03 Patella pellucida. Growth Stage 3 (GS3). 2009. Sublittoral, Farne Islands, Northumberland, England. © P.Lightfoot.

Streamlined juveniles on Laminaria hyperborea frond, orientated with prow-like anteriors probably facing from whence wave or current impact comes. The width of the blue lines remains fairly constant at 0.1mm to 0.2mm at all growth stages. These juveniles also have a variable number of red-brown rays interspersed between the blue ones, but this is a more frequent feature on adults in holdfasts; Graham & Fretter (1947) only found red-brown rays on a juvenile once in a study of 684 shells. Radula marks on the frond suggest that the limpets swing their heads in an arc as they feed.

A dark chestnut band on the distal face of the horseshoe-shaped pedal-retractor muscle can be seen through the translucent shells of four of these juveniles.

 

Concise Key id. features: 1Pp flic.kr/p/Z4FxDe

Part 1, SHELL FORMS: 2Pp flic.kr/p/Z4FxAi

Part 2, BODY & ANATOMY, BELOW.

Part, 3 HABITS & ECOLOGY: 4Pp flic.kr/p/Z4Fx9M

Sets of OTHER SPECIES: www.flickr.com/photos/56388191@N08/collections/

PDF available at www.researchgate.net/publication/346673416_Patella_pelluc...

 

GLOSSARY below.

Body description (Features visible on live animal.)

The main flesh colour is white or yellowish white. A horseshoe-shaped series of strong muscle bundles make up the pedal-retractor muscle 53Pp flic.kr/p/Z6REoG & 54Pp flic.kr/p/C3widA . Juveniles have a distinct, longitudinal, dark chestnut band on the distal face of the pedal-retractor muscle that can often be seen through the translucent juvenile shell 3Pp flic.kr/p/Z4FxtK , 4Pp flic.kr/p/Z4Fx9M & 5Pp flic.kr/p/YNrZYJ or by dissection 55Pp flic.kr/p/Z6RDUW . The chestnut band fades on adults inside holdfasts 56Pp flic.kr/p/Y8tL5H . Some of the adults on fronds retain a strongly pigmented band 57Pp flic.kr/p/Z6RDtf & 22Pp flic.kr/p/Z51FP4 , but others lose it 58Pp flic.kr/p/C3wgX9 . Although the band fades on holdfast dwellers, the anterior dorsal surface of the foot, below the head, usually becomes yellowish with brown creases. This colouration may extend faintly over more of the foot dorsally 53Pp flic.kr/p/Z6REoG . The head has short stout snout occupied by a large mouth surrounded by thick, white, wrinkled outer lips 59Pp flic.kr/p/Z6RCRd which extend the snout when protruded 60Pp flic.kr/p/Y8tJDM . A pink, internal odontophore and buccal mass can be seen through the translucent, white head 61Pp flic.kr/p/Z6RCAJ . The odontophore is separated from the outer lips by dull orange inner lips that open laterally and close to a vertical line 62Pp flic.kr/p/Z6RCAJ . When they open, the radula with rust-coloured, iron-rich teeth is protruded through the arch of the opaque white jaw 60Pp flic.kr/p/Y8tJDM . The tapered, translucent white cephalic tentacles each have a black eye with a tiny opening on the dorsum of the slightly swollen tentacle-base 61Pp flic.kr/p/Z6RCAJ . The opening is at the apex of an internal cone of black pigment that, because visible through the translucent flesh, gives the false impression of a larger opening. Some or all of the black may be pigmented retinal cells. The eye can probably differentiate light from shade, and detect the direction of the light source, but cannot discern shapes. The mantle-skirt is translucent whitish, often showing the colour of underlying shell when viewed ventrally 63Pp flic.kr/p/Y4Rjnb . The mantle cavity consists of a nuchal cavity over the head 64Pp flic.kr/p/C3wfCW and a groove, containing pallial gills, around the periphery of foot but, unlike other British Patellidae, not around the anterior of the head 65Pp flic.kr/p/Y8tH2Z . The gill-less anterior gap appears (small sample) to be proportionally of greatest extent on juveniles 66Pp flic.kr/p/C3wf4Q . A red (vestigial?) osphradium is located within the nuchal cavity next to each terminal bundle of the pedal-retractor muscle Pp53 flic.kr/p/Z6REoG . The efferent pallial vessel, less whitish than the rest of the mantle skirt, runs around the entire animal at the base of the pallial gills 65Pp flic.kr/p/Y8tH2Z and in front of the head to a point left of the head where its two ends unite in a trunk 64Pp flic.kr/p/C3wfCW that passes through the roof of the nuchal cavity to the heart. The mantle skirt is fringed with translucent, white pallial tentacles that can protrude beyond the rim of the shell when the mantle is fully extended 67Pp flic.kr/p/Y8tGwv . Along the whole length of each side of the dorsal surface of the foot there is a lateral glandular streak 53Pp flic.kr/p/Z6REoG with an overhanging upper lip. The lip has lobe-like retractile tentacles 68Pp flic.kr/p/Z6RA7A and is usually paler than the rest of the foot when the latter is tinted yellowish.

