DNA from skin biopsies taken from 13 sampling locations found "low, but significant, genetic differentiation" among H. a. amphibius, H. a. capensis, and H. a. kiboko. Neither H. a. tschadensis nor H. a. constrictus have been tested.

 

Evolution

Until 1909, naturalists classified hippos together with pigs based on molar patterns. Several lines of evidence, first from blood proteins, then from molecular systematics and DNA and the fossil record, show their closest living relatives are cetaceans (whales, dolphins, and porpoises). The common ancestor of hippos and whales branched off from Ruminantia and the rest of the even-toed ungulates; the cetacean and hippo lineages split soon afterwards.

 

Artiodactyla

Tylopoda

Artiofabula

Suina

Cetruminantia

Ruminantia

Whippomorpha

Hippopotamidae

Cetacea

  

Anthracotherium magnum from the Oligocene of Europe

The most recent theory of the origins of Hippopotamidae suggests hippos and whales shared a common semiaquatic ancestor that branched off from other artiodactyls around 60 million years ago. This hypothesised ancestral group likely split into two branches again around 54 million years ago.

 

One branch would evolve into cetaceans, possibly beginning about 52 million years ago, with the protowhale Pakicetus and other early whale ancestors collectively known as Archaeoceti. This group eventually underwent aquatic adaptation into the completely aquatic cetaceans. The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of which in the late Eocene would have resembled skinny hippos with comparatively smaller, narrower heads. All branches of the anthracotheres, except that which evolved into Hippopotamidae, became extinct during the Pliocene, leaving no descendants.

 

A rough evolutionary lineage of the hippo can thus be traced from Eocene and Oligocene species: from Anthracotherium and Elomeryx to the Miocene species Merycopotamus and Libycosaurus and finally the very latest anthracotheres in the Pliocene. These groups lived across Eurasia and Africa. The discovery of Epirigenys in East Africa, which was likely a descent of Asian anthracotheres and a sister taxon to Hippopotamidae, suggests that hippo ancestors entered Africa from Asia around 35 million years ago. An early hippopotamid is the genus Kenyapotamus, which lived in Africa from 15 to 9 million years ago. Hippopotamid species would spread across Africa and Eurasia, including the modern pygmy hippo. From 7.5 to 1.8 million years ago, a possible ancestor to the modern hippo, Archaeopotamus, lived in Africa and the Middle East. The oldest records of the genus Hippopotamus date to the Pliocene (5.3-2.6 million years ago). The oldest unambiguous records of the modern H. amphibius date to the Middle Pleistocene, though there are possible Early Pleistocene records.

 

Extinct species

Three species of Malagasy hippopotamus became extinct during the Holocene on Madagascar, the last of them within the past 1,000 years. The Malagasy hippos were smaller than the modern hippo, a likely result of the process of insular dwarfism. Fossil evidence indicates many Malagasy hippos were hunted by humans, a factor in their eventual extinction. Isolated individual Malagasy hippos may have survived in remote pockets; in 1976, villagers described a living animal called the kilopilopitsofy, which may have been a Malagasy hippo.

 

Hippopotamus gorgops from the Early Pleistocene-early Middle Pleistocene of Africa and West Asia grew considerably larger than the living hippopotamus, with an estimated body mass of over 4,000 kilograms (8,800 lb). Hippopotamus antiquus ranged throughout Europe, extending as far north as Britain during the Early and Middle Pleistocene epochs, before being replaced by the modern H. amphibius in Europe during the latter part of the Middle Pleistocene. The Pleistocene also saw a number of dwarf species evolve on several Mediterranean islands, including Crete (Hippopotamus creutzburgi), Cyprus (the Cyprus dwarf hippopotamus, Hippopotamus minor), Malta (Hippopotamus melitensis), and Sicily (Hippopotamus pentlandi). Of these, the Cyprus dwarf hippo survived until the end of the Pleistocene or early Holocene. Evidence from the archaeological site Aetokremnos continues to cause debate on whether or not the species was driven to extinction, or even encountered, by man.

 

Characteristics

The hippopotamus is a megaherbivore and is exceeded in size among land animals only by elephants and some rhinoceros species. The mean adult weight is around 1,480 kg (3,260 lb) for bulls and 1,365 kg (3,009 lb) for cows. Exceptionally large males have been recorded reaching 2,660 kg (5,860 lb). Male hippos appear to continue growing throughout their lives, while females reach maximum weight at around age 25. Hippos measure 2.90 to 5.05 m (9.5 to 16.6 ft) long, including a tail of about 35 to 56 cm (1.15 to 1.84 ft) in length and 1.30 to 1.65 m (4.3 to 5.4 ft) tall at the shoulder, with males and females ranging 1.40 to 1.65 m (4.6 to 5.4 ft) and 1.30 to 1.45 m (4.3 to 4.8 ft) tall at the shoulder respectively. The species has a typical head-body length of 3.3–3.45 m (10.8–11.3 ft) and an average standing height of 1.4 m (4.6 ft) at the shoulder.

 

Hippos have barrel-shaped bodies with short tails and legs, and an hourglass-shaped skull with a long snout.  Their skeletal structures are graviportal, adapted to carrying their enormous weight, and their dense bones and low centre of gravity allows them to sink and move along the bottom of the water. Hippopotamuses have small legs (relative to other megafauna) because the water in which they live reduces the weight burden. The toes are webbed and the pelvis rests at an angle of 45 degrees.  Though chubby-looking, hippos have little fat.  The eyes, ears, and nostrils of hippos are placed high on the roof of their skulls. This allows these organs to remain above the surface while the rest of the body is submerged.  The nostrils and ears can close when underwater while nictitating membranes cover the eyes.  The vocal folds of the hippo are more horizontally positioned, much like cetaceans. Underneath are throat tissues, where vibrations are transmitted to produce underwater calls.

  

Characteristic "yawn" of a hippo

The hippo's jaw is powered by huge masseter and digastric muscles which give them large, droopy cheeks. The jaw hinge allows the animal to open its mouth at almost 180°.  A folded orbicularis oris muscle allows the hippo to attain an extreme gape without tearing any tissue. On the lower jaw, the incisors and canines grow continuously, the former reaching 40 cm (1 ft 4 in), while the latter can grow to up to 50 cm (1 ft 8 in). The lower canines are sharpened through contact with the smaller upper canines. The canines and incisors are used mainly for combat instead of feeding. Hippos rely on their flattened, horny lips to grasp and pull grasses which are then ground by the molars.  The hippo is considered to be a pseudoruminant; it has a complex three-chambered stomach, but does not "chew cud".

  

Completely submerged hippo (San Diego Zoo)

Hippo skin is 6 cm (2 in) thick across much of its body with little hair.  The animal is mostly purplish-grey or blue-black, but brownish-pink on the underside and around the eyes and ears. Their skin secretes a natural, red-coloured sunscreen substance that is sometimes referred to as "blood sweat" but is neither blood nor sweat. This secretion is initially colourless and turns red-orange within minutes, eventually becoming brown. Two highly acidic pigments have been identified in the secretions; one red (hipposudoric acid) and one orange (norhipposudoric acid), which inhibit the growth of disease-causing bacteria and their light-absorption profile peaks in the ultraviolet range, creating a sunscreen effect. Regardless of diet, all hippos secrete these pigments so food does not appear to be their source; rather, they may be synthesised from precursors such as the amino acid tyrosine. This natural sunscreen cannot prevent the animal's skin from cracking if it stays out of water too long.

 

The testes of the males do not fully descend and a scrotum is not present. In addition, the penis retracts into the body when not erect. The genitals of the female hippos are unusual in that the vagina is ridged and the vulval vestibule has two large, protruding diverticula. Both of these have an unknown function. 

 

A hippo's lifespan is typically 40 to 50 years.  Donna the Hippo was one of the oldest living hippos in captivity. She lived at the Mesker Park Zoo in Evansville, Indiana, in the US until her death in 2012 at the age of 61. The oldest hippo ever recorded was called Bertha; she had lived in the Manila Zoo in the Philippines since it first opened in 1959. When she died in 2017, her age was estimated to be 65.

 

Distribution and status

During the Middle Pleistocene to the early Late Pleistocene (~500,000–80,000 years ago) Hippopotamus amphibius was present in Europe, extending as far north as England during the Eemian (130–115,000 years ago), with fossils also known from Portugal, Spain, Italy, and Greece. The species first entered Europe between 560,000 and 460,000 years ago, while the last H. amphibius remains in Europe are dated to about 30,000 years ago. Archaeological evidence exists of its presence in the Levant, dating to less than 3,000 years ago. The species was common in Egypt's Nile region during antiquity, but it has since been driven out. According to Pliny the Elder, in his time, the best location in Egypt for capturing this animal was in the Saite nome; the animal could still be found along the Damietta branch of the Nile after the Arab Conquest in 639. Reports of the slaughter of the last hippo in Natal Province were made at the end of the 19th century. Hippos are still found in the rivers and lakes of the northern Democratic Republic of the Congo, Uganda, Tanzania, and Kenya, north through to Ethiopia, Somalia, and Sudan, west to The Gambia, and south to South Africa.

 

Genetic evidence suggests common hippos in Africa experienced a marked population expansion during or after the Pleistocene, attributed to an increase in water bodies at the end of the era. These findings have important conservation implications, as hippo populations across the continent are currently threatened by loss of access to fresh water. Hippos are also subject to unregulated hunting and poaching. The species is included in Appendix II of the Convention on International Trade in Endangered Species (CITES) meaning international export/import (including in parts and derivatives) requires CITES documentation to be obtained and presented to border authorities.

 

As of 2017, the IUCN Red List drawn up by the International Union for Conservation of Nature (IUCN) lists the species as vulnerable, with a stable population estimated between 115,000 and 130,000 animals. The hippo population has declined most dramatically in the Democratic Republic of the Congo. By 2005, the population in Virunga National Park had dropped to 800 or 900 from around 29,000 in the mid-1970s. This decline is attributed to the disruptions caused by the Second Congo War. The poachers are believed to be Mai-Mai rebels, underpaid Congolese soldiers, and local militia groups. Reasons for poaching include the belief hippos are harmful to society, as well as financial gain. As of 2016, the Virunga hippo population appears to have increased again, possibly due to better protection from park rangers, who have worked with local fishermen. The sale of hippo meat is illegal, but black-market sales are difficult for Virunga National Park officers to track. Hippo meat is highly valued in some areas of central Africa and the teeth may be used as a replacement for elephant ivory.

 

A population of hippos exists in Colombia, descended from captive individuals that escaped from Pablo Escobar's estate after his death in 1993. Their numbers grew to 100 by the 2020s and ecologists believe the population should be eradicated, as they are breeding rapidly and are an increasing menace to humans and the environment. Attempts to control them include sterilisation and culling.

 

Behaviour and ecology

Hippos are semiaquatic and require enough water to immerse in, while being close to grass. Like most herbivores, hippos will consume a variety of plants if presented with them in captivity, but their diet in nature consists almost entirely of grass, with only minimal consumption of aquatic plants. They prefer relatively still waters with gently sloping shores, though male hippos may also be found in very small numbers in more rapid waters with rocky slopes.  Hippos mostly live in freshwater habitat, but can be found in estuaries. Despite being semiaquatic, an adult hippo is not a particularly good swimmer, nor can it float. It rarely enters deep water; when it does, the animal moves by bouncing off the bottom. An adult hippo surfaces every four to six minutes, while young need to breathe every two to three minutes.

 

Hippos spend most the day in water to stay cool and hydrated. Just before night begins, they leave the water to forage on land. Hippos usually trot to move quickly on land and can gallop at 30 km/h (19 mph) when needed. They are incapable of jumping but can walk up steep banks. A hippo will travel 3–5 km (1.9–3.1 mi) per night, eating around 40 kg (88 lb) of grass. By dawn, they are back in the water. The hippopotamus sleeps with both hemispheres of the brain resting, as in all land mammals, and usually sleeps on land or in water with the nostrils exposed. Despite this, it may be capable of sleeping while submerged, intermittently surfacing to breathe without waking. They appear to transition between different phases of sleep more quickly than other mammals.

 

Because of their size and their habit of taking the same paths to feed, hippos can have a significant impact on the land across which they walk, keeping the land clear of vegetation and depressing the ground. Over prolonged periods, hippos can divert the paths of swamps and channels. By defecating in the water, the animals also appear to pass on microbes from their gut, affecting the biogeochemical cycle. On occasion, hippos have been filmed eating carrion, usually near the water. There are other reports of meat-eating and even cannibalism and predation. Hippos' stomach anatomy lacks adaptions to carnivory and meat-eating is likely caused by lack of nutrients or just an abnormal behaviour.

 

Social life

It is challenging to study the interaction of bulls and cows because hippos are not sexually dimorphic, so cows and young bulls are almost indistinguishable in the field. Hippo pods fluctuate but can contain over 100 hippos. Although they lie close together, adults develop almost no social bonds. Males establish territories in water but not land, and these may range 250–500 m (270–550 yd) in lakes and 50–100 m (55–109 yd) in rivers. Territories are abandoned when the water dries up. The bull has breeding access to all the cows in his territory. Younger bachelors are allowed to stay as long as they defer to him. A younger male may challenge the old bull for control of the territory. Within the pods, the hippos tend to segregate by sex and status. Bachelor males lounge near other bachelors, females with other females, and the territorial male is on his own. When hippos emerge from the water to graze, they do so individually.

  

Male hippos fighting

Hippos engage in "muck-spreading" which involves defecating while spinning their tails to distribute the faeces over a greater area. Muck-spreading occurs both on land and in water and its function is not well understood. It is unlikely to serve a territorial function, as the animals only establish territories in the water. They may be used as trails between the water and grazing areas.  "Yawning" serves as a threat display. When fighting, bulls use their incisors to block each other's attacks and their large canines as offensive weapons.  When hippos become over-populated or a habitat shrinks, bulls sometimes attempt infanticide, but this behaviour is not common under normal conditions.

 

The most common hippo vocalisation is the "wheeze honk", which can travel over long distances in air. This call starts as a high-pitched squeal followed by a deeper, resonant call.  The animals can recognise the calls of other individuals. Hippos are more likely to react to the wheeze honks of strangers than to those they are more familiar with. When threatened or alarmed, they produce exhalations, and fighting bulls will bellow loudly. Hippos are recorded to produce clicks underwater which may have echolocative properties. They have the unique ability to hold their heads partially above the water and send out a cry that travels through both water and air; individuals respond both above and below water.

 

Reproduction

Cows reach sexual maturity at five to six years of age and have a gestation period of eight months. A study of endocrine systems revealed cows may begin puberty at as early as three or four years. Males reach maturity at around 7.5 years. Both conceptions and births are highest during the wet season. Male hippo always have mobile spermatozoa and can breed year-round.  After becoming pregnant, a female hippo will typically not begin ovulation again for 17 months.

  

Preserved hippopotamus fetus

Hippos mate in the water, with the cow remaining under the surface,  her head emerging periodically to draw breath. Cows give birth in seclusion and return within 10 to 14 days. Calves are born on land or shallow water weighing on average 50 kg (110 lb) and at an average length of around 127 cm (4.17 ft). The female lies on her side when nursing, which can occur underwater or on land. The young are carried on their mothers' backs in deep water. 

 

Mother hippos are very protective of their young, not allowing others to get too close. One cow was recorded protecting a calf's carcass after it had died. Calves may be temporarily kept in nurseries, guarded by one or more adults, and will play amongst themselves. Like many other large mammals, hippos are described as K-strategists, in this case typically producing just one large, well-developed infant every couple of years (rather than many small, poorly developed young several times per year, as is common among small mammals such as rodents). Calves no longer need to suckle when they are a year old. 

 

Interspecies interactions

Hippos coexist alongside a variety of large predators in their habitats. Nile crocodiles, lions, and spotted hyenas are known to prey on young hippos. Beyond these, adult hippos are not usually preyed upon by other animals due to their aggression and size. Cases where large lion prides have successfully preyed on adult hippos have been reported, but it is generally rare. Lions occasionally prey on adults at Gorongosa National Park and calves are sometimes taken at Virunga. Crocodiles are frequent targets of hippo aggression, probably because they often inhabit the same riparian habitats; crocodiles may be either aggressively displaced or killed by hippos. In turn, very large Nile crocodiles have been observed preying occasionally on calves, "half-grown" hippos, and possibly also adult female hippos. Groups of crocodiles have also been observed finishing off still-living male hippos that were previously injured in mating battles with other males.

 

Hippos occasionally visit cleaning stations in order to be cleaned of parasites by certain species of fishes. They signal their readiness for this service by opening their mouths wide. This is an example of mutualism, in which the hippo benefits from the cleaning while the fish receive food. Hippo defecation creates allochthonous deposits of organic matter along the river beds. These deposits have an unclear ecological function. A 2015 study concluded hippo dung provides nutrients from terrestrial material for fish and aquatic invertebrates, while a 2018 study found that their dung can be toxic to aquatic life in large quantities, due to absorption of dissolved oxygen in water bodies.

 

The parasitic monogenean flatworm Oculotrema hippopotami infests hippopotamus eyes, mainly the nictitating membrane. It is the only monogenean species (which normally live on fish) documented to live on a mammal.

 

Hippos and humans

The earliest evidence of human interaction with hippos comes from butchery cut marks on hippo bones found at the Bouri Formation and dated to around 160,000 years ago. 4,000–5,000 year art showing hippos being hunted have been found in the Tassili n'Ajjer Mountains of the central Sahara near Djanet. The ancient Egyptians recognised the hippo as a ferocious denizen of the Nile and representations on the tombs of nobles show the animals were hunted by humans.

 

The hippo was also known to the Greeks and Romans. The Greek historian Herodotus described the hippo in The Histories (written circa 440 BC) and the Roman naturalist Pliny the Elder wrote about the hippo in his encyclopedia Naturalis Historia (written circa 77 AD). The Yoruba people called the hippo erinmi, which means "elephant of the water". Some individual hippos have achieved international fame. Huberta became a celebrity during the Great Depression for trekking a great distance across South Africa. 

 

Attacks on humans

The hippo is considered to be extremely aggressive and has frequently been reported charging and attacking boats. Small boats can easily be capsized by hippos and passengers can be injured or killed by the animals, or drown in the water. In one 2014 case in Niger, a boat was capsized by a hippo and 13 people were killed. Hippos will often raid farm crops if the opportunity arises, and humans may come into conflict with them on these occasions. These encounters can be fatal to either humans or hippos.

 

According to the Ptolemaic historian Manetho, the pharaoh Menes was carried off and then killed by a hippopotamus.

 

In zoos

Hippos have long been popular zoo animals. The first record of hippos taken into captivity for display is dated to 3500 BC in Hierakonpolis, Egypt. The first zoo hippo in modern history was Obaysch, who arrived at the London Zoo on 25 May 1850, where he attracted up to 10,000 visitors a day and inspired a popular song, the "Hippopotamus Polka".

 

Hippos generally breed well in captivity; birth rates are lower than in the wild, but this can be attributed to zoos' desire to limit births, since hippos are relatively expensive to maintain. Starting in 2015, the Cincinnati Zoo built a US$73 million exhibit to house three adult hippos, featuring a 250,000 L (66,000 US gal) tank. Modern hippo enclosures also have a complex filtration system for the animal's waste, an underwater viewing area for the visitors, and glass that may be up to 9 cm (3.5 in) thick and capable of holding water under pressures of 31 kPa (4.5 psi).  In 1987, the Toledo Zoo saw the first underwater birth by a captive hippo. The exhibit was so popular, the logo of the Toledo Zoo was updated to feature the hippos.

 

Cultural significance

In Egyptian mythology, the god Set takes the form of a red hippopotamus and fights Horus for control of the land, but is defeated. The goddess Tawaret is depicted as a pregnant woman with a hippo head, representing fierce maternal love. The Ijaw people of the Niger Delta wore masks of aquatic animals like the hippo when practising their water spirit cults, and hippo ivory was used in the divination rituals of the Yoruba. Hippo masks were also used in Nyau funerary rituals of the Chewa of Southern Africa.[97]: 120  According to Robert Baden-Powell, Zulu warriors referred to hippos in war chants. The Behemoth from the Book of Job, is thought to be based on the hippo.

 

Hippos have been the subjects of various African folktales. According to a San story, when the Creator assigned each animal its place in nature, the hippos wanted to live in the water, but were refused out of fear they might eat all the fish. After begging and pleading, the hippos were finally allowed to live in the water on the condition they would eat grass instead of fish, and fling their dung so it can be inspected for fish bones. In a Ndebele tale, the hippo originally had long, beautiful hair, but it was set on fire by a jealous hare and the hippo had to jump into a nearby pool. The hippo lost most of his hair and was too embarrassed to leave the water.

  

The "Hippopotamus Polka"

Hippopotamuses were rarely depicted in European art during the Renaissance and Baroque periods, due to less access to specimens by Europeans. One notable exception is Peter Paul Rubens' The Hippopotamus and Crocodile Hunt (1615–1616).  Ever since Obaysch inspired the "Hippopotamus Polka", hippos have been popular animals in Western culture for their rotund appearance, which many consider comical. The Disney film Fantasia featured a ballerina hippo dancing to the opera La Gioconda. The film Hugo the Hippo is set in Tanzania and involves the title character trying to escape being slaughtered with the help of local children. The Madagascar films feature a hippo named Gloria.  Hippos even inspired a popular board game, Hungry Hungry Hippos.

 

Among the most famous poems about the hippo is "The Hippopotamus" by T. S. Eliot, where he uses the animal to represent the Catholic Church. Hippos are mentioned in the novelty Christmas song "I Want a Hippopotamus for Christmas" that became a hit for child star Gayla Peevey in 1953. They also featured in the popular "The Hippopotamous Song" by Flanders and Swann. 

 

A popular internet myth reports that hippos have pink milk. Biologist David Wynick states, "I think this is an Internet legend that is oft repeated but without any evidence for it that I can find... Like all mammals, hippos produce white or off-white milk for their young.

Giovanni Battista Piranesi

Giovanni Battista Piranesi, detto anche Giambattista (Mogliano Veneto, 4 ottobre 1720 – Roma, 9 novembre 1778), è stato un incisore, architetto e teorico dell'architettura italiano.

 

Le sue tavole incise, segnate da un'intonazione e una grafica drammatiche, appaiono improntate ad un'idea di dignità e magnificenza tutta romana, espressa attraverso la grandiosità e l'isolamento degli elementi architettonici, in modo da pervenire ad un sublime sentimento di grandezza del passato antico, pur segnato da inesorabile abbandono.

Formazione

Giovanni Battista Piranesi nacque il 4 ottobre 1720 da Angelo e da Laura Lucchesi, e fu battezzato l'8 ottobre nella parrocchia di San Moisè a Venezia. Non è supportata da documenti la tradizione che nacque a Mogliano Veneto: i genitori abitavano in Corte Barozzi a Venezia. Venne introdotto allo studio dell'architettura dal padre, esperto tagliapietre e capomastro, e dallo zio materno Matteo Lucchesi, magistrato alle acque della Serenissima e amante dell'antico sui modelli di Andrea Palladio e di Vitruvio; dal colto fratello Angelo, frate domenicano, trasse invece una certa padronanza della lingua latina e il duraturo amore per Tito Livio e la storia di Roma. Dopo una controversia con lo zio, il giovane Giovanni Battista continuò la propria formazione con Giovanni Scalfarotto, anch'egli architetto orientato verso un gusto che già preannuncia il neoclassicismo; frequentò, inoltre, la bottega di Carlo Zucchi.

 

Nel 1740 Piranesi, divenuto consapevole delle scarse possibilità lavorative che gli avrebbe offerto la capitale veneta, decise di lasciare la propria terra patria e di trasferirsi a Roma, partecipando in qualità di disegnatore alla spedizione diplomatica del nuovo ambasciatore della Serenissima Francesco Venier. Partito il 9 settembre, arrivò nell'Urbe entro il mese, all'età di soli venti anni, ottenendo un alloggio presso palazzo Venezia. Rivelando ben presto le proprie attitudini da disegnatore, dopo l'iniziale apprendistato con i pittori-scenografi Domenico e Giuseppe Valeriani e con Giovanni Battista Nolli, intorno al 1742 il Piranesi apprese i rudimenti dell'acquaforte sotto la guida di Giuseppe Vasi, titolare di una bottega calcografica che al tempo godeva a Roma di una certa popolarità. Sempre nell'Urbe, inoltre, Piranesi ebbe modo di stringersi in affettuosa amicizia con il conterraneo Antonio Corradini, con cui intorno al 1743 si recò a Napoli per studiare l'arte barocca e visitare gli scavi archeologici di Ercolano.

Ben presto Piranesi iniziò a palesare un commosso entusiasmo davanti allo spettacolo delle «parlanti ruine» dei Fori Imperiali, «che di simili non arrivai di potermene mai formare sopra i disegni, benché accuratissimi che di queste stesse ha fatto l’immortale Palladio, che io pur sempre mi teneva inanzi agli occhi». Questo interesse per le antichità romane è attestato dall'esecuzione nel 1743 della Prima parte di architetture e prospettive inventate e incise da Gio. Batta Piranesi architetto veneziano; per realizzare questa raccolta di dodici tavole, dove già si impone per le sue notevoli capacità tecniche, Piranesi si consultò con la ricca biblioteca di Nicola Giobbe, per intercessione del quale riuscì anche ad entrare in contatto con Luigi Vanvitelli e Nicola Salvi.

 

Piranesi effettuò un primo bilancio della sua carriera artistica tra il 1744 e il 1747, quando spinto dalla mancanza di riconoscimenti e dalle pressanti condizioni economiche fece temporaneamente ritorno a Venezia. In questo soggiorno, peraltro scarsamente documentato, Piranesi probabilmente volle riflettere su quanto appena compiuto dal punto di vista artistico, anche in vista di scelte future: fu, inoltre, in rapporto con Giovan Battista Tiepolo e con il Canaletto, i quali lasciarono un'impronta profonda sulla sua fantasia. Alla fine, il Piranesi decise di dedicarsi al mestiere di incisore e di stabilirsi definitivamente a Roma, aprendo bottega propria a via del Corso, di fronte all'Accademia di Francia: si trattò di una scelta ben meditata, come osservato dallo studioso Henri Focillon che commentò: Accetta volutamente di essere un incisore perché capisce di poter realizzare così le sue ambizioni di architetto, archeologo e pittore.

 

Il prestigio

All'inizio del suo definitivo insediamento il Piranesi, affascinato dalle antichità della Città Eterna, iniziò la produzione delle Vedute di Roma. Si trattava di una raccolta di tavole raffiguranti ruderi classici e monumenti antichi, anche esterni alla città (via Appia, Tivoli, Benevento), che gli assicurarono una cospicua remunerazione e anche rinomanza europea, grazie soprattutto al «grande formato delle tavole, al taglio sempre originale e prospetticamente accattivante delle composizioni, alla scelta mai scontata dei soggetti» (Treccani).

 

Si trattò questo di un periodo di profondo fermento artistico per il Piranesi, che al di là delle Vedute di Roma pubblicò diverse opere. In tal senso, si segnalano le Opere varie di architettura, prospettive, grotteschi, antichità sul gusto degli antichi romani, inventate e incise da Gio. Piranesi architetto veneziano (1750), le Camere sepolcrali degli antichi romani, le quali esistono dentro e fuori di Roma (fra il 1750 e il 1752), e la prima edizione delle Carceri, con il titolo Invenzioni capric. di carceri all’acqua forte datte in luce da Giovani Buzard in Roma mercante al Corso (1745). Un cenno a parte va fatto per queste opere, pubblicate in due edizioni nel 1745 e nel 1761: insieme alle Vedute romane, le Carceri rappresentano l'opera più famosa, diffusa, e anche remunerativa di tutta la sua produzione. Tale celebrità va ricercata nella scelta di un soggetto assai caro al mondo barocco, ma reinterpretato enfatizzandone non solo il rimando alla romanità, bensì anche il carattere onirico e inquietante, talmente forte che le Carceri furono ritenute da Marguerite Yourcenar «una delle opere più segrete che ci abbia lasciato in eredità un uomo del XVIII secolo».[

La notorietà di cui già allora il Piranesi godeva venne ulteriormente accresciuta nel periodo intercorso tra gli anni di pubblicazione delle due Carceri, ovvero il 1745 e il 1761. In questo arco di tempo, infatti, iniziò a diffondersi il fenomeno del grand tour, ovvero un lungo viaggio per le principali città d'interesse artistico e culturale dell'Europa continentale, considerato quasi d'obbligo allora per le persone del gran mondo: tappa fondamentale di questo giro era ovviamente Roma, con i suoi monumenti della civiltà antica e le sue prestigiose gallerie d'arte. In questo modo nell'Urbe si formò una cospicua comunità internazionale, e il Piranesi non tardò a diventare un punto di riferimento irrinunciabile della nuova vita artistica e intellettuale sorta in città. Importante fu l'amicizia con Thomas Hollis, gentiluomo britannico versato nelle arti presente in Italia nel 1751-53, che contribuì a consolidarne la fama e a diffonderne le opere; in virtù del prestigio raggiunto, e soprattutto grazie all'intercessione di Hollis, nel 1757 Piranesi venne perfino eletto membro onorario della Society of Antiquaries di Londra. Tra le amicizie legate al fenomeno di grand tour, comunque, si ricordano quella con l’architetto Robert Mylne, l'architetto scozzese Robert Adam, a Roma nel 1755-57 (cui Piranesi dedicò nel 1762 il Campo Marzio dell’antica Roma), l'architetto William Chambers, il pittore Thomas Jones; non mancò di fraternizzare anche con numerosi pittori francesi, fra cui Charles-Louis Clérisseau, Jean-Laurent Legeay, Jacques Gondoin, Charles de Wailly, Pierre-Louis Moreau-Desproux, e Pierre-Adrien Pâris.[

Il pontificato di Clemente XIII

Intanto Roma serbava le tracce di un nuovo movimento artistico, sorto come reazione all'edonismo del rococò e caratterizzato da un ritorno alle forme classiche: si trattava del neoclassicismo, cui Piranesi conferì un personalissimo impulso grazie alla pubblicazione dei quattro volumi delle Antichità Romane, per un totale di 252 tavole. Importante fu l'ascesa al soglio pontificio nel 1758 del veneziano Clemente XIII, nato Carlo della Torre di Rezzonico, che ben presto divenne un munifico protettore e mecenate del Piranesi. Fu proprio sotto il suo pontificato - precisamente nel 1761, al tempo delle seconde Carceri - che l'artista pubblicò Della magnificenza e architettura de’ romani, saggio storico corredato di immagini teso a sostenere la supremazia dell'architettura romana su quella greca.

