Jellyfish 2

 word jellyfish (which has been in common usage for more than a century)[6] is used to denote several different kinds of cnidarians, all of which have a basic body structure that resembles an umbrella, including scyphozoans, staurozoans (stalked jellyfish), hydrozoans, and cubozoans (box jellyfish). Some textbooks and websites refer to scyphozoans as "true jellyfish".[7][8]As jellyfish are not even vertebrates, let alone true fish, the usual word jellyfish is considered by some to be a misnomer, and American public aquariums have popularized use of the terms jellies or sea jellies instead.[9]In its broadest usage, some scientists occasionally include members of the phylum Ctenophora (comb jellies) when they are referring to jellyfish.[10] Other scientists prefer to use the more all-encompassing term "gelatinous zooplankton", when referring to these, together with other soft-bodied animals in the water column.[11]A group of jellyfish is sometimes called a bloom or a swarm.[12] "Bloom" is usually used for a large group of jellyfish that gather in a small area, but may also have a time component, referring to seasonal increases, or numbers beyond what was expected.[13] Another collective name for a group of jellyfish is a smack.[14]Jellyfish are "bloomy" by nature of their life cycles, being produced by their benthic polyps usually in the spring when sunshine and plankton increase, so they appear rather suddenly and often in large numbers, even when an ecosystem is in balance.[15] Using "swarm" usually implies some kind of active ability to stay together, which a few species such as Aurelia, the moon jelly, demonstrate.[16]Most jellyfish have a second stage to their life cycle, the planula larvae phase, following the initial egg and sperm phase. Although this is a short lived stage for jellyfish, it is an important phase when the fertilized eggs that had previously undergone embryonic development, hatch, and planulae emerge from the females mouth or brood pouch and are off on their own.[17][edit] Anatomy The major surfaces and axes of a jellyfish Most jellyfish do not have specialized digestive, osmoregulatory, central nervous, respiratory, or circulatory systems. The manubrium is a stalk-like structure hanging down from the centre of the underside, with the mouth at its tip. This opens into the gastrovascular cavity, where digestion takes place and nutrients are absorbed. It is joined to the radial canals which extend to the margin of the bell.[18] Jellyfish do not need a respiratory system since their skin is thin enough that the body is oxygenated by diffusion. They have limited control over movement, but can use their hydrostatic skeleton to navigate through contraction-pulsations of the bell-like body; some species actively swim most of the time, while others are mostly passive.[citation needed] The Jellyfish body consist of over 95% water; most of their umbrella mass is a gelatinous material—the jelly—called mesoglea which is surrounded by two layers of protective skin. The top layer is called the epidermis, and the inner layer is referred to as gastrodermis, which lines the gut.[edit] Nervous systemMost jellyfish do not have a brain or central nervous system, but employ a loose network of nerves, located in the epidermis, which is called a "nerve net". A jellyfish detects various stimuli including the touch of other animals via this nerve net, which then transmits impulses both throughout the nerve net and around a circular nerve ring, through the rhopalial lappet, located at the rim of the jellyfish body, to other nerve cells.Another counter to the "brainless jellyfish" hypothesis is that some species explicitly adapt to tidal flux to control their location. In Roscoe Bay, jellyfish ride the current at ebb tide until they hit a gravel bar, and then descend below the current. They remain in still waters waiting for the tide to rise, ascending and allowing it to sweep them back into the bay. They monitor salinity to avoid fresh water from mountain snowmelt, again by diving until they find enough salt.[4][edit] VisionSome jellyfish also have ocelli: light-sensitive organs that do not form images but which can detect light, and are used to determine up from down, responding to sunlight shining on the water's surface. These are generally pigment spot ocelli, which have some cells (not all) pigmented.Certain species of jellyfish, such as the Box jellyfish, have been revealed to be more advanced than their counterparts. The Box jellyfish has 24 eyes, two of which are capable of seeing color, and four parallel brains that act in competition, supposedly making it one of the only creatures to have a 360 degree view of its environment.