 

Con il papato di Clemente XIII si moltiplicarono per l'artista gli incarichi e i riconoscimenti ufficiali. Eletto accademico onorario di San Luca nel 1761, e cavaliere dello Speron d'oro nel 1766, nel 1761 il Piranesi fu inviato dal Papa a studiare i restauri all'interno del Pantheon; due anni dopo, nel 1763, venne invece incaricato di intervenire sul piedistallo della colonna di Marco Aurelio con una statua della Giustizia e di modificare la zona absidale della basilica di San Giovanni in Laterano, edificio già restaurato da Francesco Borromini tra il 1646 e il 1649. Fu proprio mentre si occupava della chiesa lateranense che Piranesi ricevette la sua commessa architettonica più importante: si trattava della trasformazione della piccola chiesa di Santa Maria del Priorato e della piazza antistante, su commissione del cardinale Giovanni Battista Rezzonico, nipote del pontefice e priore dell'Ordine di Malta a Roma. Il cantiere si concluse nell'ottobre 1766, restituì alla città di Roma un tempio caratterizzato da un'austera eleganza neoclassica, squisitamente settecentesca, misurata nelle strutture e negli ornati e fruttò all'artista l'onorificenza di Cavaliere dello Speron d'oro. In questi anni, inoltre, l'operosità del Piranesi si estese anche alla decorazione degli edifici della famiglia pontificia. È del 1767 la decorazione degli appartamenti al Quirinale e a Castel Gandolfo di monsignor Giovanni Battista, mentre nel 1768-69 Piranesi decorò l'appartamento in Campidoglio del senatore Abbondio, disegnando soffitti, arredi, e cornici di camini.

 

Alla maturità più tarda appartengono Il Campo Marzio dell’antica Roma (1762) e le Diverse maniere di adornare i cammini (1769), dove è testimoniata l'intensa attività di Piranesi nella lucrosa commercializzazione di camini e oggetti decorativi, già rilevata nel 1770 dal pittore Vincenzo Brenna che affermò: «Piranesi ha fatto una raccolta così grande di marmi, che oltre avere riempito tutta la sua casa ha preso moltissime botteghe nella sua strada che sono anche piene, e per tutto si lavora e tiene da trenta persone il giorno a lavorare li suoi marmi, ha guasi lasciato da incidere, e si è buttato a traficare di marmi antichi». A quest'ultima opera si collega un'antologia di oggetti d'arredo denominata Vasi, candelabri, cippi che esercitò un'influenza notevole tra i gli orafi, i bronzisti e i lapicidi.

 

Giovan Battista Piranesi morì infine il 9 novembre 1778 a Roma, stroncato da una malattia nella sua casa in strada Felice (l'attuale n. 48 di via Sistina). Fu sepolto nella chiesa di Santa Maria del Priorato, da lui progettata, per volontà del cardinale Rezzonico, con la statua del defunto realizzata su commissione della famiglia dallo scultore Giuseppe Angelini; il sepolcro era adornato anche da un candelabro marmoreo predisposto dallo stesso artista, poi confiscato da Napoleone Bonaparte durante la campagna d'Italia e ricollocato nel Louvre, dove è tuttora esposto.

Concezione artistica e stile

L'eclettismo delle sue opere e la versatilità del suo estro creativo rendono Piranesi un artista difficilmente inseribile all'interno di una schematicità dettata da una suddivisione in stili o correnti artistiche. Personalità dalla duplice matrice culturale, veneziana e romana, Piranesi presenta una fisionomia artistica assai complessa, che si può scandire in tre componenti fondamentali.

 

L'arte di Piranesi, infatti, ha radici profondamente affondate nella tradizione del rococò, del quale egli rappresenta uno degli ultimi eredi. Quest'adesione al rococò è riscontrabile non solo nella qualità del disegno, sfatto ed evocatore, ma soprattutto nella natura stessa delle sue opere, che si configurano come invenzioni capricciose (come si legge nel frontespizio delle Carceri): con questa denominazione squisitamente rococò, infatti, Piranesi voleva indicare il carattere immaginoso e inconsueto delle proprie creazioni.

 

Il nucleo del discorso artistico di Piranesi si inserisce anche all'interno del neoclassicismo. Con la sensibilità neoclassica, infatti, Piranesi condivide l'impegno metodico e teorico e la passione per l'archeologia, maturata dopo la visita degli scavi di Ercolano. Questa caratteristica della poetica piranesiana fu rapidamente colta da Marguerite Yourcenar, che in un'opera commentò:

 

«L’autore delle Vedute e delle Antichità Romane non ha certo inventato né il gusto delle rovine, né l’amore per Roma. Un secolo prima di lui, anche Poussin e Claude Gelée [Claude Lorrain] avevano scoperto Roma con occhi nuovi di stranieri; la loro opera si era nutrita di quei luoghi inesauribili. Ma mentre per un Claude Gelée, per un Poussin, Roma era stata soprattutto il mirabile sfondo di una fantasticheria personale o di un discorso di ordine generale, ed un luogo sacro anche, accuratamente purificato da ogni contingenza contemporanea, situato a mezza strada dal divino paese della Favola, è l’Urbe stessa, sotto tutti i suoi aspetti e in tutte le sue implicazioni, dalle più banali alle più insolite, che Piranesi ha fissata ad un certo momento del XVIII secolo, nelle sue migliaia di tavole, insieme aneddotiche e visionarie. Non ha solo esplorato i monumenti antichi da disegnatore che cerchi una prospettiva da riprodurre; ne ha personalmente frugato i ruderi, un po’ per reperirvi le antichità di cui faceva commercio, ma soprattutto per penetrare il segreto delle loro fondazioni, per imparare e per dimostrare come vennero costruiti. È stato archeologo in un’epoca in cui il termine stesso non era in uso corrente»

 

(Marguerite Yourcenar)

 

Sul piano teorico, invece, Piranesi si discostò dall'ambiente neoclassico, sostenendo la superiorità della civiltà romana su quella greca. In opposizione alla fazione filoellenica di Johann Joachim Winckelmann, secondo cui la perfezione nell'arte fosse stata raggiunta solo dalla cultura greca (vista come fonte originaria di quella romana), Piranesi si schierò a favore degli antichi Romani. L'architettura romana, diceva Piranesi, era superiore in virtù delle notevoli capacità tecniche e dell'esuberanza creativa, opposte alla semplice uniformità di quella greca; sostenne, inoltre, che l'architettura romana dipendeva solo da quella etrusca, negandone gli aspetti derivativi dalla Grecia e sottolineandone le origini conseguentemente italiche. Questa polemica culminò con la pubblicazione del Parere su l'architettura (1765) dove due architetti, Protopiro e Didascalo, dibattono sui rispettivi meriti dell'architettura greca e di quella romana.

 

Ciò malgrado, risulta impossibile omologare l'opera del Piranesi al nascente neoclassicismo internazionale. In effetti l'artista veneto trae dalle colossali rovine il sentimento nuovo e nostalgico di un mondo ideale, incommensurabile e grandioso, ormai perduto e corroso: questo ne fa un precursore della sensibilità romantica. Piranesi, infatti, interpreta l'antichità classica allontanandosi dalla visione distaccata di Winckelmann: le opere antiche, per l'artista veneto, non suscitano pertanto una sensazione di quiete e distaccate riflessioni, bensì provocano forti emozioni. Ne è prova la sua opera grafica, dove la struttura monumentale delle vestigia classiche effigiate è interpretata alla luce di un'inquieta sensibilità decisamente preromantica.

 

Fortuna critica

Giovanni Battista Piranesi subì fasi alterne di apprezzamento e di aperta ostilità da parte degli intellettuali e degli artisti italiani e stranieri. Non conobbe, per esempio, una buona accoglienza presso Antonio Visentini che, oltre ad aver censurato la ristrutturazione della chiesa di Santa Maria del Priorato, definì Piranesi un «povero spensierato» che «pretende di esaltar Roma sopra la Grecia al somo, e la abasò per così dire al limo… [e] sempre intende le cose fuori del suo luoco senza posata considerazione». Una critica analoga gli fu rivolta dall'architetto inglese Richard Norris che, in visita a Santa Maria del Priorato nell'aprile 1772, annotò sul suo diario che «the Church is in my Opinion very bad, a strange composition of Ornaments that mean nothing– some of which, that is to say some small parts of the Ornaments, are good, but on the whole is a part of confusion».

 

Tra gli ammiratori più ferventi vi fu lo scrittore inglese Horace Walpole, che consigliò agli studenti inglesi di studiare «i sublimi sogni del Piranesi», dedicando al maestro italiano anche un lungo paragrafo ove scrisse:

 

«Selvaggio come Salvator Rosa, fiero come Michelangelo, esuberante come Rubens, ha immaginato scene... impensabili perfino nelle Indie. Costruisce palazzi sopra ponti, templi sui palazzi, scala il cielo con montagne di edifici»

 

In effetti, Piranesi fu uno degli iniziatori dell'immaginario gotico. Si dice, infatti, che le lugubri e vastissime carceri ideate da Piranesi avessero ispirato allo stesso Walpole la stesura de Il castello di Otranto, primo esempio di romanzo gotico, e la costruzione della sua villa di Strawberry Hill. Fu in particolare a partire dallo Sturm und Drang e dalla ricezione delle prime istanze romantiche che il culto di Piranesi si ravvivò: durante la stagione del Romanticismo, infatti, furono in molti ad apprezzare e amare l'opera grafica di Piranesi. Tra gli ammiratori più significativi si riportano Samuel Taylor Coleridge e Thomas de Quincey (che individuavano nelle visioni piranesiane una prova dell'identità di sogno e creazione), Victor Hugo, Charles Baudelaire, Aldous Huxley e Marguerite Yourcenar, che dedicò all'artista italiano un'intensa biografia.

 

L'interesse per Piranesi non scemò neanche nel corso del XX secolo, quando la sua produzione grafica fu sottoposta per la prima volta a uno studio filologico sistematico e scientifico, con la pubblicazione dei due cataloghi tuttora in uso (Focillon, 1918; Hind, 1922). Notevole fu in questo periodo l'influenza esercitata dalle tavole di Piranesi sulla produzione di Maurits Cornelis Escher (le cui costruzioni impossibili presentano un evidente debito alle Carceri) e sul Surrealismo.

 

Onorificenze

Cavaliere dello Speron d'oro - nastrino per uniforme ordinariaCavaliere dello Speron d'oro

— Roma, 1766

 

Da Wikipedia, l'enciclopedia libera.

Raccolta Foto De Alvariis

Rabbits, also known as bunnies or bunny rabbits, are small mammals in the family Leporidae (along with the hare) of the order Lagomorpha (along with the pika). Oryctolagus cuniculus includes the European rabbit species and its descendants, the world's 305 breeds of domestic rabbit. Sylvilagus includes 13 wild rabbit species, among them the seven types of cottontail. The European rabbit, which has been introduced on every continent except Antarctica, is familiar throughout the world as a wild prey animal and as a domesticated form of livestock and pet. With its widespread effect on ecologies and cultures, the rabbit is, in many areas of the world, a part of daily life - as food, clothing, a companion, and a source of artistic inspiration.

 

Although once considered rodents, lagomorphs like rabbits have been discovered to have diverged separately and earlier than their rodent cousins and have a number of traits rodents lack, like two extra incisors.

 

TERMINOLOGY AND ETYMOLOGY

Male rabbits are called bucks; females are called does. An older term for an adult rabbit used until the 18th century is coney (derived ultimately from the Latin cuniculus), while rabbit once referred only to the young animals. Another term for a young rabbit is bunny, though this term is often applied informally (particularly by children) to rabbits generally, especially domestic ones. More recently, the term kit or kitten has been used to refer to a young rabbit.

 

A group of rabbits is known as a colony or nest (or, occasionally, a warren, though this more commonly refers to where the rabbits live). A group of baby rabbits produced from a single mating is referred to as a litter and a group of domestic rabbits living together is sometimes called a herd.

 

The word rabbit itself derives from the Middle English rabet, a borrowing from the Walloon robète, which was a diminutive of the French or Middle Dutch robbe.

 

TAXONOMY

Rabbits and hares were formerly classified in the order Rodentia (rodent) until 1912, when they were moved into a new order, Lagomorpha (which also includes pikas). Below are some of the genera and species of the rabbit.

 

DFFERENCES FROM HARES

Hares are precocial, born relatively mature and mobile with hair and good vision, while rabbits are altricial, born hairless and blind, and requiring closer care. Hares (and cottontail rabbits) live a relatively solitary life in a simple nest above the ground, while most rabbits live in social groups in burrows or warrens. Hares are generally larger than rabbits, with ears that are more elongated, and with hind legs that are larger and longer. Hares have not been domesticated, while descendants of the European rabbit are commonly bred as livestock and kept as pets.

 

DOMESTICATION

Rabbits have long been domesticated. Beginning in the Middle Ages, the European rabbit has been widely kept as livestock, starting in ancient Rome. Selective breeding has generated a wide variety of rabbit breeds, of which many (since the early 19th century) are also kept as pets. Some strains of rabbit have been bred specifically as research subjects.

 

As livestock, rabbits are bred for their meat and fur. The earliest breeds were important sources of meat, and so became larger than wild rabbits, but domestic rabbits in modern times range in size from dwarf to giant. Rabbit fur, prized for its softness, can be found in a broad range of coat colors and patterns, as well as lengths. The Angora rabbit breed, for example, was developed for its long, silky fur, which is often hand-spun into yarn. Other domestic rabbit breeds have been developed primarily for the commercial fur trade, including the Rex, which has a short plush coat.

 

BIOLOGY

EVOLUTION

Because the rabbit's epiglottis is engaged over the soft palate except when swallowing, the rabbit is an obligate nasal breather. Rabbits have two sets of incisor teeth, one behind the other. This way they can be distinguished from rodents, with which they are often confused. Carl Linnaeus originally grouped rabbits and rodents under the class Glires; later, they were separated as the scientific consensus is that many of their similarities were a result of convergent evolution. However, recent DNA analysis and the discovery of a common ancestor has supported the view that they do share a common lineage, and thus rabbits and rodents are now often referred to together as members of the superorder Glires.

 

MORPHOLOGY

Since speed and agility are a rabbit's main defenses against predators (including the swift fox), rabbits have large hind leg bones and well developed musculature. Though plantigrade at rest, rabbits are on their toes while running, assuming a more digitigrade posture. Rabbits use their strong claws for digging and (along with their teeth) for defense. Each front foot has four toes plus a dewclaw. Each hind foot has four toes (but no dewclaw).

 

Most wild rabbits (especially compared to hares) have relatively full, egg-shaped bodies. The soft coat of the wild rabbit is agouti in coloration (or, rarely, melanistic), which aids in camouflage. The tail of the rabbit (with the exception of the cottontail species) is dark on top and white below. Cottontails have white on the top of their tails.

 

As a result of the position of the eyes in its skull, the rabbit has a field of vision that encompasses nearly 360 degrees, with just a small blind spot at the bridge of the nose.

 

HIND LIMB ELEMENTS

The anatomy of rabbits' hind limbs are structurally similar to that of other land mammals and contribute to their specialized form of locomotion. The bones of the hind limbs consist of long bones (the femur, tibia, fibula, and phalanges) as well as short bones (the tarsals). These bones are created through endochondral ossification during development. Like most land mammals, the round head of the femur articulates with the acetabulum of the ox coxae. The femur articulates with the tibia, but not the fibula, which is fused to the tibia. The tibia and fibula articulate with the tarsals of the pes, commonly called the foot. The hind limbs of the rabbit are longer than the front limbs. This allows them to produce their hopping form of locomotion. Longer hind limbs are more capable of producing faster speeds. Hares, which have longer legs than cottontail rabbits, are able to move considerably faster. Rabbits stay just on their toes when moving; this is called Digitigrade locomotion. The hind feet have four long toes that allow for this and are webbed to prevent them from spreading when hopping. Rabbits do not have paw pads on their feet like most other animals that use digitigrade locomotion. Instead, they have coarse compressed hair that offers protection.

 

MUSCULATURE

Rabbits have muscled hind legs that allow for maximum force, maneuverability, and acceleration that is divided into three main parts; foot, thigh, and leg. The hind limbs of a rabbit are an exaggerated feature. They are much longer than the forelimbs, providing more force. Rabbits run on their toes to gain the optimal stride during locomotion. The force put out by the hind limbs is contributed to both the structural anatomy of the fusion tibia and fibula, and muscular features. Bone formation and removal, from a cellular standpoint, is directly correlated to hind limb muscles. Action pressure from muscles creates force that is then distributed through the skeletal structures. Rabbits that generate less force, putting less stress on bones are more prone to osteoporosis due to bone rarefaction. In rabbits, the more fibers in a muscle, the more resistant to fatigue. For example, hares have a greater resistance to fatigue than cottontails. The muscles of rabbit's hind limbs can be classified into four main categories: hamstrings, quadriceps, dorsiflexors, or plantar flexors. The quadriceps muscles are in charge of force production when jumping. Complementing these muscles are the hamstrings which aid in short bursts of action. These muscles play off of one another in the same way as the plantar flexors and dorsiflexors, contributing to the generation and actions associated with force.

 

EARS

Within the order lagomorphs, the ears are utilized to detect and avoid predators. In the family Leporidae, the ears are typically longer than they are wide. For example, in black tailed jack rabbits, their long ears cover a greater surface area relative to their body size that allow them to detect predators from far away. Contrasted to cotton tailed rabbits, their ears are smaller and shorter, requiring predators to be closer to detect them before they can flee. Evolution has favored rabbits having shorter ears so the larger surface area does not cause them to lose heat in more temperate regions. The opposite can be seen in rabbits that live in hotter climates, mainly because they possess longer ears that have a larger surface area that help with dispersion of heat as well as the theory that sound does not travel well in more arid air, opposed to cooler air. Therefore, longer ears are meant to aid the organism in detecting predators sooner rather than later in warmer temperatures. The rabbit is characterized by its shorter ears while hares are characterized by their longer ears. Rabbits' ears are an important structure to aid thermoregulation and detect predators due to how the outer, middle, and inner ear muscles coordinate with one another. The ear muscles also aid in maintaining balance and movement when fleeing predators.Outer ear

  

MIDDLE EAR

The middle ear is filled with three bones called ossicles and is separated by the outer eardrum in the back of the rabbit's skull. The three ossicles are called hammer, anvil, and stirrup and act to decrease sound before it hits the inner ear. In general, the ossicles act as a barrier to the inner ear for sound energy.

 

INNER EAR

Inner ear fluid called endolymph receives the sound energy. After receiving the energy, later within the inner ear there are two parts: the cochlea that utilizes sound waves from the ossicles and the vestibular apparatus that manages the rabbit's position in regards to movement. Within the cochlea there is a basilar membrane that contains sensory hair structures utilized to send nerve signals to the brain so it can recognize different sound frequencies. Within the vestibular apparatus the rabbit possesses three semicircular canals to help detect angular motion.

 

THERMOREGULATION

Thermoregulation is the process that an organism utilizes to maintain an optimal body temperature independent of external conditions. This process is carried out by the pinnae which takes up most of the rabbit's body surface and contain a vascular network and arteriovenous shunts. In a rabbit, the optimal body temperature is around 38.5–40℃. If their body temperature exceeds or does not meet this optimal temperature, the rabbit must return to homeostasis. Homeostasis of body temperature is maintained by the use of their large, highly vascularized ears that are able to change the amount of blood flow that passes through the ears.

 

Constriction and dilation of blood vessels in the ears are used to control the core body temperature of a rabbit. If the core temperature exceeds its optimal temperature greatly, blood flow is constricted to limit the amount of blood going through the vessels. With this constriction, there is only a limited amount of blood that is passing through the ears where ambient heat would be able to heat the blood that is flowing through the ears and therefore, increasing the body temperature. Constriction is also used when the ambient temperature is much lower than that of the rabbit's core body temperature. When the ears are constricted it again limits blood flow through the ears to conserve the optimal body temperature of the rabbit. If the ambient temperature is either 15 degrees above or below the optimal body temperature, the blood vessels will dilate. With the blood vessels being enlarged, the blood is able to pass through the large surface area which causes it to either heat or cool down.

 

During hot summers, the rabbit has the capability to stretch its pinnae which allows for greater surface area and increase heat dissipation. In cold winters, the rabbit does the opposite and folds its ears in order to decrease its surface area to the ambient air which would decrease their body temperature.

 

The jackrabbit has the largest ears within the Oryctolagus cuniculus group. Their ears contribute to 17% of their total body surface area. Their large pinna were evolved to maintain homeostasis while in the extreme temperatures of the desert.

 

RESPIRATORY SYSTEM

The rabbit's nasal cavity lies dorsal to the oral cavity, and the two compartments are separated by the hard and soft palate. The nasal cavity itself is separated into a left and right side by a cartilage barrier, and it is covered in fine hairs that trap dust before it can enter the respiratory tract. As the rabbit breathes, air flows in through the nostrils along the alar folds. From there, the air moves into the nasal cavity, also known as the nasopharynx, down through the trachea, through the larynx, and into the lungs. The larynx functions as the rabbit's voice box, which enables it to produce a wide variety of sounds. The trachea is a long tube embedded with cartilaginous rings that prevent the tube from collapsing as air moves in and out of the lungs. The trachea then splits into a left and right bronchus, which meet the lungs at a structure called the hilum. From there, the bronchi split into progressively more narrow and numerous branches. The bronchi branch into bronchioles, into respiratory bronchioles, and ultimately terminate at the alveolar ducts. The branching that is typically found in rabbit lungs is a clear example of monopodial branching, in which smaller branches divide out laterally from a larger central branch. The structure of the rabbit's nasal and oral cavities, necessitates breathing through the nose. This is due to the fact that the epiglottis is fixed to the backmost portion of the soft palate. Within the oral cavity, a layer of tissue sits over the opening of the glottis, which blocks airflow from the oral cavity to the trachea. The epiglottis functions to prevent the rabbit from aspirating on its food. Further, the presence of a soft and hard palate allow the rabbit to breathe through its nose while it feeds.Rabbits lungs are divided into four lobes: the cranial, middle, caudal, and accessory lobes. The right lung is made up of all four lobes, while the left lung only has two: the cranial and caudal lobes. In order to provide space for the heart, the left cranial lobe of the lungs is significantly smaller than that of the right. The diaphragm is a muscular structure that lies caudal to the lungs and contracts to facilitate respiration.

 

DIGESTION

Rabbits are herbivores that feed by grazing on grass and other leafy plants. In consequence, their diet contains large amounts of cellulose, which is hard to digest. Rabbits solve this problem via a form of hindgut fermentation. They pass two distinct types of feces: hard droppings and soft black viscous pellets, the latter of which are known as caecotrophs or "night droppings" and are immediately eaten (a behaviour known as coprophagy). Rabbits reingest their own droppings (rather than chewing the cud as do cows and numerous other herbivores) to digest their food further and extract sufficient nutrients.

 

Rabbits graze heavily and rapidly for roughly the first half-hour of a grazing period (usually in the late afternoon), followed by about half an hour of more selective feeding.[citation needed] In this time, the rabbit will also excrete many hard fecal pellets, being waste pellets that will not be reingested.[citation needed] If the environment is relatively non-threatening, the rabbit will remain outdoors for many hours, grazing at intervals.[citation needed] While out of the burrow, the rabbit will occasionally reingest its soft, partially digested pellets; this is rarely observed, since the pellets are reingested as they are produced.

 

Hard pellets are made up of hay-like fragments of plant cuticle and stalk, being the final waste product after redigestion of soft pellets. These are only released outside the burrow and are not reingested. Soft pellets are usually produced several hours after grazing, after the hard pellets have all been excreted.[citation needed] They are made up of micro-organisms and undigested plant cell walls.[citation needed]

 

Rabbits are hindgut digesters. This means that most of their digestion takes place in their large intestine and cecum. In rabbits, the cecum is about 10 times bigger than the stomach and it along with the large intestine makes up roughly 40% of the rabbit's digestive tract. The unique musculature of the cecum allows the intestinal tract of the rabbit to separate fibrous material from more digestible material; the fibrous material is passed as feces, while the more nutritious material is encased in a mucous lining as a cecotrope. Cecotropes, sometimes called "night feces", are high in minerals, vitamins and proteins that are necessary to the rabbit's health. Rabbits eat these to meet their nutritional requirements; the mucous coating allows the nutrients to pass through the acidic stomach for digestion in the intestines. This process allows rabbits to extract the necessary nutrients from their food.[35]

 

The chewed plant material collects in the large cecum, a secondary chamber between the large and small intestine containing large quantities of symbiotic bacteria that help with the digestion of cellulose and also produce certain B vitamins. The pellets are about 56% bacteria by dry weight, largely accounting for the pellets being 24.4% protein on average. The soft feces form here and contain up to five times the vitamins of hard feces. After being excreted, they are eaten whole by the rabbit and redigested in a special part of the stomach. The pellets remain intact for up to six hours in the stomach; the bacteria within continue to digest the plant carbohydrates. This double-digestion process enables rabbits to use nutrients that they may have missed during the first passage through the gut, as well as the nutrients formed by the microbial activity and thus ensures that maximum nutrition is derived from the food they eat. This process serves the same purpose in the rabbit as rumination does in cattle and sheep.

Because rabbits cannot vomit, if buildup occurs within the intestines (due often to a diet with insufficient fibre), intestinal blockage can occur.

 

REPRODUCTION

The adult male reproductive system forms the same as most mammals with the seminiferous tubular compartment containing the Sertoli cells and an adluminal compartment that contains the Leydig cells. The Leydig cells produce testosterone, which maintains libido and creates secondary sex characteristics such as the genital tubercle and penis. The Sertoli cells triggers the production of Anti-Müllerian duct hormone, which absorbs the Müllerian duct. In an adult male rabbit, the sheath of the penis is cylinder-like and can be extruded as early as two months of age. The scrotal sacs lay lateral to the penis and contain epididymal fat pads which protect the testes. Between 10 and 14 weeks, the testes descend and are able to retract into the pelvic cavity in order to thermoregulate. Furthermore, the secondary sex characteristics, such as the testes, are complex and secrete many compounds. These compounds includes fructose, citric acid, minerals, and a uniquely high amount of catalase.

 

The adult female reproductive tract is bipartite, which prevents an embryo from translocating between uteri. The two uterine horns communicate to two cervixes and forms one vaginal canal. Along with being bipartite, the female rabbit does not go through an estrus cycle, which causes mating induced ovulation.

 

The average female rabbit becomes sexually mature at 3 to 8 months of age and can conceive at any time of the year for the duration of her life. However, egg and sperm production can begin to decline after three years. During mating, the male rabbit will mount the female rabbit from behind and insert his penis into the female and make rapid pelvic hip thrusts. The encounter lasts only 20–40 seconds and after, the male will throw himself backwards off the female.

 

The rabbit gestation period is short and ranges from 28 to 36 days with an average period of 31 days. A longer gestation period will generally yield a smaller litter while shorter gestation periods will give birth to a larger litter. The size of a single litter can range from four to 12 kits allowing a female to deliver up to 60 new kits a year. After birth, the female can become pregnant again as early as the next day.

 

The mortality rates of embryos are high in rabbits and can be due to infection, trauma, poor nutrition and environmental stress so a high fertility rate is necessary to counter this.

 

SLEEP

Rabbits may appear to be crepuscular, but their natural inclination is toward nocturnal activity. In 2011, the average sleep time of a rabbit in captivity was calculated at 8.4 hours per day. As with other prey animals, rabbits often sleep with their eyes open, so that sudden movements will awaken the rabbit to respond to potential danger.

 

DISEASES

In addition to being at risk of disease from common pathogens such as Bordetella bronchiseptica and Escherichia coli, rabbits can contract the virulent, species-specific viruses RHD ("rabbit hemorrhagic disease", a form of calicivirus) or myxomatosis. Among the parasites that infect rabbits are tapeworms (such as Taenia serialis), external parasites (including fleas and mites), coccidia species, and Toxoplasma gondii. Domesticated rabbits with a diet lacking in high fiber sources, such as hay and grass, are susceptible to potentially lethal gastrointestinal stasis. Rabbits and hares are almost never found to be infected with rabies and have not been known to transmit rabies to humans.

 

Encephalitozoon cuniculi, an obligate intracellular parasite is also capable of infecting many mammals including rabbits.

 

ECOLOGY

Rabbits are prey animals and are therefore constantly aware of their surroundings. For instance, in Mediterranean Europe, rabbits are the main prey of red foxes, badgers, and Iberian lynxes. If confronted by a potential threat, a rabbit may freeze and observe then warn others in the warren with powerful thumps on the ground. Rabbits have a remarkably wide field of vision, and a good deal of it is devoted to overhead scanning. They survive predation by burrowing, hopping away in a zig-zag motion, and, if captured, delivering powerful kicks with their hind legs. Their strong teeth allow them to eat and to bite in order to escape a struggle. The longest-lived rabbit on record, a domesticated European rabbit living in Tasmania, died at age 18. The lifespan of wild rabbits is much shorter; the average longevity of an eastern cottontail, for instance, is less than one year.

 

HABITAT AND RANGE

Rabbit habitats include meadows, woods, forests, grasslands, deserts and wetlands. Rabbits live in groups, and the best known species, the European rabbit, lives in burrows, or rabbit holes. A group of burrows is called a warren.

 

More than half the world's rabbit population resides in North America. They are also native to southwestern Europe, Southeast Asia, Sumatra, some islands of Japan, and in parts of Africa and South America. They are not naturally found in most of Eurasia, where a number of species of hares are present. Rabbits first entered South America relatively recently, as part of the Great American Interchange. Much of the continent has just one species of rabbit, the tapeti, while most of South America's southern cone is without rabbits.

 

The European rabbit has been introduced to many places around the world.

 

ENVIRONMENTAL PROBLEMS

Rabbits have been a source of environmental problems when introduced into the wild by humans. As a result of their appetites, and the rate at which they breed, feral rabbit depredation can be problematic for agriculture. Gassing (fumigation of warrens), barriers (fences), shooting, snaring, and ferreting have been used to control rabbit populations, but the most effective measures are diseases such as myxomatosis (myxo or mixi, colloquially) and calicivirus. In Europe, where rabbits are farmed on a large scale, they are protected against myxomatosis and calicivirus with a genetically modified virus. The virus was developed in Spain, and is beneficial to rabbit farmers. If it were to make its way into wild populations in areas such as Australia, it could create a population boom, as those diseases are the most serious threats to rabbit survival. Rabbits in Australia and New Zealand are considered to be such a pest that land owners are legally obliged to control them.

 

AS FOOD AND CLOTHING

In some areas, wild rabbits and hares are hunted for their meat, a lean source of high quality protein. In the wild, such hunting is accomplished with the aid of trained falcons, ferrets, or dogs, as well as with snares or other traps, and rifles. A caught rabbit may be dispatched with a sharp blow to the back of its head, a practice from which the term rabbit punch is derived.

 

Wild leporids comprise a small portion of global rabbit-meat consumption. Domesticated descendants of the European rabbit (Oryctolagus cuniculus) that are bred and kept as livestock (a practice called cuniculture) account for the estimated 200 million tons of rabbit meat produced annually. Approximately 1.2 billion rabbits are slaughtered each year for meat worldwide. In 1994, the countries with the highest consumption per capita of rabbit meat were Malta with 8.89 kg, Italy with 5.71 kg, and Cyprus with 4.37 kg, falling to 0.03 kg in Japan. The figure for the United States was 0.14 kg per capita. The largest producers of rabbit meat in 1994 were China, Russia, Italy, France, and Spain. Rabbit meat was once a common commodity in Sydney, Australia, but declined after the myxomatosis virus was intentionally introduced to control the exploding population of feral rabbits in the area.

 

In the United Kingdom, fresh rabbit is sold in butcher shops and markets, and some supermarkets sell frozen rabbit meat. At farmers markets there, including the famous Borough Market in London, rabbit carcasses are sometimes displayed hanging, unbutchered (in the traditional style), next to braces of pheasant or other small game. Rabbit meat is a feature of Moroccan cuisine, where it is cooked in a tajine with "raisins and grilled almonds added a few minutes before serving". In China, rabbit meat is particularly popular in Sichuan cuisine, with its stewed rabbit, spicy diced rabbit, BBQ-style rabbit, and even spicy rabbit heads, which have been compared to spicy duck neck. Rabbit meat is comparatively unpopular elsewhere in the Asia-Pacific.

 

An extremely rare infection associated with rabbits-as-food is tularemia (also known as rabbit fever), which may be contracted from an infected rabbit. Hunters are at higher risk for tularemia because of the potential for inhaling the bacteria during the skinning process.

 

In addition to their meat, rabbits are used for their wool, fur, and pelts, as well as their nitrogen-rich manure and their high-protein milk. Production industries have developed domesticated rabbit breeds (such as the well-known Angora rabbit) to efficiently fill these needs.

In art, literature, and culture

 

Rabbits are often used as a symbol of fertility or rebirth, and have long been associated with spring and Easter as the Easter Bunny. The species' role as a prey animal with few defenses evokes vulnerability and innocence, and in folklore and modern children's stories, rabbits often appear as sympathetic characters, able to connect easily with youth of all kinds (for example, the Velveteen Rabbit, or Thumper in Bambi).

 

With its reputation as a prolific breeder, the rabbit juxtaposes sexuality with innocence, as in the Playboy Bunny. The rabbit (as a swift prey animal) is also known for its speed, agility, and endurance, symbolized (for example) by the marketing icons the Energizer Bunny and the Duracell Bunny.

 

FOLKLORE

The rabbit often appears in folklore as the trickster archetype, as he uses his cunning to outwit his enemies.

In Aztec mythology, a pantheon of four hundred rabbit gods known as Centzon Totochtin, led by Ometochtli or Two Rabbit, represented fertility, parties, and drunkenness.

In Central Africa, the common hare (Kalulu), is "inevitably described" as a trickster figure.

In Chinese folklore, rabbits accompany Chang'e on the Moon. In the Chinese New Year, the zodiacal rabbit is one of the twelve celestial animals in the Chinese zodiac. Note that the Vietnamese zodiac includes a zodiacal cat in place of the rabbit, possibly because rabbits did not inhabit Vietnam.[citation needed] The most common explanation, however, is that the ancient Vietnamese word for "rabbit" (mao) sounds like the Chinese word for "cat" (卯, mao).

In Japanese tradition, rabbits live on the Moon where they make mochi, the popular snack of mashed sticky rice. This comes from interpreting the pattern of dark patches on the moon as a rabbit standing on tiptoes on the left pounding on an usu, a Japanese mortar.

In Jewish folklore, rabbits (shfanim שפנים) are associated with cowardice, a usage still current in contemporary Israeli spoken Hebrew (similar to the English colloquial use of "chicken" to denote cowardice).

In Korean mythology, as in Japanese, rabbits live on the moon making rice cakes ("Tteok" in Korean).

In Anishinaabe traditional beliefs, held by the Ojibwe and some other Native American peoples, Nanabozho, or Great Rabbit, is an important deity related to the creation of the world.

 

A Vietnamese mythological story portrays the rabbit of innocence and youthfulness. The Gods of the myth are shown to be hunting and killing rabbits to show off their power.

Buddhism, Christianity, and Judaism have associations with an ancient circular motif called the three rabbits (or "three hares"). Its meaning ranges from "peace and tranquility", to purity or the Holy Trinity, to Kabbalistic levels of the soul or to the Jewish diaspora. The tripartite symbol also appears in heraldry and even tattoos.

 

The rabbit as trickster is a part of American popular culture, as Br'er Rabbit (from African-American folktales and, later, Disney animation) and Bugs Bunny (the cartoon character from Warner Bros.), for example.

 

Anthropomorphized rabbits have appeared in film and literature, in Alice's Adventures in Wonderland (the White Rabbit and the March Hare characters), in Watership Down (including the film and television adaptations), in Rabbit Hill (by Robert Lawson), and in the Peter Rabbit stories (by Beatrix Potter). In the 1920s, Oswald the Lucky Rabbit, was a popular cartoon character.

 

A rabbit's foot may be carried as an amulet, believed to bring protection and good luck. This belief is found in many parts of the world, with the earliest use being recorded in Europe c. 600 BC.

 

On the Isle of Portland in Dorset, UK, the rabbit is said to be unlucky and even speaking the creature's name can cause upset among older island residents. This is thought to date back to early times in the local quarrying industry where (to save space) extracted stones that were not fit for sale were set aside in what became tall, unstable walls. The local rabbits' tendency to burrow there would weaken the walls and their collapse resulted in injuries or even death. Thus, invoking the name of the culprit became an unlucky act to be avoided. In the local culture to this day, the rabbit (when he has to be referred to) may instead be called a “long ears” or “underground mutton”, so as not to risk bringing a downfall upon oneself. While it was true 50 years ago that a pub on the island could be emptied by calling out the word "rabbit", this has become more fable than fact in modern times.

 

In other parts of Britain and in North America, invoking the rabbit's name may instead bring good luck. "Rabbit rabbit rabbit" is one variant of an apotropaic or talismanic superstition that involves saying or repeating the word "rabbit" (or "rabbits" or "white rabbits" or some combination thereof) out loud upon waking on the first day of each month, because doing so will ensure good fortune for the duration of that month.

 

The "rabbit test" is a term, first used in 1949, for the Friedman test, an early diagnostic tool for detecting a pregnancy in humans. It is a common misconception (or perhaps an urban legend) that the test-rabbit would die if the woman was pregnant. This led to the phrase "the rabbit died" becoming a euphemism for a positive pregnancy test.

 

WIKIPEDIA

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

Ethiopian government is taking over and inciting girls to give up tribal customs such as scarification and lip plates. Without labret, a girl will be sold less for wedding (25 cows + 1 kalaschnikov)

Surma woman with her giant lip plate, a sign of beauty in Surma tribe, like in Mursi one. When they are ready to marry, they start to make a hole in the lip with a wood stick.

It will be kept for one night , and is removed to put a bigger one. This is very painful at this time... Few months after, the lip plate has its full size, and the girl is seen as beautiful by the men.

The lip plate made of wood or terracotta, and they have to remove the lower incisors to let some space for the disc. it's amazing to see them speak without any trouble, put it and remove it as a classic jewel.

Sometimes the lip is broken by the pressure of the lip plate. This is a very big problem for the girl cos men will consider her as ugly, she won't be able to marry anyone in the tribe apart the old men or the sick people...

The women are shaved, like the men, cos they hate hairiness!

 

© Eric Lafforgue

www.ericlafforgue.com

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

Today, July 5, 2017, is the grand opening of Land Of Lemurs at the Calgary Zoo.

 

• Lemurs are prosimians, meaning they are “more primitive”

and more like the ancestor to all primates.

• This group also contains galagos, aye-ayes, pottos, lorises

and bushbabies.

• Prosimians have:

• claws instead of fingernails

• long, bony snouts

• little teeth differentiation, except for dental comb made

of lower front incisors that are used for grooming

• incomplete eye orbitals

• smaller braincases than monkeys or apes

• shorter forelimbs than hindlimbs

• most prosimians are nocturnal, but since there are no

monkey species on Madagascar to compete

with, lemurs are also diurnal

• many of the well-known lemur species (ie: ring-tailed,

ruffed, etc.) are diurnal, but some species

have retained their nocturnal lifestyle

• Lemurs are also the only prosimians to live in social

groups (this does not apply to all species)

• Lemurs are found on Madagascar and the neighboring

ComorosIslands (where they were introduced), and

are found nowhere else on earth.

• In 2016 Madagascar is home to 104 types of lemurs but new

species are often found.

• Lemurs are an important conservation focus for their ability

to regenerate forest habitat through seed dispersal

The rock hyrax (Procavia capensis), also called rock badger, rock rabbit, and Cape hyrax, is commonly referred to in South African English as the dassie. It is one of the four living species of the order Hyracoidea, and the only living species in the genus Procavia. Like all hyraxes, it is a medium-sized (~4 kg) terrestrial mammal, with short ears and tail.

 

The closest living relatives to hyraxes are the modern-day elephants and sirenians. The rock hyrax is found across Africa and the Middle East in habitats with rock crevices into which it escapes from predators. It is the only extant terrestrial afrotherian in the Middle East. Hyraxes typically live in groups of 10–80 animals, and forage as a group. They have been reported to use sentries: one or more animals take up position on a vantage point and issue alarm calls on the approach of predators.

 

The rock hyrax has incomplete thermoregulation and is most active in the morning and evening, although its activity pattern varies substantially with season and climate.

 

Over most of its range, the rock hyrax is not endangered, and in some areas is considered a minor pest. In Ethiopia, Israel and Jordan, it is a reservoir of the leishmaniasis parasite.

 

The rock hyrax is squat and heavily built, adults reaching a length of 50 cm (20 in) and weighing around 4 kg (8.8 lb), with a slight sexual dimorphism, males being approximately 10% heavier than females. Their fur is thick and grey-brown, although this varies strongly between different environments: from dark brown in wetter habitats, to light grey in desert living individuals. Hyrax size (as measured by skull length and humerus diameter) is correlated to precipitation, probably because of the effect on preferred hyrax forage.

 

Prominent in, and apparently unique to hyraxes, is the dorsal gland, which excretes an odour used for social communication and territorial marking. The gland is most clearly visible in dominant males.

 

The head of the rock hyrax is pointed, having a short neck with rounded ears. They have long black whiskers on their muzzles. The rock hyrax has a prominent pair of long, pointed tusk-like upper incisors which are reminiscent of the elephant, to which the hyrax is distantly related. The forefeet are plantigrade, and the hind feet semi-digitigrade. The soles of the feet have large, soft pads that are kept moist with sweat-like secretions. In males, the testes are permanently abdominal, another anatomical feature that hyraxes share with their relatives elephants and sirenians.

 

Thermoregulation in the rock hyrax has been subject to much research, as their body temperature varies with a diurnal rhythm. However, animals kept in constant environmental conditions also display such variation and this internal mechanism may be related to water balance regulation.

 

The rock hyrax occurs across sub-Saharan Africa, with the exception of the Congo basin and Madagascar. A larger, longer-haired subspecies is abundant in the glacial moraines in the alpine zone of Mount Kenya. The distribution continues into southern Algeria, Libya and Egypt, and the Middle East, with populations in Israel, Jordan, Lebanon and the Arabian peninsula.

 

A mammal of similar appearance by convergent evolution, but unrelated, is the rock cavy of Brazil.

 

Rock hyraxes build dwelling holes in any type of rock with suitable cavities such as sedimentary rocks and soil. In Mount Kenya, rock hyraxes live in colonies comprising an adult male, differing numbers of adult females and immatures. They are active during the day, and sometimes during moonlit nights. The dominant male defends and watches over the group. The male also marks its territory.

 

In Africa, hyraxes are preyed on by leopards, Egyptian cobras, puff adders, rock pythons, caracals, wild dogs, hawks, and owls. Verreaux's eagle in particular is a specialist hunter of hyrax. In Israel, the rock hyrax is reportedly rarely preyed upon by terrestrial predators, as their system of sentries and their reliable refuges provide considerable protection. Hyrax remains are almost absent from the droppings of wolves in the Judean Desert.

 

Hyraxes feed on a wide variety of different plants, including Lobelia and broad-leafed plants. They also have been reported to eat insects and grubs. The rock hyraxes forage for food up to about 50 metres from their refuge, usually feeding as a group and with one or more acting as sentries from a prominent lookout position. On the approach of danger, the sentries give an alarm call, and the animals quickly retreat to their refuge.

 

They are able to go for many days without water due to the moisture they obtain through their food, but will quickly dehydrate under direct sunlight.

 

Despite their seemingly clumsy build, they are able to climb trees (although not as readily as Heterohyrax), and will readily enter residential gardens to feed on the leaves of citrus and other trees.

 

The rock hyrax also makes a loud grunting sound while moving its jaws as if chewing, and this behaviour may be a sign of aggression. Some authors have proposed that observation of this behaviour by ancient Israelites gave rise to the misconception given in Leviticus 11:4-8 that the hyrax chews the cud In fact, hyraxes are not ruminants.

 

Rock hyraxes give birth to between two and four young after a gestation period of 6–7 months (long, for their size). The young are well developed at birth with fully opened eyes and complete pelage. Young can ingest solid food after two weeks and are weaned at ten weeks. After 16 months, the rock hyraxes become sexually mature, they reach adult size at three years, and they typically live about ten years. During seasonal changes, the weight of the male reproductive organs (testis, seminal vesicles) changes due to sexual activity. A study showed that between May and January, the males were inactive sexually. From February onward, there was a dramatic increase to the weight of these organs, and the males are able to copulate.

 

Social behaviour

In a study of their social networks, it was found that hyraxes that live in more "egalitarian" groups, in which social associations are spread more evenly among group members, survive longer. In addition, hyraxes are the first non-human species in which structural balance was described. They follow "the friend of my friend is my friend" rule, and avoid unbalanced social configurations.

 

Captive rock hyraxes make more than 20 different noises and vocal signals. The most familiar signal is a high trill, given in response to perceived danger. Rock hyrax calls can provide important biological information such as size, age, social status, body weight, condition, and hormonal state of the caller, as determined by measuring their call length, patterns, complexity, and frequency. More recently, researchers have found rich syntactic structure and geographical variations in the calls of rock hyraxes, a first in the vocalization of mammalian taxa other than primates, cetaceans, and bats. Higher ranked males tend to sing more often, although the energetic cost of singing is relatively low.

 

The rock hyrax spends approximately 95% of its time resting. During this time, they can often be seen basking in the sun, which is thought to be an element of their complex thermoregulation.

 

Dispersal

Male hyraxes have been categorised into four classes: territorial, peripheral, early dispersers, and late dispersers. The territorial males are dominant. Peripheral males are more solitary and sometimes take over a group when the dominant male is missing. Early-dispersing males are juveniles that leave the birth site around 16 to 24 months of age. Late dispersers are also juvenile males, but they leave the birth site much later; around 30 or more months of age.

 

Names

They are known as dassies in South Africa, and sometimes rock rabbits. The Swahili names for them are pimbi, pelele and wibari, though the latter two names are nowadays reserved for the tree hyraxes. This species has many subspecies, many of which are also known as rock or Cape hyrax, although the former usually refers to African varieties.

 

In Arabic, the rock hyrax is called "wabr" or "tabsoun". In Hebrew, the rock hyrax is called "shafan sela", meaning rock "shafan", where the meaning of shafan is obscure, but is colloquially used as a synonym for rabbit in modern Hebrew. According to Gerald Durrell local people in Bafut, Cameroon, call the rock hyrax the n'eer.

 

Naturopathic use

Rock hyraxes produce large quantities of hyraceum—a sticky mass of dung and urine that has been employed as a South African folk remedy in the treatment of several medical disorders, including epilepsy and convulsions. Hyraceum is now being used by perfumers who tincture it in alcohol to yield a natural animal musk.

 

In culture

The rock hyrax is classified as non-kosher in the Jewish Torah. Nonetheless, it is also included in Proverbs 30:26 as one of a number of remarkable animals for being small, but exceedingly wise, in this case because "the rock badgers are a people not mighty, yet they make their homes in the cliffs".

 

In Joy Adamson's books and the movie Born Free, a rock hyrax called Pati-Pati was her companion for six years before Elsa and her siblings came along; Pati-Pati took the role of nanny and watched over them with great care.

 

The 2013 animated film Khumba features a number of rock hyraxes who sacrifice one of their own to a white black eagle.

 

The species was introduced to Jebel Hafeet, which is on the border of Oman and the United Arab Emirates.

 

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Dikhololo

South Africa

Quality prints, greeting cards and many products can be purchased at >> kaye-menner.pixels.com/featured/beach-dog-rest-time-by-ka...

 

This wonderful and friendly dog, whose name is Rambo, has been enjoying a swim in the surf and games with his orange ball, but now he seems to need a rest, just lying on the sand still with ball in mouth, with a beautiful beach scene surrounding him.

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[From Wikipedia]

The Staffordshire Bull Terrier is a medium-sized, stocky, and very muscular dog, with a similar appearance to the much larger American Staffordshire terrier and American pit bull terrier. It has a broad head (male considerably more so than female), defined occipital muscles, a relatively short fore-face, dark round eyes and a wide mouth with a clean scissor-like bite (the top incisors slightly overlap the bottom incisors). The ears are small. The cheek muscles are very pronounced. The lips show no looseness. From above, the head loosely resembles a triangle. The head tapers down to a strong well-muscled neck and shoulders placed on squarely spaced forelimbs. They are tucked up in their loins and the last 1-2 ribs of the rib-cage are usually visible. The tail resembles an old fashioned pump handle. The hind quarters are well-muscled and are what give the Stafford drive when baiting. They are coloured brindle, black, red, fawn, blue, white, or any blending of these colors with white. White with any other colour broken up over the body is known as pied. Liver-colored, black and tan dogs can occur but are rare and it is advised not to breed from either as well as those with light eyes. The exception to the light eye rule are Blue staffies; all others should have dark brown eyes even if fawn coat. The coat is smooth and clings tightly to the body giving the dog a streamlined appearance.

 

The dogs stand 36 to 41 cm (14 to 16 in) at the withers and weigh 13 to 17 kg (29 to 37 lb) for males; females are 11 to 15.4 kg (24.3 to 34.0 lb).

 

The eastern gray squirrel (Sciurus carolinensis), also known, particularly outside of North America, as simply the grey squirrel, is a tree squirrel in the genus Sciurus. It is native to eastern North America, where it is the most prodigious and ecologically essential natural forest regenerator. Widely introduced to certain places around the world, the eastern gray squirrel in Europe, in particular, is regarded as an invasive species.

 

In Europe, Sciurus carolinensis is included since 2016 in the list of Invasive Alien Species of Union concern (the Union list). This implies that this species cannot be imported, bred, transported, commercialized, or intentionally released into the environment in the whole of the European Union.

 

Distribution

Sciurus carolinensis is native to the eastern and midwestern United States, and to the southerly portions of the central provinces of Canada. In the mid-1800s the population in the midwestern United States was described as being "truly astonishing", but human predation and habitat destruction through deforestation resulted in drastic population reductions, to the point that the animal was almost absent from Illinois by 1900.

 

The native range of the eastern gray squirrel overlaps with that of the fox squirrel (Sciurus niger), with which it is sometimes confused, although the core of the fox squirrel's range is slightly more to the west. The eastern gray squirrel is found from New Brunswick, through southwestern Quebec and throughout southern Ontario plus in southern Manitoba, south to East Texas and Florida. Breeding eastern gray squirrels are found in Nova Scotia, but whether this population was introduced or came from natural range expansion is not known.

 

A prolific and adaptable species, the eastern gray squirrel has also been introduced to, and thrives in, several regions of the western United States and in 1966, this squirrel was introduced onto Vancouver Island in Western Canada in the area of Metchosin, and has spread widely from there. They are considered highly invasive and a threat to both the local ecosystem and the native squirrel, the American red squirrel.

 

Overseas, eastern gray squirrels in Europe are a concern because they have displaced some of the native squirrels there. They have been introduced into Ireland, Britain, Italy, South Africa, and Australia (where it was extirpated by 1973).

 

In Ireland, the native squirrel – also colored red – the Eurasian red squirrel S. vulgaris – has been displaced in several eastern counties, though it still remains common in the south and west of the country. The gray squirrel is also an invasive species in Britain; it has spread across the country and has largely displaced the red squirrel. That such a displacement might happen in Italy is of concern, as gray squirrels might spread to other parts of mainland Europe.

 

The generic name, Sciurus, is derived from two Greek words, skia 'shadow' and oura 'tail'. This name alludes to the squirrel sitting in the shadow of its tail. The specific epithet, carolinensis, refers to the Carolinas, where the species was first recorded and where the animal is still extremely common. In the United Kingdom and Canada, it is simply referred to as the "grey squirrel". In the US, "eastern" is used to differentiate the species from the western gray squirrel (Sciurus griseus).

 

Characteristics

The eastern gray squirrel has predominantly gray fur, but it can have a brownish color. It has a usual white underside as compared to the typical brownish-orange underside of the fox squirrel. It has a large bushy tail. Particularly in urban situations where the risk of predation is reduced, both white – and black-colored individuals are quite often found. The melanistic form, which is almost entirely black, is predominant in certain populations and in certain geographic areas, such as in large parts of southeastern Canada. Melanistic squirrels appear to exhibit a higher cold tolerance than the common gray morph; when exposed to −10 °C, black squirrels showed an 18% reduction in heat loss, a 20% reduction in basal metabolic rate, and an 11% increase to non-shivering thermogenesis capacity when compared to the common gray morph. The black coloration is caused by an incomplete dominant mutation of MC1R, where E+/E+ is a wild type squirrel, E+/EB is brown-black, and EB/EB is black.

 

The head and body length is from 23 to 30 cm (9.1 to 11.8 in), the tail from 19 to 25 cm (7.5 to 9.8 in), and the adult weight varies between 400 and 600 g (14 and 21 oz). They do not display sexual dimorphism, meaning there is no gender difference in size or coloration.

 

The tracks of an eastern gray squirrel are difficult to distinguish from the related fox squirrel and Abert's squirrel, though the latter's range is almost entirely different from the gray's. Like all squirrels, the eastern gray shows four toes on the front feet and five on the hind feet. The hind foot-pad is often not visible in the track. When bounding or moving at speed, the front foot tracks will be behind the hind foot tracks. The bounding stride can be two to three feet long.

 

The dental formula of the eastern gray squirrel is 1023/1013 (upper teeth/lower teeth).

 

1.0.2.3

1.0.1.3

× 2 = 22 total teeth.

 

Incisors exhibit indeterminate growth, meaning they grow consistently throughout life, and their cheek teeth exhibit brachydont (low-crowned teeth) and bunodont (having tubercles on crowns) structures.

 

Growth and ontogeny

Newborn gray squirrels weigh 13–18 grams and are entirely hairless and pink, although vibrissae are present at birth. 7–10 days postpartum, the skin begins to darken, just before the juvenile pelage grows in. Lower incisors erupt 19–21 days postpartum, while upper incisors erupt after 4 weeks. Cheek teeth erupt during week 6. Eyes open after 21–42 days, and ears open 3–4 weeks postpartum. Weaning is initiated around 7 weeks postpartum, and is usually finished by week 10, followed by the loss of the juvenile pelage. Full adult body mass is achieved by 8–9 months after birth.

 

Diseases

Diseases such as typhus, plague, and tularemia are spread by eastern gray squirrels. If not properly treated, these diseases have the potential to kill squirrels. When bitten or exposed to bodily fluids, humans can contract these diseases. Also carried by eastern gray squirrels are parasites such as ringworm, fleas, lice, mites, and ticks which can kill their squirrel host. Their skin may become rough, blotchy, and prone to hair loss due to the mite parasite during the chilly winter months. The parasites are not transferred to people when these squirrels reside in attics or homes. A frequent illness spread by ticks is Lyme disease. Ticks can also spread Rocky Mountain spotted fever. It can result in damage to internal organs including the heart and kidney if not properly treated. An eastern gray squirrel is susceptible to illness. They are susceptible to diseases including fibromatosis and squirrelpox. A squirrel with fibromatosis, a virus-induced illness, may grow massive skin tumors all over the body. Blindness could result from a tumor that is located close to a squirrel's eye.

 

Behavior and ecology

Like many members of the family Sciuridae, the eastern gray squirrel is a scatter-hoarder; it hoards food in numerous small caches for later recovery. Some caches are quite temporary, especially those made near the site of a sudden abundance of food which can be retrieved within hours or days for reburial in a more secure site. Others are more permanent and are not retrieved until months later. Each squirrel is estimated to make several thousand caches each season. The squirrels have very accurate spatial memory for the locations of these caches, using distant and nearby landmarks to retrieve them. Smell is used partly to uncover food caches, and also to find food in other squirrels' caches. Scent can be unreliable when the ground is too dry or covered in snow.

 

Squirrels sometimes use deceptive behavior to prevent other animals from retrieving cached food. For example, they will pretend to bury the object if they feel that they are being watched. They do this by preparing the spot as usual, for instance, digging a hole or widening a crack, miming the placement of the food, while actually concealing it in their mouths, and then covering up the "cache" as if they had deposited the object. They also hide behind vegetation while burying food or hide it high up in trees (if their rival is not arboreal). Such a complex repertoire suggests that the behaviours are not innate, and imply theory of mind thinking.

 

The eastern gray squirrel is one of very few mammalian species that can descend a tree head-first. It does this by turning its feet so the claws of its hind paws are backward-pointing and can grip the tree bark.

 

Eastern gray squirrels build a type of nest, known as a drey, in the forks of trees, consisting mainly of dry leaves and twigs. The dreys are roughly spherical, about 30 to 60 cm in diameter and are usually insulated with moss, thistledown, dried grass, and feathers to reduce heat loss. Males and females may share the same nest for short times during the breeding season, and during cold winter spells. Squirrels may share a drey to stay warm. They may also nest in the attic or exterior walls of a house, where they may be regarded as pests, as well as fire hazards due to their habit of gnawing on electrical cables. In addition, squirrels may inhabit a permanent tree den hollowed out in the trunk or a large branch of a tree.

 

Eastern gray squirrels are crepuscular, or more active during the early and late hours of the day, and tend to avoid the heat in the middle of a summer day. They do not hibernate.

 

Eastern gray squirrels can breed twice a year, but younger and less experienced mothers normally have a single litter per year in the spring. Depending on forage availability, older and more experienced females may breed again in summer. In a year of abundant food, 36% of females bear two litters, but none will do so in a year of poor food. Their breeding seasons are December to February and May to June, though this is slightly delayed in more northern latitudes. The first litter is born in February or March, the second in June or July, though, again, bearing may be advanced or delayed by a few weeks depending on climate, temperature, and forage availability. In any given breeding season, an average of 61 – 66% of females bear young. If a female fails to conceive or loses her young to unusually cold weather or predation, she re-enters estrus and has a later litter. Five days before a female enters estrus, she may attract up to 34 males from up to 500 meters away. Eastern gray squirrels exhibit a form of polyandry, in which the competing males will form a hierarchy of dominance, and the female will mate with multiple males depending on the hierarchy established.

 

Normally, one to four young are born in each litter, but the largest possible litter size is eight. The gestation period is about 44 days. The young are weaned around 10 weeks, though some may wean up to six weeks later in the wild. They begin to leave the nest after 12 weeks, with autumn born young often wintering with their mother. Only one in four squirrel kits survives to one year of age, with mortality around 55% for the following year. Mortality rates then decrease to around 30% for following years until they increase sharply at eight years of age.

 

Rarely, eastern gray females can enter estrus as early as five and a half months old, but females are not normally fertile until at least one year of age. Their mean age of first estrus is 1.25 years. The presence of a fertile male will induce ovulation in a female going through estrus. Male eastern grays are sexually mature between one and two years of age. Reproductive longevity for females appears to be over 8 years, with 12.5 years documented in North Carolina. These squirrels can live to be 20 years old in captivity, but in the wild live much shorter lives due to predation and the challenges of their habitat. At birth, their life expectancy is 1–2 years, an adult typically can live to be six, with exceptional individuals making it to 12 years.

 

Communication

As in most other mammals, communication among eastern gray squirrel individuals involves both vocalizations and posturing. The species has a quite varied repertoire of vocalizations, including a squeak similar to that of a mouse, a low-pitched noise, a chatter, and a raspy "mehr mehr mehr". Other methods of communication include tail-flicking and other gestures, including facial expressions. Tail flicking and the "kuk" or "quaa" call are used to ward off and warn other squirrels about predators, as well as to announce when a predator is leaving the area. Squirrels also make an affectionate coo-purring sound that biologists call the "muk-muk" sound. This is used as a contact sound between a mother and her kits and in adulthood, by the male when he courts the female during mating season. The use of vocal and visual communication has been shown to vary by location, based on elements such as noise pollution and the amount of open space. For instance, populations living in large cities generally rely more on the visual signals, due to the generally louder environment with more areas without much visual restriction. However, in heavily wooded areas, vocal signals are used more often due to the relatively lower noise levels and a dense canopy restricting visual range.

 

Habitat

In the wild, eastern gray squirrels can be found inhabiting large areas of mature, dense woodland ecosystems, generally covering 100 acres (40 hectares) of land. These forests usually contain large mast-producing trees such as oaks and hickories, providing ample food sources. Oak-hickory hardwood forests are generally preferred over coniferous forests due to the greater abundance of mast forage. This is why they are found only in parts of eastern Canada which do not contain boreal forest (i.e. they are found in some parts of New Brunswick, in southwestern Quebec, throughout southern Ontario and in southern Manitoba).