[19] It is suggested that the two eyes that contain cornea and retina are attached to a central nervous system which enables the four brains to process images. It is unknown how this works, as the creature has a unique central nervous system.[4][20]The eyes are suspended on stalks with heavy crystals on one end, acting like a gyroscope to orient the eyes skyward. They look upward to navigate from roots in mangrove swamps to the open lagoon and back, watching for the mangrove canopy, where they feed.[4][edit] Size The lion's mane jellyfish is arguably the longest animal in the world One of the smallest adult jellies is the Australian Irukandji, which is about the size of a fingernail.[4]The lion's mane jellyfish, Cyanea capillata, were long-cited as the largest jellyfish, and arguably the longest animal in the world, with fine, thread-like tentacles up to 36.5 metres (120 ft) long (though most are nowhere near that large).[21][22][23] They have a painful, but rarely fatal, sting.The increasingly common giant Nomura's jellyfish, Nemopilema nomurai, found in some, but not all years in the waters of Japan, Korea and China in summer and autumn is probably a much better candidate for "largest jellyfish", since the largest Nomura's jellyfish in late autumn can reach 200 centimetres (79 in) in bell (body) diameter and about 200 kilograms (440 lb) in weight, with average specimens frequently reaching 90 centimetres (35 in) in bell diameter and about 150 kilograms (330 lb) in weight.[24][25] The large bell mass of the giant Nomura's jellyfish[26] can dwarf a diver and is nearly always much greater than the up-to-100 centimetres (39 in) bell diameter Lion's Mane.[27]The rarely-encountered deep-sea jellyfish Stygiomedusa gigantea is another solid candidate for "largest jellyfish", with its 100 centimetres (39 in) wide, and thick, massive bell and four thick, "paddle-like" oral arms extending up to 6 metres (20 ft) in length,[28] very different than the typical fine, threadlike tentacles that rim the umbrella of more-typical-looking jellyfish, including the Lion's Mane.[edit] Life history See also: Biological life cycle and Developmental biology The developmental stages of scyphozoan jellyfish's life cycle. Most jellyfish undergo two distinct life history stages (body forms) during their life cycle. The first is the polypoid stage. After fertilization and initial growth, a larval form, called the planula, develops. The planula is a small larva covered with cilia. It settles onto a firm surface and develops into a polyp. The polyp is generally a small stalk with a mouth surrounded by upward-facing tentacles like miniatures of the closely related anthozoan polyps (sea anemones and corals), also of the phylum Cnidaria. This polyp may be sessile, living on the bottom or on similar substrata such as floats or boat-bottoms, or it may be free-floating or attached to tiny bits of free-living plankton[29] or rarely, fish[30] or other invertebrates. Polyps may be solitary or colonial. Most are very small, measured in millimeters. They feed continuously.Certain Box Jellyfish species sleep on the sea bed in shallow water.[31][edit] ReproductionJellyfish reproduce both sexually and asexually. Upon reaching adult size, jellyfish spawn daily if there is enough food. In most species, spawning is controlled by light, so the entire population spawns at about the same time of day, often at either dusk or dawn.[32] Jellyfish are usually either male or female (hermaphroditic specimens are occasionally found).In most cases, adults release sperm and eggs into the surrounding water, where the (unprotected) eggs are fertilized and mature into new organisms. In a few species, the sperm swim into the female's mouth fertilizing the eggs within the female's body where they remain during early development stages. In moon jellies, the eggs lodge in pits on the oral arms, which form a temporary brood chamber for the developing planula larvae.After a growth interval, the polyp begins reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. New scyphistomae may be produced by budding or form new, immature jellies called ephyrae. A few jellyfish species can produce new medusae by budding directly from the medusan stage. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. A few species of hydromedusae reproduce by fission (splitting in half).[29]In the second stage, the tiny polyps asexually produce jellyfish, each of which is also known as a medusa. Tiny jellyfish (usually only a millimeter or two across) swim away from the polyp and then grow and feed in the plankton.