 

Eastern gray squirrels generally prefer constructing their dens upon large tree branches and within the hollow trunks of trees. They also have been known to take shelter within abandoned bird nests. The dens are usually lined with moss plants, thistledown, dried grass, and feathers. These perhaps provide and assist in the insulation of the den, used to reduce heat loss. A cover to the den is usually built afterwards. Eastern grays squirrels also use dens for protection from prey and helps them look after their young. Young survive 40 percent less if they lived in a leaf nest compared to a den. Squirrels tend to claim 2-3 dens at the same time. Canopy and midstory Trees are used by squirrels to hide from predators such as hawks and owls. The typical squirrel ranges over 1.5 to 8 acres (0.61 to 3.24 ha) and tend to be smaller where more of them are found.

 

Close to human settlements, eastern gray squirrels are found in parks and back yards of houses within urban environments and in the farmlands of rural environments.

 

Ecosystem

Eastern Grey Squirrels are important to the ecosystem by eating a lot of seeds. By caching seeds, they help in the spread of tree seeds. Also, by eating truffles, they contribute to the spread of fungal spores. In addition, they are essential to the environment because they transport parasites. The ecology is influenced by the contribution of squirrels to nature. They often collect seeds and bury them for later consumption, but they often forget where have left them, and they have effectively planted those seeds. These seeds increase the diversity of trees by bringing additional trees into the environment. They are an important key to the forest ecosystem that they belong to.

 

Predation

Eastern gray squirrels predators include hawks, weasels, raccoons, bobcats, foxes, domestic and feral cats, snakes, owls, and dogs. Their primary predators are hawks, owls, and snakes. Raccoons and weasels may consume a squirrel depending on where it lives in the United States. Rattlesnakes eat squirrels in California as they are searching for food in a heavy forest. The squirrel is susceptible to be eaten by a fox in the eastern region of the United States.

 

In its introduced range in South Africa, it has been preyed on by African harrier-hawks. When a predator is approaching the eastern gray squirrel, other squirrels will inform the squirrel of the predator by sending an acoustic signal to let the squirrel know. The speed of a squirrel makes it hard for it to be captured by the predators.

 

Fossil record of the eastern gray squirrel

Twenty different Pleistocene fauna specimens contain S. carolinensis, found in Florida and dated to be as early as the late Irvingtonian period. Body size seems to have increased during the early to middle Holocene and then decreased to the present size seen today.

 

Diet

Eastern gray squirrels eat a range of foods, such as tree bark, tree buds, flowers, berries, many types of seeds and acorns, walnuts, and other nuts, like hazelnuts (see picture) and some types of fungi found in the forests, including fly agaric mushrooms and truffles. They can cause damage to trees by tearing the bark and eating the soft cambial tissue underneath. In Europe, sycamore and beech suffer the greatest damage. Mast-bearing gymnosperms such as cedar, hemlock, pine, and spruce are another food source, as well as angiosperms such as hickory, oak, and walnut. These trees produce important foods for them during the spring and fall months. The squirrels will vary the species they forage from depending on the season. The squirrels also raid gardens for wheat, tomatoes, corn, strawberries, and other garden crops. Sometimes they eat the tomato seeds and discard the rest. On occasion, eastern gray squirrels also prey upon insects, frogs, small rodents including other squirrels, and small birds, their eggs, and young. They also gnaw on bones, antlers, and turtle shells – likely as a source of minerals scarce in their normal diet. In urban and suburban areas, these squirrels scavenge for food in trash bins. However, these foods are not safe for them to digest because they include sugar, fat, as well as additives that can make them sick. Eastern gray squirrels are sometimes mistakenly thought to be herbivores, but they are omnivores.

 

Eastern gray squirrels have a high enough tolerance for humans to inhabit residential neighborhoods and raid bird feeders for millet, corn, and sunflower seeds. Some people who feed and watch birds for entertainment also intentionally feed seeds and nuts to the squirrels for the same reason. However, in the UK eastern gray squirrels can take a significant proportion of supplementary food from feeders, preventing access and reducing use by wild birds. Attraction to supplementary feeders can increase local bird nest predation, as eastern gray squirrels are more likely to forage near feeders, resulting in increased likelihood of finding nests, eggs and nestlings of small passerines.

 

Introductions and impact

The eastern gray squirrel is an introduced species in a variety of locations in western North America: in western Canada, to the southwest corner of British Columbia and to the city of Calgary, Alberta; in the United States, to the states of Washington and Oregon and, in California, to the city of San Francisco and the San Francisco Peninsula area in San Mateo and Santa Clara Counties, south of the city. It has become the most common squirrel in many urban and suburban habitats in western North America, from north of central California to southwest British Columbia.

 

By the turn of the 20th century, breeding populations of the eastern gray squirrel had been introduced into South Africa, Ireland, Italy, Australia (extirpated by 1973), and the United Kingdom.

 

In South Africa, though exotic, it is not usually considered an invasive species owing to its small range (only found in the extreme southwestern part of the Western Cape, going north as far as the small farming town of Franschhoek), as well because it inhabits urban areas and places greatly affected by humans, such as agricultural areas and exotic pine plantations. Here, it mostly eats acorns and pine seeds, although it will take indigenous and commercial fruit, as well. Even so, it is unable to use the natural vegetation (fynbos) found in the area, a factor which has helped to limit its spread. It does not come into contact with native squirrels due to geographic isolation (a native tree squirrel, Paraxerus cepapi, is found only in the savanna regions in the northeast of the country) and different habitats.

 

Gray squirrels were first introduced to Britain in the 1870s, as fashionable additions to estates. In 1921 it was reported in The Times that the Zoological Society of London had released eastern greys to breed at liberty in Regents Park:

 

A dozen years ago the Zoological Society of London obtained a number from a private collection in Bedfordshire for the purpose of inducing them to breed at liberty in the Gardens in Regent's Park. They were first kept in a large enclosure from which, when they had become used to visitors, they were allowed to pass in and out by a rope bridge to a tree. It was hoped that they would spread from the Gardens to the Park. After two or three years in which they seemed to be disappearing, they suddenly became ubiquitous...The grey squirrels are plainly happy and plainly give happiness to Londoners...On the other hand, grey squirrels, whether by taking advantage of tubes and buses, or by deliberate human connivance, have spread from London and are invading the country over very wide areas. They are said to drive out the red squirrel, to raid gardens, and to add to the anxieties of the pheasant breeder. We hope that fuller inquiry will not sustain these charges.

 

They spread rapidly across England, and then became established in both Wales and parts of southern Scotland. On mainland Britain, they have almost entirely displaced native red squirrels. Larger than red squirrels and capable of storing up to four times more fat, gray squirrels are better able to survive winter conditions. They produce more young and can live at higher densities. Gray squirrels also carry the squirrelpox virus, to which red squirrels have no immunity. When an infected gray squirrel introduces squirrelpox to a red squirrel population, its decline is 17–25 times greater than through competition alone.

 

In Ireland, the displacement of red squirrels has not been as rapid because only a single introduction occurred, in County Longford. Schemes have been introduced to control the population of gray squirrels in Ireland to encourage the native red squirrels. Eastern gray squirrels have also been introduced to Italy, and the European Union has expressed concern that they will similarly displace the red squirrel from parts of the European continent.

 

Main article: Tree squirrel § As food

Gray squirrels were eaten in earlier times by Native Americans and their meat is still popular with hunters across most of their range in North America. Today, it is still available for human consumption and is occasionally sold in the United Kingdom. However, physicians in the United States have warned that squirrel brains should not be eaten, because of the risk that they may carry Creutzfeldt–Jakob disease.

 

Displacement of red squirrels

Further information: Eastern gray squirrels in Europe

In Britain and Ireland, the eastern gray squirrel is not regulated by natural predators, other than the European pine marten, which is generally absent from England and Wales. This has aided its rapid population growth and has led to the species being classed as a pest. Measures are being devised to reduce its numbers, including a campaign starting in 2006 named "Save Our Squirrels" using the slogan "Save a red, eat a grey!" which attempted to re-introduce squirrel meat in to the local market, with celebrity chefs promoting the idea, cookbooks introducing recipes containing squirrel and the Forestry Commission providing a regular supply of squirrel meat to British restaurants, factories and butchers. In areas where relict populations of red squirrels survive, such as the islands of Anglesey, Brownsea and the Isle of Wight, programs exist to eradicate gray squirrels and prevent them from reaching these areas in order to allow red squirrel populations to recover and grow.

 

Although complex and controversial, the main factor in the eastern gray squirrel's displacement of the red squirrel is thought to be its greater fitness, hence a competitive advantage over the red squirrel on all measures. Within 15 years of the grey squirrel's introduction to a red squirrel habitat, red squirrel populations are extinct. The eastern gray squirrel tends to be larger and stronger than the red squirrel and has been shown to have a greater ability to store fat for winter. Due to the lack of trees in their native Ireland for them to reside in, red squirrels are the only species being harmed by the invasion of grey squirrels. The squirrel can, therefore, compete more effectively for a larger share of the available food, resulting in relatively lower survival and breeding rates among the red squirrel. Parapoxvirus may also be a strongly contributing factor; red squirrels have long been fatally affected by the disease, while the eastern gray squirrels are unaffected, but thought to be carriers – although how the virus is transmitted has yet to be determined. Red squirrel extinction rates can be 20–25 times greater in areas with confirmed cases of squirrel pox than they are in areas without the disease. This competitive action done between these two squirrels is reasoned to qualify the eastern gray squirrel as a keystone species because since the eastern gray squirrel is coming and wiping out the red squirrels, there would be a reduced chance of competition hence more eastern gray squirrels will come in to Ireland. However, several cases of red squirrels surviving have been reported, as they have developed an immunity – although their population is still being massively affected. The red squirrel is also less tolerant of habitat destruction and fragmentation, which has led to its population decline, while the more adaptable eastern gray squirrel has taken advantage and expanded. Methods done to control this competition between these squirrels are that red squirrels should remain in their original habitats, such as Ireland, while the grey squirrels should be kept out of these places entirely as a means of controlling this squirrel competition.

 

Similar factors appear to have been at play in the Pacific region of North America, where the native American red squirrel has been largely displaced by the eastern gray squirrel in parks and forests throughout much of the region.

 

Ironically, "fears" for the future of the eastern gray squirrel arose in 2008, as the melanistic form (black) began to spread through the southern British population. In the UK, if a "grey squirrel" (eastern gray squirrel) is trapped, under the Wildlife and Countryside Act 1981, it is illegal to release it or to allow it to escape into the wild; instead, it is legally required be "humanely dispatched".

 

In the late 1990s, Italy's National Wildlife Institute and University of Turin launched an eradication attempt to halt the spread of gray squirrels in northwest Italy, but court action by animal rights groups blocked this. Hence gray squirrels are expected to cross the Alps into France and Switzerland in the next few decades

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

Sunda Colugo (Galeopterus variegatus) with baby. The Sunda or Malayan Colugo is the most widespread of the genus Galeopterus; it occurs in parts of southern Myanmar and southern Thailand, localised areas of Laos, Cambodia and Vietnam, and has an extensive range in Peninsular Malaysia, Singapore and Sumatra where suitable forest still exists.

 

A divergent lineage of Galeopterus occurs in Java (Janečka et al, 2008), which some consider as a separate species, and the island of Borneo supports the widely-recognised Bornean Colugo Galeopterus borneanus. Mason (2016) concluded that there may actually be 6 species of Galeopterus.

 

Colugos have large eyes with excellent night vision, small ears and a pointed snout. Some of its teeth are unusual, for example the lower incisors point outwards and are comb-like in structure.

 

Their diet includes leaves and young shoots. During the day colugos rest mainly on tree trunks, generally high up, but sometimes just three metres or so from the ground; they may also make use of tree holes. At dusk they become active, gliding from tree to tree to feed.

 

Females carry a single young (rarely twins) which clings underneath; when roosting, the young often poke their head out from beneath the female. In flight, the young are carried clinging to the flight membrane. Males are smaller than females.

 

The Sunda Colugo is generally mottled grey or greenish-grey but many individuals are reddish to yellowish-orange; males are predominantly reddish.

 

Colugos are rarely heard vocalising; Lim (2007) describes calls from sparring Sunda Colugo males as "a cracking or ripping sound, like the ripping of a thick piece of cardboard".

 

In Singapore this species has proven itself to be adaptable to fragmented forests in semi-urban settings; it makes use of tree plantings next to major roads and other linear plantings to navigate between fragments of habitat. It has been observed gliding across 6-lane highways, if the median divider supports mature trees.

 

Photo by Nick Dobbs, Berjaya Resort, Langkawi 17-12-2023

The horse (Equus caballus) is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of Equus ferus. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature, close to Eohippus, into the large, single-toed animal of today. Humans began domesticating horses around 4000 BCE, and their domestication is believed to have been widespread by 3000 BCE. Horses in the subspecies caballus are domesticated, although some domesticated populations live in the wild as feral horses. These feral populations are not true wild horses, which are horses that never have been domesticated. There is an extensive, specialized vocabulary used to describe equine-related concepts, covering everything from anatomy to life stages, size, colors, markings, breeds, locomotion, and behavior.

 

Horses are adapted to run, allowing them to quickly escape predators, and possess an excellent sense of balance and a strong fight-or-flight response. Related to this need to flee from predators in the wild is an unusual trait: horses are able to sleep both standing up and lying down, with younger horses tending to sleep significantly more than adults. Female horses, called mares, carry their young for approximately 11 months and a young horse, called a foal, can stand and run shortly following birth. Most domesticated horses begin training under a saddle or in a harness between the ages of two and four. They reach full adult development by age five, and have an average lifespan of between 25 and 30 years.

 

Horse breeds are loosely divided into three categories based on general temperament: spirited "hot bloods" with speed and endurance; "cold bloods", such as draft horses and some ponies, suitable for slow, heavy work; and "warmbloods", developed from crosses between hot bloods and cold bloods, often focusing on creating breeds for specific riding purposes, particularly in Europe. There are more than 300 breeds of horse in the world today, developed for many different uses.

 

Horses and humans interact in a wide variety of sport competitions and non-competitive recreational pursuits as well as in working activities such as police work, agriculture, entertainment, and therapy. Horses were historically used in warfare, from which a wide variety of riding and driving techniques developed, using many different styles of equipment and methods of control. Many products are derived from horses, including meat, milk, hide, hair, bone, and pharmaceuticals extracted from the urine of pregnant mares. Humans provide domesticated horses with food, water, and shelter, as well as attention from specialists such as veterinarians and farriers.

 

Lifespan and life stages

Depending on breed, management and environment, the modern domestic horse has a life expectancy of 25 to 30 years. Uncommonly, a few animals live into their 40s and, occasionally, beyond. The oldest verifiable record was "Old Billy", a 19th-century horse that lived to the age of 62. In modern times, Sugar Puff, who had been listed in Guinness World Records as the world's oldest living pony, died in 2007 at age 56.

 

Regardless of a horse or pony's actual birth date, for most competition purposes a year is added to its age each January 1 of each year in the Northern Hemisphere and each August 1 in the Southern Hemisphere. The exception is in endurance riding, where the minimum age to compete is based on the animal's actual calendar age.

 

The following terminology is used to describe horses of various ages:

 

Foal

A horse of either sex less than one year old. A nursing foal is sometimes called a suckling, and a foal that has been weaned is called a weanling. Most domesticated foals are weaned at five to seven months of age, although foals can be weaned at four months with no adverse physical effects.

Yearling

A horse of either sex that is between one and two years old.

Colt

A male horse under the age of four. A common terminology error is to call any young horse a "colt", when the term actually only refers to young male horses.

Filly

A female horse under the age of four.

Mare

A female horse four years old and older.

Stallion

A non-castrated male horse four years old and older.The term "horse" is sometimes used colloquially to refer specifically to a stallion.

Gelding

A castrated male horse of any age.

In horse racing, these definitions may differ: For example, in the British Isles, Thoroughbred horse racing defines colts and fillies as less than five years old. However, Australian Thoroughbred racing defines colts and fillies as less than four years old.

 

Size and measurement

The height of horses is measured at the highest point of the withers, where the neck meets the back. This point is used because it is a stable point of the anatomy, unlike the head or neck, which move up and down in relation to the body of the horse.

 

Size varies greatly among horse breeds, as with this full-sized horse and small pony.

In English-speaking countries, the height of horses is often stated in units of hands and inches: one hand is equal to 4 inches (101.6 mm). The height is expressed as the number of full hands, followed by a point, then the number of additional inches, and ending with the abbreviation "h" or "hh" (for "hands high"). Thus, a horse described as "15.2 h" is 15 hands plus 2 inches, for a total of 62 inches (157.5 cm) in height.

 

The size of horses varies by breed, but also is influenced by nutrition. Light-riding horses usually range in height from 14 to 16 hands (56 to 64 inches, 142 to 163 cm) and can weigh from 380 to 550 kilograms (840 to 1,210 lb). Larger-riding horses usually start at about 15.2 hands (62 inches, 157 cm) and often are as tall as 17 hands (68 inches, 173 cm), weighing from 500 to 600 kilograms (1,100 to 1,320 lb). Heavy or draft horses are usually at least 16 hands (64 inches, 163 cm) high and can be as tall as 18 hands (72 inches, 183 cm) high. They can weigh from about 700 to 1,000 kilograms (1,540 to 2,200 lb).

 

The largest horse in recorded history was probably a Shire horse named Mammoth, who was born in 1848. He stood 21.2 1⁄4 hands (86.25 inches, 219 cm) high and his peak weight was estimated at 1,524 kilograms (3,360 lb). The record holder for the smallest horse ever is Thumbelina, a fully mature miniature horse affected by dwarfism. She was 43 centimetres; 4.1 hands (17 in) tall and weighed 26 kg (57 lb).

 

Ponies

Main article: Pony

Ponies are taxonomically the same animals as horses. The distinction between a horse and pony is commonly drawn on the basis of height, especially for competition purposes. However, height alone is not dispositive; the difference between horses and ponies may also include aspects of phenotype, including conformation and temperament.

 

The traditional standard for height of a horse or a pony at maturity is 14.2 hands (58 inches, 147 cm). An animal 14.2 hands (58 inches, 147 cm) or over is usually considered to be a horse and one less than 14.2 hands (58 inches, 147 cm) a pony, but there are many exceptions to the traditional standard. In Australia, ponies are considered to be those under 14 hands (56 inches, 142 cm). For competition in the Western division of the United States Equestrian Federation, the cutoff is 14.1 hands (57 inches, 145 cm). The International Federation for Equestrian Sports, the world governing body for horse sport, uses metric measurements and defines a pony as being any horse measuring less than 148 centimetres (58.27 in) at the withers without shoes, which is just over 14.2 hands (58 inches, 147 cm), and 149 centimetres (58.66 in; 14.2+1⁄2 hands), with shoes.

 

Height is not the sole criterion for distinguishing horses from ponies. Breed registries for horses that typically produce individuals both under and over 14.2 hands (58 inches, 147 cm) consider all animals of that breed to be horses regardless of their height. Conversely, some pony breeds may have features in common with horses, and individual animals may occasionally mature at over 14.2 hands (58 inches, 147 cm), but are still considered to be ponies.

 

Ponies often exhibit thicker manes, tails, and overall coat. They also have proportionally shorter legs, wider barrels, heavier bone, shorter and thicker necks, and short heads with broad foreheads. They may have calmer temperaments than horses and also a high level of intelligence that may or may not be used to cooperate with human handlers. Small size, by itself, is not an exclusive determinant. For example, the Shetland pony which averages 10 hands (40 inches, 102 cm), is considered a pony. Conversely, breeds such as the Falabella and other miniature horses, which can be no taller than 76 centimetres; 7.2 hands (30 in), are classified by their registries as very small horses, not ponies.

 

Genetics

Horses have 64 chromosomes. The horse genome was sequenced in 2007. It contains 2.7 billion DNA base pairs, which is larger than the dog genome, but smaller than the human genome or the bovine genome.

 

Colors and markings

Horses exhibit a diverse array of coat colors and distinctive markings, described by a specialized vocabulary. Often, a horse is classified first by its coat color, before breed or sex. Horses of the same color may be distinguished from one another by white markings, which, along with various spotting patterns, are inherited separately from coat color.

 

Many genes that create horse coat colors and patterns have been identified. Current genetic tests can identify at least 13 different alleles influencing coat color, and research continues to discover new genes linked to specific traits. The basic coat colors of chestnut and black are determined by the gene controlled by the Melanocortin 1 receptor, also known as the "extension gene" or "red factor", as its recessive form is "red" (chestnut) and its dominant form is black. Additional genes control suppression of black color to point coloration that results in a bay, spotting patterns such as pinto or leopard, dilution genes such as palomino or dun, as well as greying, and all the other factors that create the many possible coat colors found in horses.

 

Horses that have a white coat color are often mislabeled; a horse that looks "white" is usually a middle-aged or older gray. Grays are born a darker shade, get lighter as they age, but usually keep black skin underneath their white hair coat (with the exception of pink skin under white markings). The only horses properly called white are born with a predominantly white hair coat and pink skin, a fairly rare occurrence. Different and unrelated genetic factors can produce white coat colors in horses, including several different alleles of dominant white and the sabino-1 gene. However, there are no "albino" horses, defined as having both pink skin and red eyes.

 

Reproduction and development

Gestation lasts approximately 340 days, with an average range 320–370 days, and usually results in one foal; twins are rare. Horses are a precocial species, and foals are capable of standing and running within a short time following birth. Foals are usually born in the spring. The estrous cycle of a mare occurs roughly every 19–22 days and occurs from early spring into autumn. Most mares enter an anestrus period during the winter and thus do not cycle in this period. Foals are generally weaned from their mothers between four and six months of age.

 

Horses, particularly colts, are sometimes physically capable of reproduction at about 18 months, but domesticated horses are rarely allowed to breed before the age of three, especially females. Horses four years old are considered mature, although the skeleton normally continues to develop until the age of six; maturation also depends on the horse's size, breed, sex, and quality of care. Larger horses have larger bones; therefore, not only do the bones take longer to form bone tissue, but the epiphyseal plates are larger and take longer to convert from cartilage to bone. These plates convert after the other parts of the bones, and are crucial to development.

 

Depending on maturity, breed, and work expected, horses are usually put under saddle and trained to be ridden between the ages of two and four. Although Thoroughbred race horses are put on the track as young as the age of two in some countries, horses specifically bred for sports such as dressage are generally not put under saddle until they are three or four years old, because their bones and muscles are not solidly developed. For endurance riding competition, horses are not deemed mature enough to compete until they are a full 60 calendar months (five years) old.

 

Anatomy

The horse skeleton averages 205 bones. A significant difference between the horse skeleton and that of a human is the lack of a collarbone—the horse's forelimbs are attached to the spinal column by a powerful set of muscles, tendons, and ligaments that attach the shoulder blade to the torso. The horse's four legs and hooves are also unique structures. Their leg bones are proportioned differently from those of a human. For example, the body part that is called a horse's "knee" is actually made up of the carpal bones that correspond to the human wrist. Similarly, the hock contains bones equivalent to those in the human ankle and heel. The lower leg bones of a horse correspond to the bones of the human hand or foot, and the fetlock (incorrectly called the "ankle") is actually the proximal sesamoid bones between the cannon bones (a single equivalent to the human metacarpal or metatarsal bones) and the proximal phalanges, located where one finds the "knuckles" of a human. A horse also has no muscles in its legs below the knees and hocks, only skin, hair, bone, tendons, ligaments, cartilage, and the assorted specialized tissues that make up the hoof.

 

Hooves

Main articles: Horse hoof, Horseshoe, and Farrier

The critical importance of the feet and legs is summed up by the traditional adage, "no foot, no horse". The horse hoof begins with the distal phalanges, the equivalent of the human fingertip or tip of the toe, surrounded by cartilage and other specialized, blood-rich soft tissues such as the laminae. The exterior hoof wall and horn of the sole is made of keratin, the same material as a human fingernail. The result is that a horse, weighing on average 500 kilograms (1,100 lb), travels on the same bones as would a human on tiptoe. For the protection of the hoof under certain conditions, some horses have horseshoes placed on their feet by a professional farrier. The hoof continually grows, and in most domesticated horses needs to be trimmed (and horseshoes reset, if used) every five to eight weeks, though the hooves of horses in the wild wear down and regrow at a rate suitable for their terrain.

 

Teeth

Main article: Horse teeth

Horses are adapted to grazing. In an adult horse, there are 12 incisors at the front of the mouth, adapted to biting off the grass or other vegetation. There are 24 teeth adapted for chewing, the premolars and molars, at the back of the mouth. Stallions and geldings have four additional teeth just behind the incisors, a type of canine teeth called "tushes". Some horses, both male and female, will also develop one to four very small vestigial teeth in front of the molars, known as "wolf" teeth, which are generally removed because they can interfere with the bit. There is an empty interdental space between the incisors and the molars where the bit rests directly on the gums, or "bars" of the horse's mouth when the horse is bridled.

 

An estimate of a horse's age can be made from looking at its teeth. The teeth continue to erupt throughout life and are worn down by grazing. Therefore, the incisors show changes as the horse ages; they develop a distinct wear pattern, changes in tooth shape, and changes in the angle at which the chewing surfaces meet. This allows a very rough estimate of a horse's age, although diet and veterinary care can also affect the rate of tooth wear.

 

Digestion

Main articles: Equine digestive system and Equine nutrition

Horses are herbivores with a digestive system adapted to a forage diet of grasses and other plant material, consumed steadily throughout the day. Therefore, compared to humans, they have a relatively small stomach but very long intestines to facilitate a steady flow of nutrients. A 450-kilogram (990 lb) horse will eat 7 to 11 kilograms (15 to 24 lb) of food per day and, under normal use, drink 38 to 45 litres (8.4 to 9.9 imp gal; 10 to 12 US gal) of water. Horses are not ruminants, they have only one stomach, like humans, but unlike humans, they can digest cellulose, a major component of grass. Horses are hindgut fermenters. Cellulose fermentation by symbiotic bacteria occurs in the cecum, or "water gut", which food goes through before reaching the large intestine. Horses cannot vomit, so digestion problems can quickly cause colic, a leading cause of death. Horses do not have a gallbladder; however, they seem to tolerate high amounts of fat in their diet despite lack of a gallbladder.

 

Senses

The horses' senses are based on their status as prey animals, where they must be aware of their surroundings at all times. They have the largest eyes of any land mammal, and are lateral-eyed, meaning that their eyes are positioned on the sides of their heads. This means that horses have a range of vision of more than 350°, with approximately 65° of this being binocular vision and the remaining 285° monocular vision. Horses have excellent day and night vision, but they have two-color, or dichromatic vision; their color vision is somewhat like red-green color blindness in humans, where certain colors, especially red and related colors, appear as a shade of green.

 

Their sense of smell, while much better than that of humans, is not quite as good as that of a dog. It is believed to play a key role in the social interactions of horses as well as detecting other key scents in the environment. Horses have two olfactory centers. The first system is in the nostrils and nasal cavity, which analyze a wide range of odors. The second, located under the nasal cavity, are the vomeronasal organs, also called Jacobson's organs. These have a separate nerve pathway to the brain and appear to primarily analyze pheromones.

 

A horse's hearing is good, and the pinna of each ear can rotate up to 180°, giving the potential for 360° hearing without having to move the head. Noise impacts the behavior of horses and certain kinds of noise may contribute to stress: a 2013 study in the UK indicated that stabled horses were calmest in a quiet setting, or if listening to country or classical music, but displayed signs of nervousness when listening to jazz or rock music. This study also recommended keeping music under a volume of 21 decibels. An Australian study found that stabled racehorses listening to talk radio had a higher rate of gastric ulcers than horses listening to music, and racehorses stabled where a radio was played had a higher overall rate of ulceration than horses stabled where there was no radio playing.

 

Horses have a great sense of balance, due partly to their ability to feel their footing and partly to highly developed proprioception—the unconscious sense of where the body and limbs are at all times. A horse's sense of touch is well-developed. The most sensitive areas are around the eyes, ears, and nose. Horses are able to sense contact as subtle as an insect landing anywhere on the body.

 

Horses have an advanced sense of taste, which allows them to sort through fodder and choose what they would most like to eat, and their prehensile lips can easily sort even small grains. Horses generally will not eat poisonous plants, however, there are exceptions; horses will occasionally eat toxic amounts of poisonous plants even when there is adequate healthy food.

 

Movement

All horses move naturally with four basic gaits:

the four-beat walk, which averages 6.4 kilometres per hour (4.0 mph);

the two-beat trot or jog at 13 to 19 kilometres per hour (8.1 to 11.8 mph) (faster for harness racing horses);

the canter or lope, a three-beat gait that is 19 to 24 kilometres per hour (12 to 15 mph);

the gallop, which averages 40 to 48 kilometres per hour (25 to 30 mph), but the world record for a horse galloping over a short, sprint distance is 70.76 kilometres per hour (43.97 mph).

Besides these basic gaits, some horses perform a two-beat pace, instead of the trot. There also are several four-beat 'ambling' gaits that are approximately the speed of a trot or pace, though smoother to ride. These include the lateral rack, running walk, and tölt as well as the diagonal fox trot. Ambling gaits are often genetic in some breeds, known collectively as gaited horses. These horses replace the trot with one of the ambling gaits.

 

Behavior

Horses are prey animals with a strong fight-or-flight response. Their first reaction to a threat is to startle and usually flee, although they will stand their ground and defend themselves when flight is impossible or if their young are threatened. They also tend to be curious; when startled, they will often hesitate an instant to ascertain the cause of their fright, and may not always flee from something that they perceive as non-threatening. Most light horse riding breeds were developed for speed, agility, alertness and endurance; natural qualities that extend from their wild ancestors. However, through selective breeding, some breeds of horses are quite docile, particularly certain draft horses.

  

Horses fighting as part of herd dominance behaviour

Horses are herd animals, with a clear hierarchy of rank, led by a dominant individual, usually a mare. They are also social creatures that are able to form companionship attachments to their own species and to other animals, including humans. They communicate in various ways, including vocalizations such as nickering or whinnying, mutual grooming, and body language. Many horses will become difficult to manage if they are isolated, but with training, horses can learn to accept a human as a companion, and thus be comfortable away from other horses. However, when confined with insufficient companionship, exercise, or stimulation, individuals may develop stable vices, an assortment of bad habits, mostly stereotypies of psychological origin, that include wood chewing, wall kicking, "weaving" (rocking back and forth), and other problems.

 

Intelligence and learning

Studies have indicated that horses perform a number of cognitive tasks on a daily basis, meeting mental challenges that include food procurement and identification of individuals within a social system. They also have good spatial discrimination abilities. They are naturally curious and apt to investigate things they have not seen before. Studies have assessed equine intelligence in areas such as problem solving, speed of learning, and memory. Horses excel at simple learning, but also are able to use more advanced cognitive abilities that involve categorization and concept learning. They can learn using habituation, desensitization, classical conditioning, and operant conditioning, and positive and negative reinforcement. One study has indicated that horses can differentiate between "more or less" if the quantity involved is less than four.

 

Domesticated horses may face greater mental challenges than wild horses, because they live in artificial environments that prevent instinctive behavior whilst also learning tasks that are not natural. Horses are animals of habit that respond well to regimentation, and respond best when the same routines and techniques are used consistently. One trainer believes that "intelligent" horses are reflections of intelligent trainers who effectively use response conditioning techniques and positive reinforcement to train in the style that best fits with an individual animal's natural inclinations.

 

Temperament

Horses are mammals, and as such are warm-blooded, or endothermic creatures, as opposed to cold-blooded, or poikilothermic animals. However, these words have developed a separate meaning in the context of equine terminology, used to describe temperament, not body temperature. For example, the "hot-bloods", such as many race horses, exhibit more sensitivity and energy, while the "cold-bloods", such as most draft breeds, are quieter and calmer. Sometimes "hot-bloods" are classified as "light horses" or "riding horses", with the "cold-bloods" classified as "draft horses" or "work horses".

 

a sepia-toned engraving from an old book, showing 11 horses of different breeds and sizes in nine different illustrations

Illustration of assorted breeds; slim, light hotbloods, medium-sized warmbloods and draft and pony-type coldblood breeds

"Hot blooded" breeds include "oriental horses" such as the Akhal-Teke, Arabian horse, Barb, and now-extinct Turkoman horse, as well as the Thoroughbred, a breed developed in England from the older oriental breeds. Hot bloods tend to be spirited, bold, and learn quickly. They are bred for agility and speed. They tend to be physically refined—thin-skinned, slim, and long-legged. The original oriental breeds were brought to Europe from the Middle East and North Africa when European breeders wished to infuse these traits into racing and light cavalry horses.

 

Muscular, heavy draft horses are known as "cold bloods", as they are bred not only for strength, but also to have the calm, patient temperament needed to pull a plow or a heavy carriage full of people. They are sometimes nicknamed "gentle giants". Well-known draft breeds include the Belgian and the Clydesdale. Some, like the Percheron, are lighter and livelier, developed to pull carriages or to plow large fields in drier climates. Others, such as the Shire, are slower and more powerful, bred to plow fields with heavy, clay-based soils. The cold-blooded group also includes some pony breeds.

 

"Warmblood" breeds, such as the Trakehner or Hanoverian, developed when European carriage and war horses were crossed with Arabians or Thoroughbreds, producing a riding horse with more refinement than a draft horse, but greater size and milder temperament than a lighter breed. Certain pony breeds with warmblood characteristics have been developed for smaller riders. Warmbloods are considered a "light horse" or "riding horse".

 

Today, the term "Warmblood" refers to a specific subset of sport horse breeds that are used for competition in dressage and show jumping. Strictly speaking, the term "warm blood" refers to any cross between cold-blooded and hot-blooded breeds. Examples include breeds such as the Irish Draught or the Cleveland Bay. The term was once used to refer to breeds of light riding horse other than Thoroughbreds or Arabians, such as the Morgan horse.

 

Sleep patterns

When horses lie down to sleep, others in the herd remain standing, awake, or in a light doze, keeping watch.

Horses are able to sleep both standing up and lying down. In an adaptation from life in the wild, horses are able to enter light sleep by using a "stay apparatus" in their legs, allowing them to doze without collapsing. Horses sleep better when in groups because some animals will sleep while others stand guard to watch for predators. A horse kept alone will not sleep well because its instincts are to keep a constant eye out for danger.

 

Unlike humans, horses do not sleep in a solid, unbroken period of time, but take many short periods of rest. Horses spend four to fifteen hours a day in standing rest, and from a few minutes to several hours lying down. Total sleep time in a 24-hour period may range from several minutes to a couple of hours, mostly in short intervals of about 15 minutes each. The average sleep time of a domestic horse is said to be 2.9 hours per day.

 

Horses must lie down to reach REM sleep. They only have to lie down for an hour or two every few days to meet their minimum REM sleep requirements. However, if a horse is never allowed to lie down, after several days it will become sleep-deprived, and in rare cases may suddenly collapse as it involuntarily slips into REM sleep while still standing. This condition differs from narcolepsy, although horses may also suffer from that disorder.

 

Taxonomy and evolution

The horse adapted to survive in areas of wide-open terrain with sparse vegetation, surviving in an ecosystem where other large grazing animals, especially ruminants, could not. Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals dominant during the Tertiary period. In the past, this order contained 14 families, but only three—Equidae (the horse and related species), Tapiridae (the tapir), and Rhinocerotidae (the rhinoceroses)—have survived to the present day.

 

The earliest known member of the family Equidae was the Hyracotherium, which lived between 45 and 55 million years ago, during the Eocene period. It had 4 toes on each front foot, and 3 toes on each back foot. The extra toe on the front feet soon disappeared with the Mesohippus, which lived 32 to 37 million years ago. Over time, the extra side toes shrank in size until they vanished. All that remains of them in modern horses is a set of small vestigial bones on the leg below the knee, known informally as splint bones. Their legs also lengthened as their toes disappeared until they were a hooved animal capable of running at great speed. By about 5 million years ago, the modern Equus had evolved. Equid teeth also evolved from browsing on soft, tropical plants to adapt to browsing of drier plant material, then to grazing of tougher plains grasses. Thus proto-horses changed from leaf-eating forest-dwellers to grass-eating inhabitants of semi-arid regions worldwide, including the steppes of Eurasia and the Great Plains of North America.

 

By about 15,000 years ago, Equus ferus was a widespread holarctic species. Horse bones from this time period, the late Pleistocene, are found in Europe, Eurasia, Beringia, and North America. Yet between 10,000 and 7,600 years ago, the horse became extinct in North America. The reasons for this extinction are not fully known, but one theory notes that extinction in North America paralleled human arrival. Another theory points to climate change, noting that approximately 12,500 years ago, the grasses characteristic of a steppe ecosystem gave way to shrub tundra, which was covered with unpalatable plants.

 

Wild species surviving into modern times

Three tan-colored horses with upright manes. Two horses nip and paw at each other, while the third moves towards the camera. They stand in an open, rocky grassland, with forests in the distance.

 

Main article: Wild horse

A truly wild horse is a species or subspecies with no ancestors that were ever successfully domesticated. Therefore, most "wild" horses today are actually feral horses, animals that escaped or were turned loose from domestic herds and the descendants of those animals. Only two wild subspecies, the tarpan and the Przewalski's horse, survived into recorded history and only the latter survives today.

 

The Przewalski's horse (Equus ferus przewalskii), named after the Russian explorer Nikolai Przhevalsky, is a rare Asian animal. It is also known as the Mongolian wild horse; Mongolian people know it as the taki, and the Kyrgyz people call it a kirtag. The subspecies was presumed extinct in the wild between 1969 and 1992, while a small breeding population survived in zoos around the world. In 1992, it was reestablished in the wild by the conservation efforts of numerous zoos. Today, a small wild breeding population exists in Mongolia. There are additional animals still maintained at zoos throughout the world.

 

The question of whether the Przewalski's horse was ever domesticated was challenged in 2018 when DNA studies of horses found at Botai culture sites revealed captured animals with DNA markers of an ancestor to the Przewalski's horse. The study concluded that the Botai animals appear to have been an independent domestication attempt and apparently unsuccessful, as these genetic markers do not appear in modern domesticated horses. However, the question of whether all Przewalski's horses descend from this population is also unresolved, as only one of seven modern Przewalski's horses in the study shared this ancestry.

 

The tarpan or European wild horse (Equus ferus ferus) was found in Europe and much of Asia. It survived into the historical era, but became extinct in 1909, when the last captive died in a Russian zoo. Thus, the genetic line was lost. Attempts have been made to recreate the tarpan, which resulted in horses with outward physical similarities, but nonetheless descended from domesticated ancestors and not true wild horses.

 

Periodically, populations of horses in isolated areas are speculated to be relict populations of wild horses, but generally have been proven to be feral or domestic. For example, the Riwoche horse of Tibet was proposed as such, but testing did not reveal genetic differences from domesticated horses. Similarly, the Sorraia of Portugal was proposed as a direct descendant of the Tarpan on the basis of shared characteristics, but genetic studies have shown that the Sorraia is more closely related to other horse breeds, and that the outward similarity is an unreliable measure of relatedness.

 

Other modern equids

Main article: Equus (genus)

Besides the horse, there are six other species of genus Equus in the Equidae family. These are the ass or donkey, Equus asinus; the mountain zebra, Equus zebra; plains zebra, Equus quagga; Grévy's Zebra, Equus grevyi; the kiang, Equus kiang; and the onager, Equus hemionus.

 

Horses can crossbreed with other members of their genus. The most common hybrid is the mule, a cross between a "jack" (male donkey) and a mare. A related hybrid, a hinny, is a cross between a stallion and a "jenny" (female donkey). Other hybrids include the zorse, a cross between a zebra and a horse. With rare exceptions, most hybrids are sterile and cannot reproduce.

 

Main articles: History of horse domestication theories and Domestication of the horse

Domestication of the horse most likely took place in central Asia prior to 3500 BCE. Two major sources of information are used to determine where and when the horse was first domesticated and how the domesticated horse spread around the world. The first source is based on palaeological and archaeological discoveries; the second source is a comparison of DNA obtained from modern horses to that from bones and teeth of ancient horse remains.

 

The earliest archaeological evidence for the domestication of the horse comes from sites in Ukraine and Kazakhstan, dating to approximately 4000–3500 BCE. By 3000 BCE, the horse was completely domesticated and by 2000 BCE there was a sharp increase in the number of horse bones found in human settlements in northwestern Europe, indicating the spread of domesticated horses throughout the continent. The most recent, but most irrefutable evidence of domestication comes from sites where horse remains were interred with chariots in graves of the Sintashta and Petrovka cultures c. 2100 BCE.

 

A 2021 genetic study suggested that most modern domestic horses descend from the lower Volga-Don region. Ancient horse genomes indicate that these populations influenced almost all local populations as they expanded rapidly throughout Eurasia, beginning about 4,200 years ago. It also shows that certain adaptations were strongly selected due to riding, and that equestrian material culture, including Sintashta spoke-wheeled chariots spread with the horse itself.

 

Domestication is also studied by using the genetic material of present-day horses and comparing it with the genetic material present in the bones and teeth of horse remains found in archaeological and palaeological excavations. The variation in the genetic material shows that very few wild stallions contributed to the domestic horse, while many mares were part of early domesticated herds. This is reflected in the difference in genetic variation between the DNA that is passed on along the paternal, or sire line (Y-chromosome) versus that passed on along the maternal, or dam line (mitochondrial DNA). There are very low levels of Y-chromosome variability, but a great deal of genetic variation in mitochondrial DNA. There is also regional variation in mitochondrial DNA due to the inclusion of wild mares in domestic herds. Another characteristic of domestication is an increase in coat color variation. In horses, this increased dramatically between 5000 and 3000 BCE.

 

Before the availability of DNA techniques to resolve the questions related to the domestication of the horse, various hypotheses were proposed. One classification was based on body types and conformation, suggesting the presence of four basic prototypes that had adapted to their environment prior to domestication. Another hypothesis held that the four prototypes originated from a single wild species and that all different body types were entirely a result of selective breeding after domestication. However, the lack of a detectable substructure in the horse has resulted in a rejection of both hypotheses.

 

Main article: Feral horse

Feral horses are born and live in the wild, but are descended from domesticated animals. Many populations of feral horses exist throughout the world. Studies of feral herds have provided useful insights into the behavior of prehistoric horses, as well as greater understanding of the instincts and behaviors that drive horses that live in domesticated conditions.

 

There are also semi-feral horses in many parts of the world, such as Dartmoor and the New Forest in the UK, where the animals are all privately owned but live for significant amounts of time in "wild" conditions on undeveloped, often public, lands. Owners of such animals often pay a fee for grazing rights.

 

Main articles: Horse breed, List of horse breeds, and Horse breeding

The concept of purebred bloodstock and a controlled, written breed registry has come to be particularly significant and important in modern times. Sometimes purebred horses are incorrectly or inaccurately called "thoroughbreds". Thoroughbred is a specific breed of horse, while a "purebred" is a horse (or any other animal) with a defined pedigree recognized by a breed registry. Horse breeds are groups of horses with distinctive characteristics that are transmitted consistently to their offspring, such as conformation, color, performance ability, or disposition. These inherited traits result from a combination of natural crosses and artificial selection methods. Horses have been selectively bred since their domestication. An early example of people who practiced selective horse breeding were the Bedouin, who had a reputation for careful practices, keeping extensive pedigrees of their Arabian horses and placing great value upon pure bloodlines. These pedigrees were originally transmitted via an oral tradition. In the 14th century, Carthusian monks of southern Spain kept meticulous pedigrees of bloodstock lineages still found today in the Andalusian horse.

 

Breeds developed due to a need for "form to function", the necessity to develop certain characteristics in order to perform a particular type of work. Thus, a powerful but refined breed such as the Andalusian developed as riding horses with an aptitude for dressage. Heavy draft horses were developed out of a need to perform demanding farm work and pull heavy wagons. Other horse breeds had been developed specifically for light agricultural work, carriage and road work, various sport disciplines, or simply as pets. Some breeds developed through centuries of crossing other breeds, while others descended from a single foundation sire, or other limited or restricted foundation bloodstock. One of the earliest formal registries was General Stud Book for Thoroughbreds, which began in 1791 and traced back to the foundation bloodstock for the breed. There are more than 300 horse breeds in the world today.

 

Interaction with humans

Worldwide, horses play a role within human cultures and have done so for millennia. Horses are used for leisure activities, sports, and working purposes. The Food and Agriculture Organization (FAO) estimates that in 2008, there were almost 59,000,000 horses in the world, with around 33,500,000 in the Americas, 13,800,000 in Asia and 6,300,000 in Europe and smaller portions in Africa and Oceania. There are estimated to be 9,500,000 horses in the United States alone. The American Horse Council estimates that horse-related activities have a direct impact on the economy of the United States of over $39 billion, and when indirect spending is considered, the impact is over $102 billion. In a 2004 "poll" conducted by Animal Planet, more than 50,000 viewers from 73 countries voted for the horse as the world's 4th favorite animal.

 

Communication between human and horse is paramount in any equestrian activity; to aid this process horses are usually ridden with a saddle on their backs to assist the rider with balance and positioning, and a bridle or related headgear to assist the rider in maintaining control. Sometimes horses are ridden without a saddle, and occasionally, horses are trained to perform without a bridle or other headgear. Many horses are also driven, which requires a harness, bridle, and some type of vehicle.

 

Main articles: Equestrianism, Horse racing, Horse training, and Horse tack

Historically, equestrians honed their skills through games and races. Equestrian sports provided entertainment for crowds and honed the excellent horsemanship that was needed in battle. Many sports, such as dressage, eventing, and show jumping, have origins in military training, which were focused on control and balance of both horse and rider. Other sports, such as rodeo, developed from practical skills such as those needed on working ranches and stations. Sport hunting from horseback evolved from earlier practical hunting techniques. Horse racing of all types evolved from impromptu competitions between riders or drivers. All forms of competition, requiring demanding and specialized skills from both horse and rider, resulted in the systematic development of specialized breeds and equipment for each sport. The popularity of equestrian sports through the centuries has resulted in the preservation of skills that would otherwise have disappeared after horses stopped being used in combat.

 

Horses are trained to be ridden or driven in a variety of sporting competitions. Examples include show jumping, dressage, three-day eventing, competitive driving, endurance riding, gymkhana, rodeos, and fox hunting. Horse shows, which have their origins in medieval European fairs, are held around the world. They host a huge range of classes, covering all of the mounted and harness disciplines, as well as "In-hand" classes where the horses are led, rather than ridden, to be evaluated on their conformation. The method of judging varies with the discipline, but winning usually depends on style and ability of both horse and rider. Sports such as polo do not judge the horse itself, but rather use the horse as a partner for human competitors as a necessary part of the game. Although the horse requires specialized training to participate, the details of its performance are not judged, only the result of the rider's actions—be it getting a ball through a goal or some other task. Examples of these sports of partnership between human and horse include jousting, in which the main goal is for one rider to unseat the other, and buzkashi, a team game played throughout Central Asia, the aim being to capture a goat carcass while on horseback.

 

Horse racing is an equestrian sport and major international industry, watched in almost every nation of the world. There are three types: "flat" racing; steeplechasing, i.e. racing over jumps; and harness racing, where horses trot or pace while pulling a driver in a small, light cart known as a sulky. A major part of horse racing's economic importance lies in the gambling associated with it.

 

Work

There are certain jobs that horses do very well, and no technology has yet developed to fully replace them. For example, mounted police horses are still effective for certain types of patrol duties and crowd control. Cattle ranches still require riders on horseback to round up cattle that are scattered across remote, rugged terrain. Search and rescue organizations in some countries depend upon mounted teams to locate people, particularly hikers and children, and to provide disaster relief assistance. Horses can also be used in areas where it is necessary to avoid vehicular disruption to delicate soil, such as nature reserves. They may also be the only form of transport allowed in wilderness areas. Horses are quieter than motorized vehicles. Law enforcement officers such as park rangers or game wardens may use horses for patrols, and horses or mules may also be used for clearing trails or other work in areas of rough terrain where vehicles are less effective.

 

Although machinery has replaced horses in many parts of the world, an estimated 100 million horses, donkeys and mules are still used for agriculture and transportation in less developed areas. This number includes around 27 million working animals in Africa alone. Some land management practices such as cultivating and logging can be efficiently performed with horses. In agriculture, less fossil fuel is used and increased environmental conservation occurs over time with the use of draft animals such as horses. Logging with horses can result in reduced damage to soil structure and less damage to trees due to more selective logging.

 

Main article: Horses in warfare

Horses have been used in warfare for most of recorded history. The first archaeological evidence of horses used in warfare dates to between 4000 and 3000 BCE, and the use of horses in warfare was widespread by the end of the Bronze Age. Although mechanization has largely replaced the horse as a weapon of war, horses are still seen today in limited military uses, mostly for ceremonial purposes, or for reconnaissance and transport activities in areas of rough terrain where motorized vehicles are ineffective. Horses have been used in the 21st century by the Janjaweed militias in the War in Darfur.

 

Entertainment and culture

Modern horses are often used to reenact many of their historical work purposes. Horses are used, complete with equipment that is authentic or a meticulously recreated replica, in various live action historical reenactments of specific periods of history, especially recreations of famous battles. Horses are also used to preserve cultural traditions and for ceremonial purposes. Countries such as the United Kingdom still use horse-drawn carriages to convey royalty and other VIPs to and from certain culturally significant events. Public exhibitions are another example, such as the Budweiser Clydesdales, seen in parades and other public settings, a team of draft horses that pull a beer wagon similar to that used before the invention of the modern motorized truck.

 

Horses are frequently used in television, films and literature. They are sometimes featured as a major character in films about particular animals, but also used as visual elements that assure the accuracy of historical stories. Both live horses and iconic images of horses are used in advertising to promote a variety of products. The horse frequently appears in coats of arms in heraldry, in a variety of poses and equipment. The mythologies of many cultures, including Greco-Roman, Hindu, Islamic, and Germanic, include references to both normal horses and those with wings or additional limbs, and multiple myths also call upon the horse to draw the chariots of the Moon and Sun. The horse also appears in the 12-year cycle of animals in the Chinese zodiac related to the Chinese calendar.

 

Horses serve as the inspiration for many modern automobile names and logos, including the Ford Pinto, Ford Bronco, Ford Mustang, Hyundai Equus, Hyundai Pony, Mitsubishi Starion, Subaru Brumby, Mitsubishi Colt/Dodge Colt, Pinzgauer, Steyr-Puch Haflinger, Pegaso, Porsche, Rolls-Royce Camargue, Ferrari, Carlsson, Kamaz, Corre La Licorne, Iran Khodro, Eicher, and Baojun. Indian TVS Motor Company also uses a horse on their motorcycles & scooters.

 

Therapeutic use

People of all ages with physical and mental disabilities obtain beneficial results from an association with horses. Therapeutic riding is used to mentally and physically stimulate disabled persons and help them improve their lives through improved balance and coordination, increased self-confidence, and a greater feeling of freedom and independence. The benefits of equestrian activity for people with disabilities has also been recognized with the addition of equestrian events to the Paralympic Games and recognition of para-equestrian events by the International Federation for Equestrian Sports (FEI). Hippotherapy and therapeutic horseback riding are names for different physical, occupational, and speech therapy treatment strategies that use equine movement. In hippotherapy, a therapist uses the horse's movement to improve their patient's cognitive, coordination, balance, and fine motor skills, whereas therapeutic horseback riding uses specific riding skills.

 

Horses also provide psychological benefits to people whether they actually ride or not. "Equine-assisted" or "equine-facilitated" therapy is a form of experiential psychotherapy that uses horses as companion animals to assist people with mental illness, including anxiety disorders, psychotic disorders, mood disorders, behavioral difficulties, and those who are going through major life changes. There are also experimental programs using horses in prison settings. Exposure to horses appears to improve the behavior of inmates and help reduce recidivism when they leave.

 

Products

Horses are raw material for many products made by humans throughout history, including byproducts from the slaughter of horses as well as materials collected from living horses.

 

Products collected from living horses include mare's milk, used by people with large horse herds, such as the Mongols, who let it ferment to produce kumis. Horse blood was once used as food by the Mongols and other nomadic tribes, who found it a convenient source of nutrition when traveling. Drinking their own horses' blood allowed the Mongols to ride for extended periods of time without stopping to eat. The drug Premarin is a mixture of estrogens extracted from the urine of pregnant mares (pregnant mares' urine), and was previously a widely used drug for hormone replacement therapy. The tail hair of horses can be used for making bows for string instruments such as the violin, viola, cello, and double bass.

 

Horse meat has been used as food for humans and carnivorous animals throughout the ages. Approximately 5 million horses are slaughtered each year for meat worldwide. It is eaten in many parts of the world, though consumption is taboo in some cultures, and a subject of political controversy in others. Horsehide leather has been used for boots, gloves, jackets, baseballs, and baseball gloves. Horse hooves can also be used to produce animal glue. Horse bones can be used to make implements. Specifically, in Italian cuisine, the horse tibia is sharpened into a probe called a spinto, which is used to test the readiness of a (pig) ham as it cures. In Asia, the saba is a horsehide vessel used in the production of kumis.

 

Main article: Horse care

Checking teeth and other physical examinations are an important part of horse care.

Horses are grazing animals, and their major source of nutrients is good-quality forage from hay or pasture. They can consume approximately 2% to 2.5% of their body weight in dry feed each day. Therefore, a 450-kilogram (990 lb) adult horse could eat up to 11 kilograms (24 lb) of food. Sometimes, concentrated feed such as grain is fed in addition to pasture or hay, especially when the animal is very active. When grain is fed, equine nutritionists recommend that 50% or more of the animal's diet by weight should still be forage.

 

Horses require a plentiful supply of clean water, a minimum of 38 to 45 litres (10 to 12 US gal) per day. Although horses are adapted to live outside, they require shelter from the wind and precipitation, which can range from a simple shed or shelter to an elaborate stable.

 

Horses require routine hoof care from a farrier, as well as vaccinations to protect against various diseases, and dental examinations from a veterinarian or a specialized equine dentist. If horses are kept inside in a barn, they require regular daily exercise for their physical health and mental well-being. When turned outside, they require well-maintained, sturdy fences to be safely contained. Regular grooming is also helpful to help the horse maintain good health of the hair coat and underlying skin.

 

Climate change

As of 2019, there are around 17 million horses in the world. Healthy body temperature for adult horses is in the range between 37.5 and 38.5 °C (99.5 and 101.3 °F), which they can maintain while ambient temperatures are between 5 and 25 °C (41 and 77 °F). However, strenuous exercise increases core body temperature by 1 °C (1.8 °F)/minute, as 80% of the energy used by equine muscles is released as heat. Along with bovines and primates, equines are the only animal group which use sweating as their primary method of thermoregulation: in fact, it can account for up to 70% of their heat loss, and horses sweat three times more than humans while undergoing comparably strenuous physical activity. Unlike humans, this sweat is created not by eccrine glands but by apocrine glands. In hot conditions, horses during three hours of moderate-intersity exercise can loss 30 to 35 L of water and 100g of sodium, 198 g of choloride and 45 g of potassium. In another difference from humans, their sweat is hypertonic, and contains a protein called latherin, which enables it to spread across their body easier, and to foam, rather than to drip off. These adaptations are partly to compensate for their lower body surface-to-mass ratio, which makes it more difficult for horses to passively radiate heat. Yet, prolonged exposure to very hot and/or humid conditions will lead to consequences such as anhidrosis, heat stroke, or brain damage, potentially culminating in death if not addressed with measures like cold water applications. Additionally, around 10% of incidents associated with horse transport have been attributed to heat stress. These issues are expected to worsen in the future.

 

African horse sickness (AHS) is a viral illness with a mortality close to 90% in horses, and 50% in mules. A midge, Culicoides imicola, is the primary vector of AHS, and its spread is expected to benefit from climate change. The spillover of Hendra virus from its flying fox hosts to horses is also likely to increase, as future warming would expand the hosts' geographic range. It has been estimated that under the "moderate" and high climate change scenarios, RCP4.5 and RCP8.5, the number of threatened horses would increase by 110,000 and 165,000, respectively, or by 175 and 260%

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

A Hippo enjoying some nice, cool mud

Questo disegno sarà realizzato da un incisore, completamente a mano, in pochissimi millimetri di metallo.

E poi inserito nel complesso meccanismo di un orologio da polso.

Le mani dell'uomo fanno miracoli.

Saguis no terreno da Imprensa Nacional, em Brasília-DF, Brasil.

 

Um sagui[1][2] (do tupi sauín), soim ou mico são as designações comuns dadas a várias espécies de pequenos macacos pertencentes à família Callitrichidae. A palavra sagui tem origem no tupi e sua pronúncia é feita observando-se o som da vogal "u".

Estes primatas são representados por várias espécies em território brasileiro. Todos os quais possuem o dedo polegar da mão muito curto e não oponível, as unhas em forma de garras, e dentes molares de fórmula 2/2. São espécies de pequeno porte e de cauda longa.

São os menores símios do mundo, estão dispersos por toda a América do Sul e vivem geralmente em bandos que se hospedam em árvores, como os esquilos. Travessos e ágeis, movem-se em saltos bruscos, emitindo guinchos e assobios que são ouvidos de longe.

  

Sagui-de-tufos-brancos

Espécies

 

Família Callitrichidae

Callithrix jacchus - Sagui-de-tufos-brancos

Callithrix penicillata - Sagui-de-tufos-pretos

Callithrix kuhlii - Sagui-de-wied

Callithrix geoffroyi - Sagui-de-cara-branca

Callithrix flaviceps - Sagui-da-serra

Callithrix aurita - Sagui-da-serra-escuro

Callithrix argentata - Sagui-branco

Callithrix nigriceps - Sagui-de-cabeça-preta

Callithrix humeralifera - Sagui-de-santarém

Saguinus fuscicollis - Sagui-de-cara-suja

Saguinus imperator - Sagui-imperador

Saguinus labiatus - Sagui-de-bigode

Saguinus mystax - Sagui-de-boca-branca

Saguinus oedipus - Sagui-de-cabeça-branca

Saguinus bicolor - Sagui-de-coleira

Família Callimiconidae

Callimico goeldi - Sagui-goeldi

Referências

 

↑ michaelis.uol.com.br/moderno/portugues/index.php?lingua=p...

↑ Desde 1 de janeiro de 2009, em virtude da vigência do Acordo Ortográfico de 1990, a palavra não é mais grafada com trema (sagüi).

  

O sagüi (português brasileiro) ou sagui (português europeu) (AO 1990: sagui), soim, mico, marmoset (em inglês) ou tamarim (em inglês) são as designações comuns dadas a várias espécies de pequenos macacos pertencentes à família Callitrichidae.

Estes primatas são representados por várias espécies em território brasileiro. Todos os quais possuem o dedo polegar da mão muito curto e não oponível, as unhas em forma de garras, e dentes molares de fórmula 2/2. São espécies de pequeno porte e de cauda longa.

São os menores símios do mundo, estão dispersos por toda a América do Sul e vivem geralmente em bandos que se hospedam, como os esquilos em árvores. Travessos e ágeis, movem-se a saltos bruscos, emitindo guinchos e assobios que são ouvidos de longe.

 

Following, a text, in english, from Wikipedia the free encyclopédia:

 

Black-tufted marmoset at "Imprensa Nacional" (National Press)

The black-tufted marmoset (Callithrix penicillata), also known as Mico-estrela in Portuguese, is a species of New World monkey that lives primarily in the Neo-tropical gallery forests of the Brazilian Central Plateau. It ranges from Bahia to Paraná,[3] and as far inland as Goiás, between 14 and 25 degrees south of the equator. This marmoset typically resides in rainforests, living an arboreal life high in the trees, but below the canopy. They are only rarely spotted near the ground.

Physical description:

The black-tufted marmoset is characterized by black tufts of hair around their ears. It typically has some sparse white hairs on its face. It usually has a brown or black head and its limbs and upper body are gray, as well as its abdomen, while its rump and underside are usually black. Its tail is ringed with black and white and is not prehensile, but is used for balance. It does not have an opposable thumb and its nails tend to have a claw-like appearance. The black-tufted marmoset reaches a size of 19 to 22 cm and weighs up to 350 g.

Behavior:

Diurnal and arboreal, the black-tufted marmoset has a lifestyle very similar to other marmosets. It typically lives in family groups of 2 to 14. The groups usually consist of a reproductive couple and their offspring. Twins are very common among this species and the males, as well as juvenile offspring, often assist the female in the raising of the young.

Though the black-tufted marmoset lives in small family groups, it is believed that they share their food source, sap trees, with other marmoset groups. Scent marking does occur within these groups, but it is believed that the marking is to deter other species rather than other black-tufted marmoset groups, because other groups typically ignore these markings. They also appear to be migratory, often moving in relation to the wet or dry seasons, however, the extent of their migration is unknown.

Though communication between black-tufted marmosets has not been studied thoroughly, it is believed that it communicates through vocalizations. It has known predator-specific cries and appears to vocalize frequently outside of predator cries.

Food and predation:

The Black-tufted Marmoset diet consists primarily of tree sap which it gets by nibbling the bark with its long lower incisors. In periods of drought, it will also include fruit and insects in its diet. In periods of serious drought it has also been known to eat small arthropods, molluscs, bird eggs, baby birds and small vertebrates.