[citation needed] Medusae have a radially symmetric, umbrella-shaped body called a bell, which is usually supplied with marginal tentacles - fringe-like protrusions from the bell's border that capture prey. A few species of jellyfish do not have the polyp portion of the life cycle, but go from jellyfish to the next generation of jellyfish through direct development of fertilized eggs.[edit] LifespanJellyfish lifespans typically range from a few hours (in the case of some very small hydromedusae) to several months. Life span and maximum size varies by species. One unusual species is reported to live as long as 30 years. Another species, Turritopsis dohrnii as T. nutricula, may be effectively immortal because of its ability to transform between medusa and polyp, thereby escaping death.[33] Most large coastal jellyfish live 2 to 6 months, during which they grow from a millimeter or two to many centimeters in diameter.[edit] Ecology [edit] FeedingJellies are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish, ingesting and voiding through the same convenient hole in the middle of the bell. Jellies hunt passively using their tentacles as drift nets. Should a fish brush against the often invisible extensions, the pressure activates the nematocysts, protecting the predator’s delicate tissue. Prey must be dead before ingestion, or it could puncture the jellyfish' stomach.[4][edit] PredationOther species of jellyfish are among the most common and important jellyfish predators, some of which specialize in jellies. Other predators include tuna, shark, swordfish, sea turtles and at least one species of Pacific salmon. Sea birds sometimes pick symbiotic crustaceans from the jellyfish bells near the sea's surface, inevitably feeding also on the jellyfish hosts of these amphipods or young crabs and shrimp.[edit] Blooms Aurelia sp., occurs in large quantities in most of the world's coastal waters. Members of this genus are nearly identical to each other. Jellyfish blooms can grow to 100,000 individuals. Bloom formation is a complex process that depends on ocean currents, nutrients, sunshine, temperature, season, prey availability, reduced predation and oxygen concentrations. Ocean currents tend to congregate jellyfish into large swarms or "blooms", consisting of hundreds or thousands of individuals. Blooms can also result from unusually high populations in some years. Jellyfish are better able to survive in nutrient-rich, oxygen-poor water than competitors, and thus can feast on plankton without competition. Jellyfish may also benefit from saltier waters, as saltier waters contain more iodine, which is necessary for polyps to turn into jellyfish. Rising sea temperatures caused by climate change may also contribute to jellyfish blooms, because many species of jellyfish are relatively better able to survive in warmer waters.[34]Scientists have little quantitative data of historic or current jellyfish populations beyond "impressions" in the public memory.[15] Recent speculation about increases in jellyfish populations are based on no "before" data.One hypothesis is that the global increase in jellyfish bloom frequency may stem from human impact. In some locations jellyfish may be filling ecological niches formerly occupied by now overfished creatures, but this hypothesis lacks supporting data.[15] Youngbluth states that "jellyfish feed on the same kinds of prey as adult and young fish, so if fish are removed from the equation, jellyfish are likely to move in."[35]Some jellyfish populations that have shown clear increases in the past few decades are "invasive" species, newly arrived from other habitats: examples include the Black Sea and the Caspian Sea, the Baltic Sea, the eastern Mediterranean coasts of Egypt and Israel,[36] and the American Gulf coast.[citation needed] Invasive populations can expand rapidly because they often face no predators in the new habitat.Increased nutrients, ascribed to agricultural runoff, have been cited as to jellyfish proliferation. Graham states, "ecosystems in which there are high levels of nutrients ... provide nourishment for the small organisms on which jellyfish feed. In waters where there is eutrophication, low oxygen levels often result, favoring jellyfish as they thrive in less oxygen-rich water than fish can tolerate. The fact that jellyfish are increasing is a symptom of something happening in the ecosystem."[35][edit] Population Posted from

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