Large birds of prey are the greatest threat to the black-tufted marmoset, however, snakes and wild cats also pose a danger to them. Predator-specific vocalizations and visual scanning are its only anti-predation techniques.

Reproduction:

The black-tufted marmoset is monogamous and lives in family groups. It reproduces twice a year, producing 1 to 4 offspring, though most often just twins. Its gestation period is 150 days and offspring are weaned after 8 weeks. There is considerable parental investment by this species, with both parents, as well as older juveniles, helping to raise the young. The offspring are extremely dependent on their parents and though they are sexually mature at 18 months, they typically do not mate until much later, staying with their family group until they do.

Ecosystem roles and conservation status:

The black-tufted marmoset is a mutualist with many species of fruit trees because it distributes the seeds from the fruit it consumes throughout the forests. However, it is a parasite on other species of trees because it creates sores in trees in order to extract sap, while offering no apparent benefit to the trees. Though this marmoset is not a main food source to any specific species, it is a food source to a number of different species, specifically large birds of prey, wild cats, and snakes.

While there are no known negative effects of marmosets towards humans, it carries specific positive effects by being a highly valuable exotic pet. It is also used in zoo exhibits and scientific research.

The black-tufted marmoset is listed as having no special status on the IUCN Red List or the United States Endangered Species Act List. It is listed in Appendix II of CITES and is not currently considered an endangered or threatened species.

Antoon van Dyck (Antwerp, 1599 - London, 1641) - Paulus Pontius (engraver) - Peter Paul Rubens - Van Dyck exhibition Court Painter - Turin, Royal Museums - Palatine Hall of the Sabauda Gallery

The groundhog is the largest sciurid in its geographical range, typically measuring 40 to 65 cm (16 to 26 in) long (including a 15 cm (6 in) tail) and weighing 2 to 4 kg (4 to 9 lb). In areas with fewer natural predators and large amounts of alfalfa, groundhogs can grow to 80 cm (30 in) and 14 kg (31 lb). Groundhogs are well adapted for digging, with short but powerful limbs and curved, thick claws. Unlike other sciurids, the groundhog's spine is curved, more like that of a mole, and the tail is comparably shorter as well — only about one-fourth of body length. Suited to their temperate habitat, groundhogs are covered with two coats of fur: a dense grey undercoat and a longer coat of banded guard hairs that gives the groundhog its distinctive "frosted" appearance.

The groundhog prefers open country and the edges of woodland, and is rarely far from a burrow entrance. Since the clearing of forests provided it with much more suitable habitat, the groundhog population is probably higher now than it was before the arrival of European settlers in North America. Groundhogs are often hunted for sport, which tends to control their numbers. However, their ability to reproduce quickly has tended to mitigate the depopulating effects of sport hunting. As a consequence, the groundhog is a familiar animal to many people in the United States and Canada

Despite their heavy-bodied appearance, groundhogs are accomplished swimmers and occasionally climb trees when escaping predators or when they want to survey their surroundings. They prefer to retreat to their burrows when threatened; if the burrow is invaded, the groundhog tenaciously defends itself with its two large incisors and front claws. Groundhogs are generally agonistic and territorial among their own species, and may skirmish to establish dominance. Outside their burrow, individuals are alert when not actively feeding. It is common to see one or more nearly-motionless individuals standing erect on their hind feet watching for danger. When alarmed, they use a high-pitched whistle to warn the rest of the colony, hence the name "whistle-pig" Groundhogs may squeal when fighting, seriously injured, or caught by a predator. Other sounds groundhogs may make are low barks and a sound produced by grinding their teeth. When groundhogs are frightened, the hairs of the tail stand straight up, giving the tail the appearance of a hair brush.

 

Source:Wikipedia

Mother and bay otter bonding, Morro Bay California. he sea otter (Enhydra lutris) is a marine mammal native to the coasts of the northern and eastern North Pacific Ocean. Adult sea otters typically weigh between 14 and 45 kg (30 and 100 lb), making them the heaviest members of the weasel family, but among[3] the smallest marine mammals. Unlike most marine mammals, the sea otter's primary form of insulation is an exceptionally thick coat of fur, the densest in the animal kingdom. Although it can walk on land, the sea otter is capable of living exclusively in the ocean.

 

The sea otter inhabits nearshore environments, where it dives to the sea floor to forage. It preys mostly on marine invertebrates such as sea urchins, various mollusks and crustaceans, and some species of fish. Its foraging and eating habits are noteworthy in several respects. Its use of rocks to dislodge prey and to open shells makes it one of the few mammal species to use tools. In most of its range, it is a keystone species, controlling sea urchin populations which would otherwise inflict extensive damage to kelp forest ecosystems.[4] Its diet includes prey species that are also valued by humans as food, leading to conflicts between sea otters and fisheries.

 

Sea otters, whose numbers were once estimated at 150,000–300,000, were hunted extensively for their fur between 1741 and 1911, and the world population fell to 1,000–2,000 individuals living in a fraction of their historic range.[5] A subsequent international ban on hunting, sea otter conservation efforts, and reintroduction programs into previously populated areas have contributed to numbers rebounding, and the species occupies about two-thirds of its former range. The recovery of the sea otter is considered an important success in marine conservation, although populations in the Aleutian Islands and California have recently declined or have plateaued at depressed levels. For these reasons, the sea otter remains classified as an endangered species.

 

Evolution

The sea otter is the heaviest (the giant otter is longer, but significantly slimmer) member of the family Mustelidae,[6] a diverse group that includes the 13 otter species and terrestrial animals such as weasels, badgers, and minks. It is unique among the mustelids in not making dens or burrows, in having no functional anal scent glands,[7] and in being able to live its entire life without leaving the water.[8] The only living member of the genus Enhydra, the sea otter is so different from other mustelid species that, as recently as 1982, some scientists believed it was more closely related to the earless seals.[9] Genetic analysis indicates the sea otter and its closest extant relatives, which include the African speckle-throated otter, Eurasian otter, African clawless otter and Asian small-clawed otter, shared an ancestor approximately 5 million years ago.[10]

 

Fossil evidence indicates the Enhydra lineage became isolated in the North Pacific approximately 2 million years ago, giving rise to the now-extinct Enhydra macrodonta and the modern sea otter, Enhydra lutris.[11] One related species has been described, Enhydra reevei, from the Pleistocene of East Anglia.[12] The modern sea otter evolved initially in northern Hokkaidō and Russia, and then spread east to the Aleutian Islands, mainland Alaska, and down the North American coast.[13] In comparison to cetaceans, sirenians, and pinnipeds, which entered the water approximately 50, 40, and 20 million years ago, respectively, the sea otter is a relative newcomer to a marine existence.[14] In some respects, though, the sea otter is more fully adapted to water than pinnipeds, which must haul out on land or ice to give birth.[15] The full genome of the northern sea otter (Enhydra lutris kenyoni) was sequenced in 2017 and may allow for examination of the sea otter's evolutionary divergence from terrestrial mustelids.[16]

 

Taxonomy

Lutrinae

Pteronura (giant otter)

 

Lontra (4 species)

 

Enhydra (sea otter)

 

Hydrictis

(spotted-necked otter)

 

Lutra (2 species)

 

Aonyx

(African clawless)

 

Amblonyx

(Asian small-clawed)

 

Lutrogale

(smooth-coated)

 

Cladogram showing relationships between sea otters and other otters[17][18]

The first scientific description of the sea otter is contained in the field notes of Georg Steller from 1751, and the species was described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae.[19] Originally named Lutra marina, it underwent numerous name changes before being accepted as Enhydra lutris in 1922.[11] The generic name Enhydra, derives from the Ancient Greek en/εν "in" and hydra/ύδρα "water",[20] meaning "in the water", and the Latin word lutris, meaning "otter".[21] It was formerly sometimes referred to as the "sea beaver".[22]

 

Subspecies

Three subspecies of the sea otter are recognized with distinct geographical distributions. Enhydra lutris lutris (nominate), the Asian sea otter, ranges across Russia's Kuril Islands northeast of Japan, and the Commander Islands in the northwestern Pacific Ocean. In the eastern Pacific Ocean, E. l. kenyoni, the northern sea otter, is found from Alaska's Aleutian Islands to Oregon and E. l. nereis, the southern sea otter, is native to central and southern California.[23] The Asian sea otter is the largest subspecies and has a slightly wider skull and shorter nasal bones than both other subspecies. Northern sea otters possess longer mandibles (lower jaws) while southern sea otters have longer rostrums and smaller teeth.[24][25]

 

Description

 

A sea otter's thick fur makes its body appear plumper on land than in the water.

 

Skull of a sea otter

The sea otter is one of the smallest marine mammal species, but it is the heaviest mustelid.[8] Male sea otters usually weigh 22 to 45 kg (49 to 99 lb) and are 1.2 to 1.5 m (3 ft 11 in to 4 ft 11 in) in length, though specimens up to 54 kg (119 lb) have been recorded.[26] Females are smaller, weighing 14 to 33 kg (31 to 73 lb) and measuring 1.0 to 1.4 m (3 ft 3 in to 4 ft 7 in) in length.[27] For its size, the male otter's baculum is very large, massive and bent upwards, measuring 150 mm (5+7⁄8 in) in length and 15 mm (9⁄16 in) at the base.[28]

 

Unlike most other marine mammals, the sea otter has no blubber and relies on its exceptionally thick fur to keep warm.[29] With up to 150,000 strands of hair per square centimetre (970,000/in2), its fur is the densest of any animal.[30] The fur consists of long, waterproof guard hairs and short underfur; the guard hairs keep the dense underfur layer dry.[27] There is an air compartment between the thick fur and the skin where air is trapped and heated by the body.[31] Cold water is kept completely away from the skin and heat loss is limited.[27] However, a potential disadvantage of this form of insulation is compression of the air layer as the otter dives, thereby reducing the insulating quality of fur at depth when the animal forages.[31] The fur is thick year-round, as it is shed and replaced gradually rather than in a distinct molting season.[32] As the ability of the guard hairs to repel water depends on utmost cleanliness, the sea otter has the ability to reach and groom the fur on any part of its body, taking advantage of its loose skin and an unusually supple skeleton.[33] The coloration of the pelage is usually deep brown with silver-gray speckles, but it can range from yellowish or grayish brown to almost black.[34] In adults, the head, throat, and chest are lighter in color than the rest of the body.[34]

 

The sea otter displays numerous adaptations to its marine environment. The nostrils and small ears can close.[35] The hind feet, which provide most of its propulsion in swimming, are long, broadly flattened, and fully webbed.[36] The fifth digit on each hind foot is longest, facilitating swimming while on its back, but making walking difficult.[37] The tail is fairly short, thick, slightly flattened, and muscular. The front paws are short with retractable claws, with tough pads on the palms that enable gripping slippery prey.[38] The bones show osteosclerosis, increasing their density to reduce buoyancy.[39]

 

The sea otter presents an insight into the evolutionary process of the mammalian invasion of the aquatic environment, which has occurred numerous times over the course of mammalian evolution.[40] Having only returned to the sea about 3 million years ago,[41] sea otters represent a snapshot at the earliest point of the transition from fur to blubber. In sea otters, fur is still advantageous, given their small nature and division of lifetime between the aquatic and terrestrial environments.[42] However, as sea otters evolve and adapt to spending more and more of their lifetimes in the sea, the convergent evolution of blubber suggests that the reliance on fur for insulation would be replaced by a dependency on blubber. This is particularly true due to the diving nature of the sea otter; as dives become lengthier and deeper, the air layer's ability to retain heat or buoyancy decreases,[31] while blubber remains efficient at both of those functions.[42] Blubber can also additionally serve as an energy source for deep dives,[43] which would most likely prove advantageous over fur in the evolutionary future of sea otters.

 

The sea otter propels itself underwater by moving the rear end of its body, including its tail and hind feet, up and down,[36] and is capable of speeds of up to 9 kilometres per hour (5.6 mph).[6] When underwater, its body is long and streamlined, with the short forelimbs pressed closely against the chest.[44] When at the surface, it usually floats on its back and moves by sculling its feet and tail from side to side.[45] At rest, all four limbs can be folded onto the torso to conserve heat, whereas on particularly hot days, the hind feet may be held underwater for cooling.[46] The sea otter's body is highly buoyant because of its large lung capacity – about 2.5 times greater than that of similar-sized land mammals[47] – and the air trapped in its fur. The sea otter walks with a clumsy, rolling gait on land, and can run in a bounding motion.[37]

 

Long, highly sensitive whiskers and front paws help the sea otter find prey by touch when waters are dark or murky.[48] Researchers have noted when they approach in plain view, sea otters react more rapidly when the wind is blowing towards the animals, indicating the sense of smell is more important than sight as a warning sense.[49] Other observations indicate the sea otter's sense of sight is useful above and below the water, although not as good as that of seals.[50] Its hearing is neither particularly acute nor poor.[51]

 

An adult's 32 teeth, particularly the molars, are flattened and rounded for crushing rather than cutting food.[52] Seals and sea otters are the only carnivores with two pairs of lower incisor teeth rather than three;[53] the adult dental formula is

3.1.3.1

2.1.3.2

.[54] The teeth and bones are sometimes stained purple as a result of ingesting sea urchins.[55] The sea otter has a metabolic rate two or three times that of comparatively sized terrestrial mammals. It must eat an estimated 25 to 38% of its own body weight in food each day to burn the calories necessary to counteract the loss of heat due to the cold water environment.[56][57] Its digestive efficiency is estimated at 80 to 85%,[58] and food is digested and passed in as little as three hours.[29] Most of its need for water is met through food, although, in contrast to most other marine mammals, it also drinks seawater. Its relatively large kidneys enable it to derive fresh water from sea water and excrete concentrated urine.[59]

 

Behavior

 

Sensitive vibrissae and forepaws enable sea otters to find prey (like this purple sea urchin) using their sense of touch.

The sea otter is diurnal. It has a period of foraging and eating in the morning, starting about an hour before sunrise, then rests or sleeps in mid-day.[60] Foraging resumes for a few hours in the afternoon and subsides before sunset, and a third foraging period may occur around midnight.[60] Females with pups appear to be more inclined to feed at night.[60] Observations of the amount of time a sea otter must spend each day foraging range from 24 to 60%, apparently depending on the availability of food in the area.[61]

 

Sea otters spend much of their time grooming, which consists of cleaning the fur, untangling knots, removing loose fur, rubbing the fur to squeeze out water and introduce air, and blowing air into the fur. To casual observers, it appears as if the animals are scratching, but they are not known to have lice or other parasites in the fur.[62] When eating, sea otters roll in the water frequently, apparently to wash food scraps from their fur.[63]

  

A sea otter grooming itself by rubbing its dense coat.

Foraging

See also: Physiology of underwater diving

The sea otter hunts in short dives, often to the sea floor. Although it can hold its breath for up to five minutes,[35] its dives typically last about one minute and not more than four.[27] It is the only marine animal capable of lifting and turning over rocks, which it often does with its front paws when searching for prey.[63] The sea otter may also pluck snails and other organisms from kelp and dig deep into underwater mud for clams.[63] It is the only marine mammal that catches fish with its forepaws rather than with its teeth.[29]

  

A sea otter in captivity in Japan, 2015

Under each foreleg, the sea otter has a loose pouch of skin that extends across the chest. In this pouch (preferentially the left one), the animal stores collected food to bring to the surface. This pouch also holds a rock, unique to the otter, that is used to break open shellfish and clams.[64] At the surface, the sea otter eats while floating on its back, using its forepaws to tear food apart and bring it to its mouth. It can chew and swallow small mussels with their shells, whereas large mussel shells may be twisted apart.[65] It uses its lower incisor teeth to access the meat in shellfish.[66] To eat large sea urchins, which are mostly covered with spines, the sea otter bites through the underside where the spines are shortest, and licks the soft contents out of the urchin's shell.[65]

 

The sea otter's use of rocks when hunting and feeding makes it one of the few mammal species to use tools.[67] To open hard shells, it may pound its prey with both paws against a rock on its chest. To pry an abalone off its rock, it hammers the abalone shell using a large stone, with observed rates of 45 blows in 15 seconds.[27] Releasing an abalone, which can cling to rock with a force equal to 4,000 times its own body weight, requires multiple dives.[27]

 

Social structure

 

Sleeping sea otters holding paws at the Vancouver Aquarium[68] are kept afloat by their naturally high buoyancy.

 

Southern sea otters playing with one another at the Elkhorn Slough National Estuarine Research Reserve.

Although each adult and independent juvenile forages alone, sea otters tend to rest together in single-sex groups called rafts. A raft typically contains 10 to 100 animals, with male rafts being larger than female ones.[69] The largest raft ever seen contained over 2000 sea otters. To keep from drifting out to sea when resting and eating, sea otters may wrap themselves in kelp.[70]

 

A male sea otter is most likely to mate if he maintains a breeding territory in an area that is also favored by females.[71] As autumn is the peak breeding season in most areas, males typically defend their territory only from spring to autumn.[71] During this time, males patrol the boundaries of their territories to exclude other males,[71] although actual fighting is rare.[69] Adult females move freely between male territories, where they outnumber adult males by an average of five to one.[71] Males that do not have territories tend to congregate in large, male-only groups,[71] and swim through female areas when searching for a mate.[72]

 

The species exhibits a variety of vocal behaviors. The cry of a pup is often compared to that of a gull.[73] Females coo when they are apparently content; males may grunt instead.[74] Distressed or frightened adults may whistle, hiss, or in extreme circumstances, scream.[73] Although sea otters can be playful and sociable, they are not considered to be truly social animals.[75] They spend much time alone, and each adult can meet its own hunting, grooming, and defense needs.[75]

 

Reproduction and life cycle

 

While mating the male bites the nose of the female, often bloodying and scarring it.

Sea otters are polygynous: males have multiple female partners, typically those that inhabit their territory. If no territory is established, they seek out females in estrus. When a male sea otter finds a receptive female, the two engage in playful and sometimes aggressive behavior. They bond for the duration of estrus, or 3 days. The male holds the female's head or nose with his jaws during copulation. Visible scars are often present on females from this behavior.[6][76]

 

Births occur year-round, with peaks between May and June in northern populations and between January and March in southern populations.[77] Gestation appears to vary from four to twelve months, as the species is capable of delayed implantation followed by four months of pregnancy.[77] In California, sea otters usually breed every year, about twice as often as those in Alaska.[78]

 

Birth usually takes place in the water and typically produces a single pup weighing 1.4 to 2.3 kilograms (3 lb 1 oz to 5 lb 1 oz).[79] Twins occur in 2% of births; however, usually only one pup survives.[6] At birth, the eyes are open, ten teeth are visible, and the pup has a thick coat of baby fur.[80] Mothers have been observed to lick and fluff a newborn for hours; after grooming, the pup's fur retains so much air, the pup floats like a cork and cannot dive.[81] The fluffy baby fur is replaced by adult fur after about 13 weeks.[19]

  

A mother floats with her pup on her chest. Georg Steller wrote, "They embrace their young with an affection that is scarcely credible."[82]

Nursing lasts six to eight months in Californian populations and four to twelve months in Alaska, with the mother beginning to offer bits of prey at one to two months.[83] The milk from a sea otter's two abdominal nipples is rich in fat and more similar to the milk of other marine mammals than to that of other mustelids.[84] A pup, with guidance from its mother, practices swimming and diving for several weeks before it is able to reach the sea floor. Initially, the objects it retrieves are of little food value, such as brightly colored starfish and pebbles.[64] Juveniles are typically independent at six to eight months, but a mother may be forced to abandon a pup if she cannot find enough food for it;[85] at the other extreme, a pup may be nursed until it is almost adult size.[79] Pup mortality is high, particularly during an individual's first winter – by one estimate, only 25% of pups survive their first year.[85] Pups born to experienced mothers have the highest survival rates.[86]

 

Females perform all tasks of feeding and raising offspring, and have occasionally been observed caring for orphaned pups.[87] Much has been written about the level of devotion of sea otter mothers for their pups – a mother gives her infant almost constant attention, cradling it on her chest away from the cold water and attentively grooming its fur.[88] When foraging, she leaves her pup floating on the water, sometimes wrapped in kelp to keep it from floating away;[89] if the pup is not sleeping, it cries loudly until she returns.[90] Mothers have been known to carry their pups for days after the pups' deaths.[82]

 

Females become sexually mature at around three or four years of age and males at around five; however, males often do not successfully breed until a few years later.[91] A captive male sired offspring at age 19.[79] In the wild, sea otters live to a maximum age of 23 years,[27] with lifespans ranging from 10 to 15 years for males and 15–20 years for females.[92] Several captive individuals have lived past 20 years, and a female at the Seattle Aquarium named Etika died at the age of 28 years.[93] Sea otters in the wild often develop worn teeth, which may account for their apparently shorter lifespans.[94]

 

Population and distribution

Sea otters live in coastal waters 15 to 23 metres (49 to 75 ft) deep,[95] and usually stay within a kilometre (2⁄3 mi) of the shore.[96] They are found most often in areas with protection from the most severe ocean winds, such as rocky coastlines, thick kelp forests, and barrier reefs.[97] Although they are most strongly associated with rocky substrates, sea otters can also live in areas where the sea floor consists primarily of mud, sand, or silt.[98] Their northern range is limited by ice, as sea otters can survive amidst drift ice but not land-fast ice.[99] Individuals generally occupy a home range a few kilometres long, and remain there year-round.[100]

 

The sea otter population is thought to have once been 150,000 to 300,000,[22] stretching in an arc across the North Pacific from northern Japan to the central Baja California Peninsula in Mexico. The fur trade that began in the 1740s reduced the sea otter's numbers to an estimated 1,000 to 2,000 members in 13 colonies. Hunting records researched by historian Adele Ogden place the westernmost limit of the hunting grounds off the northern Japanese island of Hokkaido and the easternmost limit off Punta Morro Hermosa about 21+1⁄2 miles (34.6 km) south of Punta Eugenia, Baja California's westernmost headland in Mexico.[101]

 

In about two-thirds of its former range, the species is at varying levels of recovery, with high population densities in some areas and threatened populations in others. Sea otters currently have stable populations in parts of the Russian east coast, Alaska, British Columbia, Washington, and California, with reports of recolonizations in Mexico and Japan.[102] Population estimates made between 2004 and 2007 give a worldwide total of approximately 107,000 sea otters.[19][103][104][105][106]

 

Japan

Adele Ogden wrote in The California Sea Otter Trade that western sea otter were hunted "from Yezo northeastward past the Kuril Group and Kamchatka to the Aleutian Chain".[101] "Yezo" refers to the island province of Hokkaido, in northern Japan, where the country’s only confirmed population of western sea otter resides.[1] Sightings have been documented in the waters of Cape Nosappu, Erimo, Hamanaka and Nemuro, among other locations in the region. [107]

 

Russia

Currently, the most stable and secure part of the western sea otter's range is along the Russian Far East coastline, in the northwestern Pacific waters off of the country (namely Kamchatka and Sakhalin Island), occasionally being seen in and around the Sea of Okhotsk.[108] Before the 19th century, around 20,000 to 25,000 sea otters lived near the Kuril Islands, with more near Kamchatka and the Commander Islands. After the years of the Great Hunt, the population in these areas, currently part of Russia, was only 750.[103] By 2004, sea otters had repopulated all of their former habitat in these areas, with an estimated total population of about 27,000. Of these, about 19,000 are at the Kurils, 2,000 to 3,500 at Kamchatka and another 5,000 to 5,500 at the Commander Islands.[103] Growth has slowed slightly, suggesting the numbers are reaching carrying capacity.[103]

 

British Columbia

Along the North American coast south of Alaska, the sea otter's range is discontinuous. A remnant population survived off Vancouver Island into the 20th century, but it died out despite the 1911 international protection treaty, with the last sea otter taken near Kyuquot in 1929. From 1969 to 1972, 89 sea otters were flown or shipped from Alaska to the west coast of Vancouver Island. This population increased to over 5,600 in 2013 with an estimated annual growth rate of 7.2%, and their range on the island's west coast extended north to Cape Scott and across the Queen Charlotte Strait to the Broughton Archipelago and south to Clayoquot Sound and Tofino.[109][110] In 1989, a separate colony was discovered in the central British Columbia coast. It is not known if this colony, which numbered about 300 animals in 2004, was founded by transplanted otters or was a remnant population that had gone undetected.[105] By 2013, this population exceeded 1,100 individuals, was increasing at an estimated 12.6% annual rate, and its range included Aristazabal Island, and Milbanke Sound south to Calvert Island.[109] In 2008, Canada determined the status of sea otters to be "special concern".[111][112]

 

United States

Alaska

Alaska is the central area of the sea otter's range. In 1973, the population in Alaska was estimated at between 100,000 and 125,000 animals.[113] By 2006, though, the Alaska population had fallen to an estimated 73,000 animals.[104] A massive decline in sea otter populations in the Aleutian Islands accounts for most of the change; the cause of this decline is not known, although orca predation is suspected.[114] The sea otter population in Prince William Sound was also hit hard by the Exxon Valdez oil spill, which killed thousands of sea otters in 1989.[63]

 

Washington

In 1969 and 1970, 59 sea otters were translocated from Amchitka Island to Washington, and released near La Push and Point Grenville. The translocated population is estimated to have declined to between 10 and 43 individuals before increasing, reaching 208 individuals in 1989. As of 2017, the population was estimated at over 2,000 individuals, and their range extends from Point Grenville in the south to Cape Flattery in the north and east to Pillar Point along the Strait of Juan de Fuca.[19]

 

In Washington, sea otters are found almost exclusively on the outer coasts. They can swim as close as six feet off shore along the Olympic coast. Reported sightings of sea otters in the San Juan Islands and Puget Sound almost always turn out to be North American river otters, which are commonly seen along the seashore. However, biologists have confirmed isolated sightings of sea otters in these areas since the mid-1990s.[19]

 

Oregon

The last native sea otter in Oregon was probably shot and killed in 1906. In 1970 and 1971, a total of 95 sea otters were transplanted from Amchitka Island, Alaska to the Southern Oregon coast. However, this translocation effort failed and otters soon again disappeared from the state.[115] In 2004, a male sea otter took up residence at Simpson Reef off of Cape Arago for six months. This male is thought to have originated from a colony in Washington, but disappeared after a coastal storm.[116] On 18 February 2009, a male sea otter was spotted in Depoe Bay off the Oregon Coast. It could have traveled to the state from either California or Washington.[117]

 

California

 

California's remote areas of coastline sheltered small colonies of sea otters through the fur trade. The 50 that survived in California, which were rediscovered in 1938, have since reproduced to almost 3,000.

The historic population of California sea otters was estimated at 16,000 before the fur trade decimated the population, leading to their assumed extinction. Today's population of California sea otters are the descendants of a single colony of about 50 sea otters located near Bixby Creek Bridge in March 1938 by Howard G. Sharpe, owner of the nearby Rainbow Lodge on Bixby Bridge in Big Sur.[118][119][120] Their principal range has gradually expanded and extends from Pigeon Point in San Mateo County to Santa Barbara County.[121]

 

Sea otters were once numerous in San Francisco Bay.[122][123] Historical records revealed the Russian-American Company snuck Aleuts into San Francisco Bay multiple times, despite the Spanish capturing or shooting them while hunting sea otters in the estuaries of San Jose, San Mateo, San Bruno and around Angel Island.[101] The founder of Fort Ross, Ivan Kuskov, finding otters scarce on his second voyage to Bodega Bay in 1812, sent a party of Aleuts to San Francisco Bay, where they met another Russian party and an American party, and caught 1,160 sea otters in three months.[124] By 1817, sea otters in the area were practically eliminated and the Russians sought permission from the Spanish and the Mexican governments to hunt further and further south of San Francisco.[125] In 1833, fur trappers George Nidever and George Yount canoed "along the Petaluma side of [the] Bay, and then proceeded to the San Joaquin River", returning with sea otter, beaver, and river otter pelts.[126] Remnant sea otter populations may have survived in the bay until 1840, when the Rancho Punta de Quentin was granted to Captain John B. R. Cooper, a sea captain from Boston, by Mexican Governor Juan Bautista Alvarado along with a license to hunt sea otters, reportedly then prevalent at the mouth of Corte Madera Creek.[127]

 

In the late 1980s, the USFWS relocated about 140 southern sea otters to San Nicolas Island in southern California, in the hope of establishing a reserve population should the mainland be struck by an oil spill. To the surprise of biologists, the majority of the San Nicolas sea otters swam back to the mainland.[128] Another group of twenty swam 74 miles (119 km) north to San Miguel Island, where they were captured and removed.[129] By 2005, only 30 sea otters remained at San Nicolas,[130] although they were slowly increasing as they thrived on the abundant prey around the island.[128] The plan that authorized the translocation program had predicted the carrying capacity would be reached within five to 10 years.[131] The spring 2016 count at San Nicolas Island was 104 sea otters, continuing a 5-year positive trend of over 12% per year.[132] Sea otters were observed twice in Southern California in 2011, once near Laguna Beach and once at Zuniga Point Jetty, near San Diego. These are the first documented sightings of otters this far south in 30 years.[133]

 

When the USFWS implemented the translocation program, it also attempted, in 1986, to implement "zonal management" of the Californian population. To manage the competition between sea otters and fisheries, it declared an "otter-free zone" stretching from Point Conception to the Mexican border. In this zone, only San Nicolas Island was designated as sea otter habitat, and sea otters found elsewhere in the area were supposed to be captured and relocated. These plans were abandoned after many translocated otters died and also as it proved impractical to capture the hundreds of otters which ignored regulations and swam into the zone.[134] However, after engaging in a period of public commentary in 2005, the Fish and Wildlife Service failed to release a formal decision on the issue.[130] Then, in response to lawsuits filed by the Santa Barbara-based Environmental Defense Center and the Otter Project, on 19 December 2012 the USFWS declared that the "no otter zone" experiment was a failure, and will protect the otters re-colonizing the coast south of Point Conception as threatened species.[135] Although abalone fisherman blamed the incursions of sea otters for the decline of abalone, commercial abalone fishing in southern California came to an end from overfishing in 1997, years before significant otter moved south of Point Conception. In addition, white abalone (Haliotis sorenseni), a species never overlapping with sea otter, had declined in numbers 99% by 1996, and became the first marine invertebrate to be federally listed as endangered.[136]

 

Although the southern sea otter's range has continuously expanded from the remnant population of about 50 individuals in Big Sur since protection in 1911, from 2007 to 2010, the otter population and its range contracted and since 2010 has made little progress.[137][138] As of spring 2010, the northern boundary had moved from about Tunitas Creek to a point 2 kilometres (1.2 mi) southeast of Pigeon Point, and the southern boundary has moved along the Gaviota Coast from approximately Coal Oil Point to Gaviota State Park.[139] A toxin called microcystin, produced by a type of cyanobacteria (Microcystis), seems to be concentrated in the shellfish the otters eat, poisoning them. Cyanobacteria are found in stagnant water enriched with nitrogen and phosphorus from septic tank and agricultural fertilizer runoff, and may be flushed into the ocean when streamflows are high in the rainy season.[140][141] A record number of sea otter carcasses were found on California's coastline in 2010, with increased shark attacks an increasing component of the mortality.[142] Great white sharks do not consume relatively fat-poor sea otters but shark-bitten carcasses have increased from 8% in the 1980s to 15% in the 1990s and to 30% in 2010 and 2011.[143]

 

For southern sea otters to be considered for removal from threatened species listing, the U.S. Fish and Wildlife Service (USFWS) determined that the population should exceed 3,090 for three consecutive years.[137] In response to recovery efforts, the population climbed steadily from the mid-20th century through the early 2000s, then remained relatively flat from 2005 to 2014 at just under 3,000. There was some contraction from the northern (now Pigeon Point) and southern limits of the sea otter's range during the end of this period, circumstantially related to an increase in lethal shark bites, raising concerns that the population had reached a plateau.[144] However, the population increased markedly from 2015 to 2016, with the United States Geological Survey (USGS) California sea otter survey 3-year average reaching 3,272 in 2016, the first time it exceeded the threshold for delisting from the Endangered Species Act (ESA).[132] If populations continued to grow and ESA delisting occurred, southern sea otters would still be fully protected by state regulations and the Marine Mammal Protection Act, which set higher thresholds for protection, at approximately 8,400 individuals.[145] However, ESA delisting seems unlikely due to a precipitous population decline recorded in the spring 2017 USGS sea otter survey count, from the 2016 high of 3,615 individuals to 2,688, a loss of 25% of the California sea otter population.[146]

 

Mexico

Historian Adele Ogden described sea otters are particularly abundant in "Lower California", now the Baja California Peninsula, where "seven bays...were main centers". The southernmost limit was Punta Morro Hermoso about 21+1⁄2 miles (34.6 km) south of Punta Eugenia, in turn a headland at the southwestern end of Sebastián Vizcaíno Bay, on the west coast of the Baja Peninsula. Otter were also taken from San Benito Island, Cedros Island, and Isla Natividad in the Bay.[101] By the early 1900s, Baja's sea otters were extirpated by hunting. In a 1997 survey, small numbers of sea otters, including pups, were reported by local fishermen, but scientists could not confirm these accounts.[147] However, male and female otters have been confirmed by scientists off shores of the Baja Peninsula in a 2014 study, who hypothesize that otter dispersed there beginning in 2005. These sea otters may have dispersed from San Nicolas Island, which is 300 kilometres (190 mi) away, as individuals have been recorded traversing distances of over 800 kilometres (500 mi). Genetic analysis of most of these animals were consistent with California, i.e. United States, otter origins, however one otter had a haplotype not previously reported, and could represent a remnant of the original native Mexican otter population.[148]

 

Ecology

Diet

High energetic requirements of sea otter metabolism require them to consume at least 20% of their body weight a day.[31] Surface swimming and foraging are major factors in their high energy expenditure due to drag on the surface of the water when swimming and the thermal heat loss from the body during deep dives when foraging.[149][31] Sea otter muscles are specially adapted to generate heat without physical activity.[150]

 

Sea otters consume over 100 prey species.[151] In most of its range, the sea otter's diet consists almost exclusively of marine benthic invertebrates, including sea urchins (such as Strongylocentrotus franciscanus and S. purpuratus), fat innkeeper worms, a variety of bivalves such as clams, mussels (such as Mytilus edulis), and scallops (such as Crassadoma gigantea), abalone, limpets (such as Diodora aspera), chitons (such as Katharina tunicata), other mollusks, crustaceans, and snails.[151][152][153] Its prey ranges in size from tiny limpets and crabs to giant octopuses.[151] Where prey such as sea urchins, clams, and abalone are present in a range of sizes, sea otters tend to select larger items over smaller ones of similar type.[151] In California, they have been noted to ignore Pismo clams smaller than 3 inches (76 mm) across.[154]

 

In a few northern areas, fish are also eaten. In studies performed at Amchitka Island in the 1960s, where the sea otter population was at carrying capacity, 50% of food found in sea otter stomachs was fish.[155] The fish species were usually bottom-dwelling and sedentary or sluggish forms, such as Hemilepidotus hemilepidotus and family Tetraodontidae.[155] However, south of Alaska on the North American coast, fish are a negligible or extremely minor part of the sea otter's diet.[19][156] Contrary to popular depictions, sea otters rarely eat starfish, and any kelp that is consumed apparently passes through the sea otter's system undigested.[157]

 

The individuals within a particular area often differ in their foraging methods and prey types, and tend to follow the same patterns as their mothers.[158] The diet of local populations also changes over time, as sea otters can significantly deplete populations of highly preferred prey such as large sea urchins, and prey availability is also affected by other factors such as fishing by humans.[19] Sea otters can thoroughly remove abalone from an area except for specimens in deep rock crevices,[159] however, they never completely wipe out a prey species from an area.[160] A 2007 Californian study demonstrated, in areas where food was relatively scarce, a wider variety of prey was consumed. Surprisingly, though, the diets of individuals were more specialized in these areas than in areas where food was plentiful.[128]

 

As a keystone species

 

Sea otters control herbivore populations, ensuring sufficient coverage of kelp in kelp forests

Sea otters are a classic example of a keystone species; their presence affects the ecosystem more profoundly than their size and numbers would suggest. They keep the population of certain benthic (sea floor) herbivores, particularly sea urchins, in check.[4] Sea urchins graze on the lower stems of kelp, causing the kelp to drift away and die.[161] Loss of the habitat and nutrients provided by kelp forests leads to profound cascade effects on the marine ecosystem. North Pacific areas that do not have sea otters often turn into urchin barrens, with abundant sea urchins and no kelp forest.[6] Kelp forests are extremely productive ecosystems. Kelp forests sequester (absorb and capture) CO2 from the atmosphere through photosynthesis. Sea otters may help mitigate effects of climate change by their cascading trophic influence[162]

 

Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems,[163] and similar changes have been observed as sea otter populations recovered in the Aleutian and Commander Islands and the Big Sur coast of California[164] However, some kelp forest ecosystems in California have also thrived without sea otters, with sea urchin populations apparently controlled by other factors.[164] The role of sea otters in maintaining kelp forests has been observed to be more important in areas of open coast than in more protected bays and estuaries.[164]

 

Sea otters affect rocky ecosystems that are dominated by mussel beds by removing mussels from rocks. This allows space for competing species and increases species diversity.[164]

 

Predators

Leading mammalian predators of this species include orcas and sea lions, and bald eagles may grab pups from the surface of the water. Young predators may kill an otter and not eat it.[67] On land, young sea otters may face attack from bears and coyotes. In California, great white sharks are their primary predator.[165] In Katmai National Park, grey wolves have been recorded to hunt and kill sea otters.[166]

 

Urban runoff transporting cat feces into the ocean brings Toxoplasma gondii, an obligate parasite of felids, which has killed sea otters.[167] Parasitic infections of Sarcocystis neurona are also associated with human activity.[16] According to the U.S. Geological Survey and the CDC, northern sea otters off Washington have been infected with the H1N1 flu virus and "may be a newly identified animal host of influenza viruses".[168]

 

Relationship with humans

Fur trade

 

Aleut men in Unalaska in 1896 used waterproof kayak gear and garments to hunt sea otters.

Sea otters have the thickest fur of any mammal, which makes them a common target for many hunters. Archaeological evidence indicates that for thousands of years, indigenous peoples have hunted sea otters for food and fur. Large-scale hunting, part of the Maritime Fur Trade, which would eventually kill approximately one million sea otters, began in the 18th century when hunters and traders began to arrive from all over the world to meet foreign demand for otter pelts, which were one of the world's most valuable types of fur.[22]

 

In the early 18th century, Russians began to hunt sea otters in the Kuril Islands[22] and sold them to the Chinese at Kyakhta. Russia was also exploring the far northern Pacific at this time, and sent Vitus Bering to map the Arctic coast and find routes from Siberia to North America. In 1741, on his second North Pacific voyage, Bering was shipwrecked off Bering Island in the Commander Islands, where he and many of his crew died. The surviving crew members, which included naturalist Georg Steller, discovered sea otters on the beaches of the island and spent the winter hunting sea otters and gambling with otter pelts. They returned to Siberia, having killed nearly 1,000 sea otters, and were able to command high prices for the pelts.[169] Thus began what is sometimes called the "Great Hunt", which would continue for another hundred years. The Russians found the sea otter far more valuable than the sable skins that had driven and paid for most of their expansion across Siberia. If the sea otter pelts brought back by Bering's survivors had been sold at Kyakhta prices they would have paid for one tenth the cost of Bering's expedition.[170]

  

Pelt sales (in thousands) in the London fur market – the decline beginning in the 1880s reflects dwindling sea otter populations.[171]

Russian fur-hunting expeditions soon depleted the sea otter populations in the Commander Islands, and by 1745, they began to move on to the Aleutian Islands. The Russians initially traded with the Aleuts inhabitants of these islands for otter pelts, but later enslaved the Aleuts, taking women and children hostage and torturing and killing Aleut men to force them to hunt. Many Aleuts were either murdered by the Russians or died from diseases the hunters had introduced.[172][disputed – discuss] The Aleut population was reduced, by the Russians' own estimate, from 20,000 to 2,000.[173] By the 1760s, the Russians had reached Alaska. In 1799, Tsar Paul I consolidated the rival fur-hunting companies into the Russian-American Company, granting it an imperial charter and protection, and a monopoly over trade rights and territorial acquisition. Under Aleksander I, the administration of the merchant-controlled company was transferred to the Imperial Navy, largely due to the alarming reports by naval officers of native abuse; in 1818, the indigenous peoples of Alaska were granted civil rights equivalent to a townsman status in the Russian Empire.[174]

 

Other nations joined in the hunt in the south. Along the coasts of what is now Mexico and California, Spanish explorers bought sea otter pelts from Native Americans and sold them in Asia.[172] In 1778, British explorer Captain James Cook reached Vancouver Island and bought sea otter furs from the First Nations people. When Cook's ship later stopped at a Chinese port, the pelts rapidly sold at high prices, and were soon known as "soft gold". As word spread, people from all over Europe and North America began to arrive in the Pacific Northwest to trade for sea otter furs.[175]

 

Russian hunting expanded to the south, initiated by American ship captains, who subcontracted Russian supervisors and Aleut hunters[176] in what are now Washington, Oregon, and California. Between 1803 and 1846, 72 American ships were involved in the otter hunt in California, harvesting an estimated 40,000 skins and tails, compared to only 13 ships of the Russian-American Company, which reported 5,696 otter skins taken between 1806 and 1846.[177] In 1812, the Russians founded an agricultural settlement at what is now Fort Ross in northern California, as their southern headquarters.[175] Eventually, sea otter populations became so depleted, commercial hunting was no longer viable. It had stopped in the Aleutian Islands, by 1808, as a conservation measure imposed by the Russian-American Company. Further restrictions were ordered by the company in 1834.[178] When Russia sold Alaska to the United States in 1867, the Alaska population had recovered to over 100,000, but Americans resumed hunting and quickly extirpated the sea otter again.[179] Prices rose as the species became rare. During the 1880s, a pelt brought $105 to $165 in the London market, but by 1903, a pelt could be worth as much as $1,125.[79] In 1911, Russia, Japan, Great Britain (for Canada) and the United States signed the Treaty for the Preservation and Protection of Fur Seals, imposing a moratorium on the harvesting of sea otters.[180] So few remained, perhaps only 1,000–2,000 individuals in the wild, that many believed the species would become extinct.[19]

 

Recovery and conservation

Main article: Sea otter conservation

 

In the wake of the March 1989 Exxon Valdez oil spill, heavy sheens of oil covered large areas of Prince William Sound.

During the 20th century, sea otter numbers rebounded in about two-thirds of their historic range, a recovery considered one of the greatest successes in marine conservation.[181] However, the IUCN still lists the sea otter as an endangered species, and describes the significant threats to sea otters as oil pollution, predation by orcas, poaching, and conflicts with fisheries – sea otters can drown if entangled in fishing gear.[1] The hunting of sea otters is no longer legal except for limited harvests by indigenous peoples in the United States.[182] Poaching was a serious concern in the Russian Far East immediately after the collapse of the Soviet Union in 1991; however, it has declined significantly with stricter law enforcement and better economic conditions.[108]

 

The most significant threat to sea otters is oil spills,[67] to which they are particularly vulnerable, since they rely on their fur to keep warm. When their fur is soaked with oil, it loses its ability to retain air, and the animals can quickly die from hypothermia.[67] The liver, kidneys, and lungs of sea otters also become damaged after they inhale oil or ingest it when grooming.[67] The Exxon Valdez oil spill of 24 March 1989 killed thousands of sea otters in Prince William Sound, and as of 2006, the lingering oil in the area continues to affect the population.[183] Describing the public sympathy for sea otters that developed from media coverage of the event, a U.S. Fish and Wildlife Service spokesperson wrote:

 

As a playful, photogenic, innocent bystander, the sea otter epitomized the role of victim ... cute and frolicsome sea otters suddenly in distress, oiled, frightened, and dying, in a losing battle with the oil.[19]

The small geographic ranges of the sea otter populations in California, Washington, and British Columbia mean a single major spill could be catastrophic for that state or province.[19][57][63] Prevention of oil spills and preparation to rescue otters if one happens is a major focus for conservation efforts. Increasing the size and range of sea otter populations would also reduce the risk of an oil spill wiping out a population.[19] However, because of the species' reputation for depleting shellfish resources, advocates for commercial, recreational, and subsistence shellfish harvesting have often opposed allowing the sea otter's range to increase, and there have even been instances of fishermen and others illegally killing them.[184]

 

In the Aleutian Islands, a massive and unexpected disappearance of sea otters has occurred in recent decades. In the 1980s, the area was home to an estimated 55,000 to 100,000 sea otters, but the population fell to around 6,000 animals by 2000.[185] The most widely accepted, but still controversial, hypothesis is that killer whales have been eating the otters. The pattern of disappearances is consistent with a rise in predation, but there has been no direct evidence of orcas preying on sea otters to any significant extent.[114]

 

Another area of concern is California, where recovery began to fluctuate or decline in the late 1990s.[186] Unusually high mortality rates amongst adult and subadult otters, particularly females, have been reported.[106] In 2017 the US Geological Survey found a 3% drop in the sea otter population of the California coast. This number still keeps them on track for removal from the endangered species list, although just barely.[187] Necropsies of dead sea otters indicate diseases, particularly Toxoplasma gondii and acanthocephalan parasite infections, are major causes of sea otter mortality in California.[188] The Toxoplasma gondii parasite, which is often fatal to sea otters, is carried by wild and domestic cats and may be transmitted by domestic cat droppings flushed into the ocean via sewage systems.[188][189] Although disease has clearly contributed to the deaths of many of California's sea otters, it is not known why the California population is apparently more affected by disease than populations in other areas.[188]

  

Sea otters off the coast of Washington, within the Olympic Coast National Marine Sanctuary

Sea otter habitat is preserved through several protected areas in the United States, Russia and Canada. In marine protected areas, polluting activities such as dumping of waste and oil drilling are typically prohibited.[190] An estimated 1,200 sea otters live within the Monterey Bay National Marine Sanctuary, and more than 500 live within the Olympic Coast National Marine Sanctuary.[191][192]

 

Economic impact

Some of the sea otter's preferred prey species, particularly abalone, clams, and crabs, are also food sources for humans. In some areas, massive declines in shellfish harvests have been blamed on the sea otter, and intense public debate has taken place over how to manage the competition between sea otters and humans for seafood.[193]

 

The debate is complicated because sea otters have sometimes been held responsible for declines of shellfish stocks that were more likely caused by overfishing, disease, pollution, and seismic activity.[63][194] Shellfish declines have also occurred in many parts of the North American Pacific coast that do not have sea otters, and conservationists sometimes note the existence of large concentrations of shellfish on the coast is a recent development resulting from the fur trade's near-extirpation of the sea otter.[194] Although many factors affect shellfish stocks, sea otter predation can deplete a fishery to the point where it is no longer commercially viable.[193] Scientists agree that sea otters and abalone fisheries cannot exist in the same area,[193] and the same is likely true for certain other types of shellfish, as well.[185]

 

Many facets of the interaction between sea otters and the human economy are not as immediately felt. Sea otters have been credited with contributing to the kelp harvesting industry via their well-known role in controlling sea urchin populations; kelp is used in the production of diverse food and pharmaceutical products.[195] Although human divers harvest red sea urchins both for food and to protect the kelp, sea otters hunt more sea urchin species and are more consistently effective in controlling these populations.[196] E. lutris is a controlling predator of the red king crab (Paralithodes camtschaticus) in the Bering Sea, which would otherwise be out of control as it is in its invasive range, the Barents Sea.[197] (Berents otters, Lutra lutra, occupy the same ecological niche and so are believed to help to control them in the Berents but this has not been studied.)[197] The health of the kelp forest ecosystem is significant in nurturing populations of fish, including commercially important fish species.[195] In some areas, sea otters are popular tourist attractions, bringing visitors to local hotels, restaurants, and sea otter-watching expeditions.[195]

 

Roles in human cultures

 

Aleut carving of a sea otter hunt

Left: Aleut sea otter amulet in the form of a mother with pup. Above: Aleut carving of a sea otter hunt on a whalebone spear. Both items are on display at the Peter the Great Museum of Anthropology and Ethnography in St. Petersburg. Articles depicting sea otters were considered to have magical properties.[198]

 

For many maritime indigenous cultures throughout the North Pacific, especially the Ainu in the Kuril Islands, the Koryaks and Itelmen of Kamchatka, the Aleut in the Aleutian Islands, the Haida of Haida Gwaii[199] and a host of tribes on the Pacific coast of North America, the sea otter has played an important role as a cultural, as well as material, resource. In these cultures, many of which have strongly animist traditions full of legends and stories in which many aspects of the natural world are associated with spirits, the sea otter was considered particularly kin to humans. The Nuu-chah-nulth, Haida, and other First Nations of coastal British Columbia used the warm and luxurious pelts as chiefs' regalia. Sea otter pelts were given in potlatches to mark coming-of-age ceremonies, weddings, and funerals.[68] The Aleuts carved sea otter bones for use as ornaments and in games, and used powdered sea otter baculum as a medicine for fever.[200]

 

Among the Ainu, the otter is portrayed as an occasional messenger between humans and the creator.[201] The sea otter is a recurring figure in Ainu folklore. A major Ainu epic, the Kutune Shirka, tells the tale of wars and struggles over a golden sea otter. Versions of a widespread Aleut legend tell of lovers or despairing women who plunge into the sea and become otters.[202] These links have been associated with the many human-like behavioral features of the sea otter, including apparent playfulness, strong mother-pup bonds and tool use, yielding to ready anthropomorphism.[203] The beginning of commercial exploitation had a great impact on the human, as well as animal, populations. The Ainu and Aleuts have been displaced or their numbers are dwindling, while the coastal tribes of North America, where the otter is in any case greatly depleted, no longer rely as intimately on sea mammals for survival.[204]

 

Since the mid-1970s, the beauty and charisma of the species have gained wide appreciation, and the sea otter has become an icon of environmental conservation.[186] The round, expressive face and soft, furry body of the sea otter are depicted in a wide variety of souvenirs, postcards, clothing, and stuffed toys.[205]

 

Aquariums and zoos

Sea otters can do well in captivity, and are featured in over 40 public aquariums and zoos.[206] The Seattle Aquarium became the first institution to raise sea otters from conception to adulthood with the birth of Tichuk in 1979, followed by three more pups in the early 1980s.[207] In 2007, a YouTube video of two sea otters holding paws drew 1.5 million viewers in two weeks, and had over 22 million views as of July 2022.[208] Filmed five years previously at the Vancouver Aquarium, it was YouTube's most popular animal video at the time, although it has since been surpassed. The lighter-colored otter in the video is Nyac, a survivor of the 1989 Exxon Valdez oil spill.[209] Nyac died in September 2008, at the age of 20.[210] Milo, the darker one, died of lymphoma in January 2012.[211]

 

Current conservation

Sea otters, being a known keystone species, need a humanitarian effort to be protected from endangerment through "unregulated human exploitation".[212] This species has increasingly been impacted by the large oil spills and environmental degradation caused by overfishing and entanglement in fishing gear.[213] Current efforts have been made in legislation: the international Fur Seal Treaty, The Endangered Species Act, IUCN/The World Conservation Union, Convention on international Trade in Endangered Species of Wild Fauna and Flora, and the Marine Mammal Protection Act of 1972. Other conservation efforts are done through reintroduction and zoological parks. Wikipedia

 

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

The Pokhot live in the Baringo and Western Pokot districts of Kenya and in Uganda.

There are two main sub-groups depending of their location and way of life. The first group consist of the Hill Pokot who live in the rainy highlands in the west and in the central south, and are mainly farmers and pastoralists. The second group is made up of the Plains Pokot who live in dry and infertile plains, with their cattles. A homestead is composed of one or more buildings for a man, his wife and children; eventual co-wives live in separate houses. The role of the community in teaching children ethical rules. Most of the Pokot are nomadic and thus have interacted with different peoples, incorporating their social customs.The Pokot are very proud of their culture. The Songs, storytelling, and decorative arts, especially bodily decoration, are very appreciated. They adorn the body with beads, hairstyling, scarification, and the removal of the lower central incisors. Pokot girls wear a beaded necklace made of the stems of an asparagus tree. Most Pokot have some knowledge of herbal medicine, so they often use these treatments along with those of the hospitals. They belong to the Kenya's Nilotic-speaking peoples. .

For the Pokot, the universe has two realms: the above is the realm of the most powerful deities—Tororot, Asis (sun), and llat (rain); and the below is the one where live humans, animals, and plants. Humans are responsible for the realm that they inhabit, but they rely upon divinities to achieve and maintain peace and prosperity. They worship many deities like the sun, moon and believe in the spirit of death.The Pokot communicate with their deities through prayer and sacrifice. They perform it during ethnic festivals and dances. Oracles are responsible for maintaining the spiritual balance within the community. They are superstitious and believe in sorcery, so sometimes they call on shielding lucky sorcery. They have prophets, either male or female, who foresee advise, usually by the means of animal sacrifices. His or her ability is considered as a divine gift. Clan histories recount the changes of location, through poetry and song, emphasizing the vulnerability of humans and the importance of supernatural powers to help them overcome hunger, thirst, and even death. Ceremonies mark the transitions in the people's social lives. Among these are: the cleansing of a couple expecting their first child; the cleansing of newborn infants and their mothers; the cleansing of twins and other children who are born under unusual circumstances; male and female initiation; marriage; sapana, a coming-of-age ceremony for men; and summer-solstice, harvest, and healing ceremonies. The most important rite of passage for most Pokot is circumcision for boys and clitoridectomy for girls. These rites consist of a series of neighborhood-based ceremonies, emphasizing the importance of having a good behavior. When boys are circumcised, they acquire membership in one of eight age sets. Women do not have age-sets. After excisions, for several months, girls have a white painting on their face and wear a hood made of blackened leather with charcoal and oil. This means they are untouchable until the lepan ceremony, that marks the passage to womanhood. Unlike other tribes, the Pokot keep the affiliation to their clan throughout their lives, there is no disruption with marriage. Surprisingly, the agreement before marriage is made by gift giving, from the groom and his family to the bride and her family, often over a period of years (and not the contrary). It often implies the gift of a combination of livestock, goods, and cash to the bride's family, and the allotment of milk cows and rights to land to the bride. The bond between a husband and wife lasts for 3 generations, after what marriages can take place again between the two groups. Polygamy exists but is not prevalent among men before 40. The spirits of the elder anticipate reincarnation in their living descendants: when a child is said to resemble the elder, the same name is given. Disputes are resolved in neighborhood councils and in government courts. Some of the sanctions include shaming, cursing, and bewitching.

 

© Eric Lafforgue

www.ericlafforgue.com

 

Fish, any of approximately 34,000 species of vertebrate animals (phylum Chordata) found in the fresh and salt waters of the world. Living species range from the primitive jawless lampreys and hagfishes through the cartilaginous sharks, skates, and rays to the abundant and diverse bony fishes. Most fish species are cold-blooded; however, one species, the opah (Lampris guttatus), is warm-blooded.

 

The term fish is applied to a variety of vertebrates of several evolutionary lines. It describes a life-form rather than a taxonomic group. As members of the phylum Chordata, fish share certain features with other vertebrates. These features are gill slits at some point in the life cycle, a notochord, or skeletal supporting rod, a dorsal hollow nerve cord, and a tail. Living fishes represent some five classes, which are as distinct from one another as are the four classes of familiar air-breathing animals—amphibians, reptiles, birds, and mammals. For example, the jawless fishes (Agnatha) have gills in pouches and lack limb girdles. Extant agnathans are the lampreys and the hagfishes. As the name implies, the skeletons of fishes of the class Chondrichthyes (from chondr, “cartilage,” and ichthyes, “fish”) are made entirely of cartilage. Modern fish of this class lack a swim bladder, and their scales and teeth are made up of the same placoid material. Sharks, skates, and rays are examples of cartilaginous fishes. The bony fishes are by far the largest class. Examples range from the tiny seahorse to the 450-kg (1,000-pound) blue marlin, from the flattened soles and flounders to the boxy puffers and ocean sunfishes. Unlike the scales of the cartilaginous fishes, those of bony fishes, when present, grow throughout life and are made up of thin overlapping plates of bone. Bony fishes also have an operculum that covers the gill slits.

 

The study of fishes, the science of ichthyology, is of broad importance. Fishes are of interest to humans for many reasons, the most important being their relationship with and dependence on the environment. A more obvious reason for interest in fishes is their role as a moderate but important part of the world’s food supply. This resource, once thought unlimited, is now realized to be finite and in delicate balance with the biological, chemical, and physical factors of the aquatic environment. Overfishing, pollution, and alteration of the environment are the chief enemies of proper fisheries management, both in fresh waters and in the ocean. (For a detailed discussion of the technology and economics of fisheries, see commercial fishing.) Another practical reason for studying fishes is their use in disease control. As predators on mosquito larvae, they help curb malaria and other mosquito-borne diseases.

 

Fishes are valuable laboratory animals in many aspects of medical and biological research. For example, the readiness of many fishes to acclimate to captivity has allowed biologists to study behaviour, physiology, and even ecology under relatively natural conditions. Fishes have been especially important in the study of animal behaviour, where research on fishes has provided a broad base for the understanding of the more flexible behaviour of the higher vertebrates. The zebra fish is used as a model in studies of gene expression.

 

There are aesthetic and recreational reasons for an interest in fishes. Millions of people keep live fishes in home aquariums for the simple pleasure of observing the beauty and behaviour of animals otherwise unfamiliar to them. Aquarium fishes provide a personal challenge to many aquarists, allowing them to test their ability to keep a small section of the natural environment in their homes. Sportfishing is another way of enjoying the natural environment, also indulged in by millions of people every year. Interest in aquarium fishes and sportfishing supports multimillion-dollar industries throughout the world.

 

Fishes have been in existence for more than 450 million years, during which time they have evolved repeatedly to fit into almost every conceivable type of aquatic habitat. In a sense, land vertebrates are simply highly modified fishes: when fishes colonized the land habitat, they became tetrapod (four-legged) land vertebrates. The popular conception of a fish as a slippery, streamlined aquatic animal that possesses fins and breathes by gills applies to many fishes, but far more fishes deviate from that conception than conform to it. For example, the body is elongate in many forms and greatly shortened in others; the body is flattened in some (principally in bottom-dwelling fishes) and laterally compressed in many others; the fins may be elaborately extended, forming intricate shapes, or they may be reduced or even lost; and the positions of the mouth, eyes, nostrils, and gill openings vary widely. Air breathers have appeared in several evolutionary lines.

 

Many fishes are cryptically coloured and shaped, closely matching their respective environments; others are among the most brilliantly coloured of all organisms, with a wide range of hues, often of striking intensity, on a single individual. The brilliance of pigments may be enhanced by the surface structure of the fish, so that it almost seems to glow. A number of unrelated fishes have actual light-producing organs. Many fishes are able to alter their coloration—some for the purpose of camouflage, others for the enhancement of behavioral signals.

 

Fishes range in adult length from less than 10 mm (0.4 inch) to more than 20 metres (60 feet) and in weight from about 1.5 grams (less than 0.06 ounce) to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C (100 °F), others in cold Arctic seas a few degrees below 0 °C (32 °F) or in cold deep waters more than 4,000 metres (13,100 feet) beneath the ocean surface. The structural and, especially, the physiological adaptations for life at such extremes are relatively poorly known and provide the scientifically curious with great incentive for study.

 

Almost all natural bodies of water bear fish life, the exceptions being very hot thermal ponds and extremely salt-alkaline lakes, such as the Dead Sea in Asia and the Great Salt Lake in North America. The present distribution of fishes is a result of the geological history and development of Earth as well as the ability of fishes to undergo evolutionary change and to adapt to the available habitats. Fishes may be seen to be distributed according to habitat and according to geographical area. Major habitat differences are marine and freshwater. For the most part, the fishes in a marine habitat differ from those in a freshwater habitat, even in adjacent areas, but some, such as the salmon, migrate from one to the other. The freshwater habitats may be seen to be of many kinds. Fishes found in mountain torrents, Arctic lakes, tropical lakes, temperate streams, and tropical rivers will all differ from each other, both in obvious gross structure and in physiological attributes. Even in closely adjacent habitats where, for example, a tropical mountain torrent enters a lowland stream, the fish fauna will differ. The marine habitats can be divided into deep ocean floors (benthic), mid-water oceanic (bathypelagic), surface oceanic (pelagic), rocky coast, sandy coast, muddy shores, bays, estuaries, and others. Also, for example, rocky coastal shores in tropical and temperate regions will have different fish faunas, even when such habitats occur along the same coastline.

 

Although much is known about the present geographical distribution of fishes, far less is known about how that distribution came about. Many parts of the fish fauna of the fresh waters of North America and Eurasia are related and undoubtedly have a common origin. The faunas of Africa and South America are related, extremely old, and probably an expression of the drifting apart of the two continents. The fauna of southern Asia is related to that of Central Asia, and some of it appears to have entered Africa. The extremely large shore-fish faunas of the Indian and tropical Pacific oceans comprise a related complex, but the tropical shore fauna of the Atlantic, although containing Indo-Pacific components, is relatively limited and probably younger. The Arctic and Antarctic marine faunas are quite different from each other. The shore fauna of the North Pacific is quite distinct, and that of the North Atlantic more limited and probably younger. Pelagic oceanic fishes, especially those in deep waters, are similar the world over, showing little geographical isolation in terms of family groups. The deep oceanic habitat is very much the same throughout the world, but species differences do exist, showing geographical areas determined by oceanic currents and water masses.

 

All aspects of the life of a fish are closely correlated with adaptation to the total environment, physical, chemical, and biological. In studies, all the interdependent aspects of fish, such as behaviour, locomotion, reproduction, and physical and physiological characteristics, must be taken into account.

 

Correlated with their adaptation to an extremely wide variety of habitats is the extremely wide variety of life cycles that fishes display. The great majority hatch from relatively small eggs a few days to several weeks or more after the eggs are scattered in the water. Newly hatched young are still partially undeveloped and are called larvae until body structures such as fins, skeleton, and some organs are fully formed. Larval life is often very short, usually less than a few weeks, but it can be very long, some lampreys continuing as larvae for at least five years. Young and larval fishes, before reaching sexual maturity, must grow considerably, and their small size and other factors often dictate that they live in a habitat different than that of the adults. For example, most tropical marine shore fishes have pelagic larvae. Larval food also is different, and larval fishes often live in shallow waters, where they may be less exposed to predators.

 

After a fish reaches adult size, the length of its life is subject to many factors, such as innate rates of aging, predation pressure, and the nature of the local climate. The longevity of a species in the protected environment of an aquarium may have nothing to do with how long members of that species live in the wild. Many small fishes live only one to three years at the most. In some species, however, individuals may live as long as 10 or 20 or even 100 years.

 

Fish behaviour is a complicated and varied subject. As in almost all animals with a central nervous system, the nature of a response of an individual fish to stimuli from its environment depends upon the inherited characteristics of its nervous system, on what it has learned from past experience, and on the nature of the stimuli. Compared with the variety of human responses, however, that of a fish is stereotyped, not subject to much modification by “thought” or learning, and investigators must guard against anthropomorphic interpretations of fish behaviour.

 

Fishes perceive the world around them by the usual senses of sight, smell, hearing, touch, and taste and by special lateral line water-current detectors. In the few fishes that generate electric fields, a process that might best be called electrolocation aids in perception. One or another of these senses often is emphasized at the expense of others, depending upon the fish’s other adaptations. In fishes with large eyes, the sense of smell may be reduced; others, with small eyes, hunt and feed primarily by smell (such as some eels).

 

Specialized behaviour is primarily concerned with the three most important activities in the fish’s life: feeding, reproduction, and escape from enemies. Schooling behaviour of sardines on the high seas, for instance, is largely a protective device to avoid enemies, but it is also associated with and modified by their breeding and feeding requirements. Predatory fishes are often solitary, lying in wait to dart suddenly after their prey, a kind of locomotion impossible for beaked parrot fishes, which feed on coral, swimming in small groups from one coral head to the next. In addition, some predatory fishes that inhabit pelagic environments, such as tunas, often school.

 

Sleep in fishes, all of which lack true eyelids, consists of a seemingly listless state in which the fish maintains its balance but moves slowly. If attacked or disturbed, most can dart away. A few kinds of fishes lie on the bottom to sleep. Most catfishes, some loaches, and some eels and electric fishes are strictly nocturnal, being active and hunting for food during the night and retiring during the day to holes, thick vegetation, or other protective parts of the environment.

 

Communication between members of a species or between members of two or more species often is extremely important, especially in breeding behaviour (see below Reproduction). The mode of communication may be visual, as between the small so-called cleaner fish and a large fish of a very different species. The larger fish often allows the cleaner to enter its mouth to remove gill parasites. The cleaner is recognized by its distinctive colour and actions and therefore is not eaten, even if the larger fish is normally a predator. Communication is often chemical, signals being sent by specific chemicals called pheromones.

 

Many fishes have a streamlined body and swim freely in open water. Fish locomotion is closely correlated with habitat and ecological niche (the general position of the animal to its environment).

 

Many fishes in both marine and fresh waters swim at the surface and have mouths adapted to feed best (and sometimes only) at the surface. Often such fishes are long and slender, able to dart at surface insects or at other surface fishes and in turn to dart away from predators; needlefishes, halfbeaks, and topminnows (such as killifish and mosquito fish) are good examples. Oceanic flying fishes escape their predators by gathering speed above the water surface, with the lower lobe of the tail providing thrust in the water. They then glide hundreds of yards on enlarged, winglike pectoral and pelvic fins. South American freshwater flying fishes escape their enemies by jumping and propelling their strongly keeled bodies out of the water.

 

So-called mid-water swimmers, the most common type of fish, are of many kinds and live in many habitats. The powerful fusiform tunas and the trouts, for example, are adapted for strong, fast swimming, the tunas to capture prey speedily in the open ocean and the trouts to cope with the swift currents of streams and rivers. The trout body form is well adapted to many habitats. Fishes that live in relatively quiet waters such as bays or lake shores or slow rivers usually are not strong, fast swimmers but are capable of short, quick bursts of speed to escape a predator. Many of these fishes have their sides flattened, examples being the sunfish and the freshwater angelfish of aquarists. Fish associated with the bottom or substrate usually are slow swimmers. Open-water plankton-feeding fishes almost always remain fusiform and are capable of rapid, strong movement (for example, sardines and herrings of the open ocean and also many small minnows of streams and lakes).

 

Bottom-living fishes are of many kinds and have undergone many types of modification of their body shape and swimming habits. Rays, which evolved from strong-swimming mid-water sharks, usually stay close to the bottom and move by undulating their large pectoral fins. Flounders live in a similar habitat and move over the bottom by undulating the entire body. Many bottom fishes dart from place to place, resting on the bottom between movements, a motion common in gobies. One goby relative, the mudskipper, has taken to living at the edge of pools along the shore of muddy mangrove swamps. It escapes its enemies by flipping rapidly over the mud, out of the water. Some catfishes, synbranchid eels, the so-called climbing perch, and a few other fishes venture out over damp ground to find more promising waters than those that they left. They move by wriggling their bodies, sometimes using strong pectoral fins; most have accessory air-breathing organs. Many bottom-dwelling fishes live in mud holes or rocky crevices. Marine eels and gobies commonly are found in such habitats and for the most part venture far beyond their cavelike homes. Some bottom dwellers, such as the clingfishes (Gobiesocidae), have developed powerful adhesive disks that enable them to remain in place on the substrate in areas such as rocky coasts, where the action of the waves is great.

 

The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

 

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female’s vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally—for example, the claspers of many sharks and rays.

 

In the females the eggs are formed in two ovaries (sometimes only one) and pass through the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but are shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous). Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species are born sexually mature, although they are not fully grown.

 

Some fishes are hermaphroditic—an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare.

 

Successful reproduction and, in many cases, defense of the eggs and the young are assured by rather stereotypical but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

 

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in the gravel beds where they themselves hatched (anadromous fishes). Some, such as the freshwater eels (family Anguillidae), live and grow to maturity in fresh water and migrate to the sea to spawn (catadromous fishes). Other fishes undertake shorter migrations from lakes into streams, within the ocean, or enter spawning habitats that they do not ordinarily occupy in other ways.

 

The basic structure and function of the fish body are similar to those of all other vertebrates. The usual four types of tissues are present: surface or epithelial, connective (bone, cartilage, and fibrous tissues, as well as their derivative, blood), nerve, and muscle tissues. In addition, the fish’s organs and organ systems parallel those of other vertebrates.

 

The typical fish body is streamlined and spindle-shaped, with an anterior head, a gill apparatus, and a heart, the latter lying in the midline just below the gill chamber. The body cavity, containing the vital organs, is situated behind the head in the lower anterior part of the body. The anus usually marks the posterior termination of the body cavity and most often occurs just in front of the base of the anal fin. The spinal cord and vertebral column continue from the posterior part of the head to the base of the tail fin, passing dorsal to the body cavity and through the caudal (tail) region behind the body cavity. Most of the body is of muscular tissue, a high proportion of which is necessitated by swimming. In the course of evolution this basic body plan has been modified repeatedly into the many varieties of fish shapes that exist today.

 

The skeleton forms an integral part of the fish’s locomotion system, as well as serving to protect vital parts. The internal skeleton consists of the skull bones (except for the roofing bones of the head, which are really part of the external skeleton), the vertebral column, and the fin supports (fin rays). The fin supports are derived from the external skeleton but will be treated here because of their close functional relationship to the internal skeleton. The internal skeleton of cyclostomes, sharks, and rays is of cartilage; that of many fossil groups and some primitive living fishes is mostly of cartilage but may include some bone. In place of the vertebral column, the earliest vertebrates had a fully developed notochord, a flexible stiff rod of viscous cells surrounded by a strong fibrous sheath. During the evolution of modern fishes the rod was replaced in part by cartilage and then by ossified cartilage. Sharks and rays retain a cartilaginous vertebral column; bony fishes have spool-shaped vertebrae that in the more primitive living forms only partially replace the notochord. The skull, including the gill arches and jaws of bony fishes, is fully, or at least partially, ossified. That of sharks and rays remains cartilaginous, at times partially replaced by calcium deposits but never by true bone.

 

The supportive elements of the fins (basal or radial bones or both) have changed greatly during fish evolution. Some of these changes are described in the section below (Evolution and paleontology). Most fishes possess a single dorsal fin on the midline of the back. Many have two and a few have three dorsal fins. The other fins are the single tail and anal fins and paired pelvic and pectoral fins. A small fin, the adipose fin, with hairlike fin rays, occurs in many of the relatively primitive teleosts (such as trout) on the back near the base of the caudal fin.

 

The skin of a fish must serve many functions. It aids in maintaining the osmotic balance, provides physical protection for the body, is the site of coloration, contains sensory receptors, and, in some fishes, functions in respiration. Mucous glands, which aid in maintaining the water balance and offer protection from bacteria, are extremely numerous in fish skin, especially in cyclostomes and teleosts. Since mucous glands are present in the modern lampreys, it is reasonable to assume that they were present in primitive fishes, such as the ancient Silurian and Devonian agnathans. Protection from abrasion and predation is another function of the fish skin, and dermal (skin) bone arose early in fish evolution in response to this need. It is thought that bone first evolved in skin and only later invaded the cartilaginous areas of the fish’s body, to provide additional support and protection. There is some argument as to which came first, cartilage or bone, and fossil evidence does not settle the question. In any event, dermal bone has played an important part in fish evolution and has different characteristics in different groups of fishes. Several groups are characterized at least in part by the kind of bony scales they possess.

 

Scales have played an important part in the evolution of fishes. Primitive fishes usually had thick bony plates or thick scales in several layers of bone, enamel, and related substances. Modern teleost fishes have scales of bone, which, while still protective, allow much more freedom of motion in the body. A few modern teleosts (some catfishes, sticklebacks, and others) have secondarily acquired bony plates in the skin. Modern and early sharks possessed placoid scales, a relatively primitive type of scale with a toothlike structure, consisting of an outside layer of enamel-like substance (vitrodentine), an inner layer of dentine, and a pulp cavity containing nerves and blood vessels. Primitive bony fishes had thick scales of either the ganoid or the cosmoid type. Cosmoid scales have a hard, enamel-like outer layer, an inner layer of cosmine (a form of dentine), and then a layer of vascular bone (isopedine). In ganoid scales the hard outer layer is different chemically and is called ganoin. Under this is a cosminelike layer and then a vascular bony layer. The thin, translucent bony scales of modern fishes, called cycloid and ctenoid (the latter distinguished by serrations at the edges), lack enameloid and dentine layers.

 

Skin has several other functions in fishes. It is well supplied with nerve endings and presumably receives tactile, thermal, and pain stimuli. Skin is also well supplied with blood vessels. Some fishes breathe in part through the skin, by the exchange of oxygen and carbon dioxide between the surrounding water and numerous small blood vessels near the skin surface.

 

Skin serves as protection through the control of coloration. Fishes exhibit an almost limitless range of colours. The colours often blend closely with the surroundings, effectively hiding the animal. Many fishes use bright colours for territorial advertisement or as recognition marks for other members of their own species, or sometimes for members of other species. Many fishes can change their colour to a greater or lesser degree, by movement of pigment within the pigment cells (chromatophores). Black pigment cells (melanophores), of almost universal occurrence in fishes, are often juxtaposed with other pigment cells. When placed beneath iridocytes or leucophores (bearing the silvery or white pigment guanine), melanophores produce structural colours of blue and green. These colours are often extremely intense, because they are formed by refraction of light through the needlelike crystals of guanine. The blue and green refracted colours are often relatively pure, lacking the red and yellow rays, which have been absorbed by the black pigment (melanin) of the melanophores. Yellow, orange, and red colours are produced by erythrophores, cells containing the appropriate carotenoid pigments. Other colours are produced by combinations of melanophores, erythrophores, and iridocytes.

 

The major portion of the body of most fishes consists of muscles. Most of the mass is trunk musculature, the fin muscles usually being relatively small. The caudal fin is usually the most powerful fin, being moved by the trunk musculature. The body musculature is usually arranged in rows of chevron-shaped segments on each side. Contractions of these segments, each attached to adjacent vertebrae and vertebral processes, bends the body on the vertebral joint, producing successive undulations of the body, passing from the head to the tail, and producing driving strokes of the tail. It is the latter that provides the strong forward movement for most fishes.

 

The digestive system, in a functional sense, starts at the mouth, with the teeth used to capture prey or collect plant foods. Mouth shape and tooth structure vary greatly in fishes, depending on the kind of food normally eaten. Most fishes are predacious, feeding on small invertebrates or other fishes and have simple conical teeth on the jaws, on at least some of the bones of the roof of the mouth, and on special gill arch structures just in front of the esophagus. The latter are throat teeth. Most predacious fishes swallow their prey whole, and the teeth are used for grasping and holding prey, for orienting prey to be swallowed (head first) and for working the prey toward the esophagus. There are a variety of tooth types in fishes. Some fishes, such as sharks and piranhas, have cutting teeth for biting chunks out of their victims. A shark’s tooth, although superficially like that of a piranha, appears in many respects to be a modified scale, while that of the piranha is like that of other bony fishes, consisting of dentine and enamel. Parrot fishes have beaklike mouths with short incisor-like teeth for breaking off coral and have heavy pavementlike throat teeth for crushing the coral. Some catfishes have small brushlike teeth, arranged in rows on the jaws, for scraping plant and animal growth from rocks. Many fishes (such as the Cyprinidae or minnows) have no jaw teeth at all but have very strong throat teeth.

 

Some fishes gather planktonic food by straining it from their gill cavities with numerous elongate stiff rods (gill rakers) anchored by one end to the gill bars. The food collected on these rods is passed to the throat, where it is swallowed. Most fishes have only short gill rakers that help keep food particles from escaping out the mouth cavity into the gill chamber.

 

Once reaching the throat, food enters a short, often greatly distensible esophagus, a simple tube with a muscular wall leading into a stomach. The stomach varies greatly in fishes, depending upon the diet. In most predacious fishes it is a simple straight or curved tube or pouch with a muscular wall and a glandular lining. Food is largely digested there and leaves the stomach in liquid form.

 

Between the stomach and the intestine, ducts enter the digestive tube from the liver and pancreas. The liver is a large, clearly defined organ. The pancreas may be embedded in it, diffused through it, or broken into small parts spread along some of the intestine. The junction between the stomach and the intestine is marked by a muscular valve. Pyloric ceca (blind sacs) occur in some fishes at this junction and have a digestive or absorptive function or both.

 

The intestine itself is quite variable in length, depending upon the fish’s diet. It is short in predacious forms, sometimes no longer than the body cavity, but long in herbivorous forms, being coiled and several times longer than the entire length of the fish in some species of South American catfishes. The intestine is primarily an organ for absorbing nutrients into the bloodstream. The larger its internal surface, the greater its absorptive efficiency, and a spiral valve is one method of increasing its absorption surface.

 

Sharks, rays, chimaeras, lungfishes, surviving chondrosteans, holosteans, and even a few of the more primitive teleosts have a spiral valve or at least traces of it in the intestine. Most modern teleosts have increased the area of the intestinal walls by having numerous folds and villi (fingerlike projections) somewhat like those in humans. Undigested substances are passed to the exterior through the anus in most teleost fishes. In lungfishes, sharks, and rays, it is first passed through the cloaca, a common cavity receiving the intestinal opening and the ducts from the urogenital system.

 

Oxygen and carbon dioxide dissolve in water, and most fishes exchange dissolved oxygen and carbon dioxide in water by means of the gills. The gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by the gill arches and filled with blood vessels, which give gills a bright red colour. Water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place. The gills are protected by a gill cover in teleosts and many other fishes but by flaps of skin in sharks, rays, and some of the older fossil fish groups. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.

 

Most modern fishes have a hydrostatic (ballast) organ, called the swim bladder, that lies in the body cavity just below the kidney and above the stomach and intestine. It originated as a diverticulum of the digestive canal. In advanced teleosts, especially the acanthopterygians, the bladder has lost its connection with the digestive tract, a condition called physoclistic. The connection has been retained (physostomous) by many relatively primitive teleosts. In several unrelated lines of fishes, the bladder has become specialized as a lung or, at least, as a highly vascularized accessory breathing organ. Some fishes with such accessory organs are obligate air breathers and will drown if denied access to the surface, even in well-oxygenated water. Fishes with a hydrostatic form of swim bladder can control their depth by regulating the amount of gas in the bladder. The gas, mostly oxygen, is secreted into the bladder by special glands, rendering the fish more buoyant; the gas is absorbed into the bloodstream by another special organ, reducing the overall buoyancy and allowing the fish to sink. Some deep-sea fishes may have oils, rather than gas, in the bladder. Other deep-sea and some bottom-living forms have much-reduced swim bladders or have lost the organ entirely.

 

The swim bladder of fishes follows the same developmental pattern as the lungs of land vertebrates. There is no doubt that the two structures have the same historical origin in primitive fishes. More or less intermediate forms still survive among the more primitive types of fishes, such as the lungfishes Lepidosiren and Protopterus.

 

The circulatory, or blood vascular, system consists of the heart, the arteries, the capillaries, and the veins. It is in the capillaries that the interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle the blood goes to a bulbous structure at the base of a ventral aorta just below the gills. The blood passes to the afferent (receiving) arteries of the gill arches and then to the gill capillaries. There waste gases are given off to the environment, and oxygen is absorbed. The oxygenated blood enters efferent (exuant) arteries of the gill arches and then flows into the dorsal aorta. From there blood is distributed to the tissues and organs of the body. One-way valves prevent backflow. The circulation of fishes thus differs from that of the reptiles, birds, and mammals in that oxygenated blood is not returned to the heart prior to distribution to the other parts of the body.

 

The primary excretory organ in fishes, as in other vertebrates, is the kidney. In fishes some excretion also takes place in the digestive tract, skin, and especially the gills (where ammonia is given off). Compared with land vertebrates, fishes have a special problem in maintaining their internal environment at a constant concentration of water and dissolved substances, such as salts. Proper balance of the internal environment (homeostasis) of a fish is in a great part maintained by the excretory system, especially the kidney.

 

The kidney, gills, and skin play an important role in maintaining a fish’s internal environment and checking the effects of osmosis. Marine fishes live in an environment in which the water around them has a greater concentration of salts than they can have inside their body and still maintain life. Freshwater fishes, on the other hand, live in water with a much lower concentration of salts than they require inside their bodies. Osmosis tends to promote the loss of water from the body of a marine fish and absorption of water by that of a freshwater fish. Mucus in the skin tends to slow the process but is not a sufficient barrier to prevent the movement of fluids through the permeable skin. When solutions on two sides of a permeable membrane have different concentrations of dissolved substances, water will pass through the membrane into the more concentrated solution, while the dissolved chemicals move into the area of lower concentration (diffusion).

 

The kidney of freshwater fishes is often larger in relation to body weight than that of marine fishes. In both groups the kidney excretes wastes from the body, but the kidney of freshwater fishes also excretes large amounts of water, counteracting the water absorbed through the skin. Freshwater fishes tend to lose salt to the environment and must replace it. They get some salt from their food, but the gills and skin inside the mouth actively absorb salt from water passed through the mouth. This absorption is performed by special cells capable of moving salts against the diffusion gradient. Freshwater fishes drink very little water and take in little water with their food.

 

Marine fishes must conserve water, and therefore their kidneys excrete little water. To maintain their water balance, marine fishes drink large quantities of seawater, retaining most of the water and excreting the salt. Most nitrogenous waste in marine fishes appears to be secreted by the gills as ammonia. Marine fishes can excrete salt by clusters of special cells (chloride cells) in the gills.

 

There are several teleosts—for example, the salmon—that travel between fresh water and seawater and must adjust to the reversal of osmotic gradients. They adjust their physiological processes by spending time (often surprisingly little time) in the intermediate brackish environment.

 

Marine hagfishes, sharks, and rays have osmotic concentrations in their blood about equal to that of seawater and so do not have to drink water nor perform much physiological work to maintain their osmotic balance. In sharks and rays the osmotic concentration is kept high by retention of urea in the blood. Freshwater sharks have a lowered concentration of urea in the blood.

 

Endocrine glands secrete their products into the bloodstream and body tissues and, along with the central nervous system, control and regulate many kinds of body functions. Cyclostomes have a well-developed endocrine system, and presumably it was well developed in the early Agnatha, ancestral to modern fishes. Although the endocrine system in fishes is similar to that of higher vertebrates, there are numerous differences in detail. The pituitary, the thyroid, the suprarenals, the adrenals, the pancreatic islets, the sex glands (ovaries and testes), the inner wall of the intestine, and the bodies of the ultimobranchial gland make up the endocrine system in fishes. There are some others whose function is not well understood. These organs regulate sexual activity and reproduction, growth, osmotic pressure, general metabolic activities such as the storage of fat and the utilization of foodstuffs, blood pressure, and certain aspects of skin colour. Many of these activities are also controlled in part by the central nervous system, which works with the endocrine system in maintaining the life of a fish. Some parts of the endocrine system are developmentally, and undoubtedly evolutionarily, derived from the nervous system.

 

As in all vertebrates, the nervous system of fishes is the primary mechanism coordinating body activities, as well as integrating these activities in the appropriate manner with stimuli from the environment. The central nervous system, consisting of the brain and spinal cord, is the primary integrating mechanism. The peripheral nervous system, consisting of nerves that connect the brain and spinal cord to various body organs, carries sensory information from special receptor organs such as the eyes, internal ears, nares (sense of smell), taste glands, and others to the integrating centres of the brain and spinal cord. The peripheral nervous system also carries information via different nerve cells from the integrating centres of the brain and spinal cord. This coded information is carried to the various organs and body systems, such as the skeletal muscular system, for appropriate action in response to the original external or internal stimulus. Another branch of the nervous system, the autonomic nervous system, helps to coordinate the activities of many glands and organs and is itself closely connected to the integrating centres of the brain.

 

The brain of the fish is divided into several anatomical and functional parts, all closely interconnected but each serving as the primary centre of integrating particular kinds of responses and activities. Several of these centres or parts are primarily associated with one type of sensory perception, such as sight, hearing, or smell (olfaction).

 

The sense of smell is important in almost all fishes. Certain eels with tiny eyes depend mostly on smell for location of food. The olfactory, or nasal, organ of fishes is located on the dorsal surface of the snout. The lining of the nasal organ has special sensory cells that perceive chemicals dissolved in the water, such as substances from food material, and send sensory information to the brain by way of the first cranial nerve. Odour also serves as an alarm system. Many fishes, especially various species of freshwater minnows, react with alarm to a chemical released from the skin of an injured member of their own species.

 

Many fishes have a well-developed sense of taste, and tiny pitlike taste buds or organs are located not only within their mouth cavities but also over their heads and parts of their body. Catfishes, which often have poor vision, have barbels (“whiskers”) that serve as supplementary taste organs, those around the mouth being actively used to search out food on the bottom. Some species of naturally blind cave fishes are especially well supplied with taste buds, which often cover most of their body surface.

 

Sight is extremely important in most fishes. The eye of a fish is basically like that of all other vertebrates, but the eyes of fishes are extremely varied in structure and adaptation. In general, fishes living in dark and dim water habitats have large eyes, unless they have specialized in some compensatory way so that another sense (such as smell) is dominant, in which case the eyes will often be reduced. Fishes living in brightly lighted shallow waters often will have relatively small but efficient eyes. Cyclostomes have somewhat less elaborate eyes than other fishes, with skin stretched over the eyeball perhaps making their vision somewhat less effective. Most fishes have a spherical lens and accommodate their vision to far or near subjects by moving the lens within the eyeball. A few sharks accommodate by changing the shape of the lens, as in land vertebrates. Those fishes that are heavily dependent upon the eyes have especially strong muscles for accommodation. Most fishes see well, despite the restrictions imposed by frequent turbidity of the water and by light refraction.

 

Fossil evidence suggests that colour vision evolved in fishes more than 300 million years ago, but not all living fishes have retained this ability. Experimental evidence indicates that many shallow-water fishes, if not all, have colour vision and see some colours especially well, but some bottom-dwelling shore fishes live in areas where the water is sufficiently deep to filter out most if not all colours, and these fishes apparently never see colours. When tested in shallow water, they apparently are unable to respond to colour differences.

 

Sound perception and balance are intimately associated senses in a fish. The organs of hearing are entirely internal, located within the skull, on each side of the brain and somewhat behind the eyes. Sound waves, especially those of low frequencies, travel readily through water and impinge directly upon the bones and fluids of the head and body, to be transmitted to the hearing organs. Fishes readily respond to sound; for example, a trout conditioned to escape by the approach of fishermen will take flight upon perceiving footsteps on a stream bank even if it cannot see a fisherman. Compared with humans, however, the range of sound frequencies heard by fishes is greatly restricted. Many fishes communicate with each other by producing sounds in their swim bladders, in their throats by rasping their teeth, and in other ways.

 

A fish or other vertebrate seldom has to rely on a single type of sensory information to determine the nature of the environment around it. A catfish uses taste and touch when examining a food object with its oral barbels. Like most other animals, fishes have many touch receptors over their body surface. Pain and temperature receptors also are present in fishes and presumably produce the same kind of information to a fish as to humans. Fishes react in a negative fashion to stimuli that would be painful to human beings, suggesting that they feel a sensation of pain.

 

An important sensory system in fishes that is absent in other vertebrates (except some amphibians) is the lateral line system. This consists of a series of heavily innervated small canals located in the skin and bone around the eyes, along the lower jaw, over the head, and down the mid-side of the body, where it is associated with the scales. Intermittently along these canals are located tiny sensory organs (pit organs) that apparently detect changes in pressure. The system allows a fish to sense changes in water currents and pressure, thereby helping the fish to orient itself to the various changes that occur in the physical environment.

  

You can guess what we'll be singing this Christmas no doubt...

While women take care of the food, kids, house and water, men talk about the strategic decisions!

The Pokhot live in the Baringo and Western Pokot districts of Kenya and in Uganda.

There are two main sub-groups depending of their location and way of life. The first group consist of the Hill Pokot who live in the rainy highlands in the west and in the central south, and are mainly farmers and pastoralists. The second group is made up of the Plains Pokot who live in dry and infertile plains, with their cattles. A homestead is composed of one or more buildings for a man, his wife and children; eventual co-wives live in separate houses. The role of the community in teaching children ethical rules. Most of the Pokot are nomadic and thus have interacted with different peoples, incorporating their social customs.The Pokot are very proud of their culture. The Songs, storytelling, and decorative arts, especially bodily decoration, are very appreciated. They adorn the body with beads, hairstyling, scarification, and the removal of the lower central incisors. Pokot girls wear a beaded necklace made of the stems of an asparagus tree. Most Pokot have some knowledge of herbal medicine, so they often use these treatments along with those of the hospitals. They belong to the Kenya's Nilotic-speaking peoples. .

For the Pokot, the universe has two realms: the above is the realm of the most powerful deities—Tororot, Asis (sun), and llat (rain); and the below is the one where live humans, animals, and plants. Humans are responsible for the realm that they inhabit, but they rely upon divinities to achieve and maintain peace and prosperity. They worship many deities like the sun, moon and believe in the spirit of death.The Pokot communicate with their deities through prayer and sacrifice. They perform it during ethnic festivals and dances. Oracles are responsible for maintaining the spiritual balance within the community. They are superstitious and believe in sorcery, so sometimes they call on shielding lucky sorcery. They have prophets, either male or female, who foresee advise, usually by the means of animal sacrifices. His or her ability is considered as a divine gift. Clan histories recount the changes of location, through poetry and song, emphasizing the vulnerability of humans and the importance of supernatural powers to help them overcome hunger, thirst, and even death. Ceremonies mark the transitions in the people's social lives. Among these are: the cleansing of a couple expecting their first child; the cleansing of newborn infants and their mothers; the cleansing of twins and other children who are born under unusual circumstances; male and female initiation; marriage; sapana, a coming-of-age ceremony for men; and summer-solstice, harvest, and healing ceremonies. The most important rite of passage for most Pokot is circumcision for boys and clitoridectomy for girls. These rites consist of a series of neighborhood-based ceremonies, emphasizing the importance of having a good behavior. When boys are circumcised, they acquire membership in one of eight age sets. Women do not have age-sets. After excisions, for several months, girls have a white painting on their face and wear a hood made of blackened leather with charcoal and oil. This means they are untouchable until the lepan ceremony, that marks the passage to womanhood. Unlike other tribes, the Pokot keep the affiliation to their clan throughout their lives, there is no disruption with marriage. Surprisingly, the agreement before marriage is made by gift giving, from the groom and his family to the bride and her family, often over a period of years (and not the contrary). It often implies the gift of a combination of livestock, goods, and cash to the bride's family, and the allotment of milk cows and rights to land to the bride. The bond between a husband and wife lasts for 3 generations, after what marriages can take place again between the two groups. Polygamy exists but is not prevalent among men before 40. The spirits of the elder anticipate reincarnation in their living descendants: when a child is said to resemble the elder, the same name is given. Disputes are resolved in neighborhood councils and in government courts. Some of the sanctions include shaming, cursing, and bewitching.

 

© Eric Lafforgue

www.ericlafforgue.com