58Pp flic.kr/p/C3wgX9 . The sole is pure white on juveniles and early adults on fronds, often becoming yellowish, especially at the anterior, on frond dwelling, large adults 69Pp flic.kr/p/Y8tEYF . Adults in holdfasts have white soles even when the dorsal surface of the foot is yellowish with brown creases 65Pp flic.kr/p/Y8tH2Z

The intensity of colour on coloured soles is diminished when the sole is expanded laterally 70Pp flic.kr/p/Z9Bf6x . The anterior of the foot is bilaminate with the dorsal lamina extending beyond the sole 59Pp flic.kr/p/Z6RCRd . The anterior-pedal mucous gland is between the laminae. As with other patellids, there is no penis as fertilization is external.

 

Internal functional anatomy

When the body is removed from the shell the entire mantle is visible. The peripheral mantle-skirt is white translucent and may reveal the white foot below 71Pp flic.kr/p/Zjfznw . The rest of the mantle is semi-transparent with a fine spray of black pigment of varying intensity. Many features of the viscera can be discerned through it.

Blood circulation and respiration

Image 72Pp flic.kr/p/YhjbgA illustrates paragraph 1.

After it has oxygenated organs in the visceral mass, the colourless blood, depleted of oxygen, collects in visceral sinuses. It then passes in vessels through gaps in the pedal-retractor muscle into an afferent branchial vessel that runs through the mantle skirt around the edge of the distal face of the muscle . The afferent branchial vessel distributes blood to all the gill leaflets which re-oxygenate it as it passes through them. The oxygenated blood passes into a large efferent pallial vessel (a.k.a. efferent branchial vessel) and passes forwards on both right and left sides of the animal. The

flow of blood in the efferent pallial vessel (e.p.v.) on the right continues round the front of the head to join the left of the e.p.v. in a trunk that enters the left side of the nuchal cavity to connect with the heart in the pericardial cavity behind the left of the nuchal cavity 55Pp flic.kr/p/Z6RDUW & 73Pp flic.kr/p/Zjfz8y . The heart consists of three parallel chambers; the auricle, ventricle and bulbous aorta. Blood is pumped out via the bulbous aorta to oxygenate organs in the head and visceral hump before returning to the visceral sinuses to recommence the cycle.

The gills consist of a large number of leaflets, alternating large and small, hanging from the roof of the pallial groove 74Pp flic.kr/p/YhjaWs . The leaflets vary in shape with free ends that can be rounded, flat or pointed, but all narrow where they attach to the roof of the pallial groove and have a deep and densely ciliated groove around the rim 75Pp flic.kr/p/YhjaS9 . The respiratory inhalant water current is created by the motion of cilia on each leaflet of the gills drawing water in all around the shell, except at the gill-less anterior (X5000 scanning electron microscope images of cilia on a chiton at flic.kr/p/qQB5zj ). Sediment entering with the water is trapped by mucus from the lateral glandular streak 53Pp flic.kr/p/Z6REoG and carried forwards and out by the exhalant current along the pallial groove. The oxygen-bearing water is drawn across the surface of the leaflets in the opposite direction to the blood flow within, so creating a counter current-system (Fretter and Graham, 1994). The dense cilia in the rim-groove are reported (Fretter and Graham, 1994) to propel debris particles caught on the leaflet surface to the free end where they drop off, but this would seem unlikely for those in holdfasts feeding upside down into the core of the stipe. After passing the leaflets, the current, now exhalant, is propelled by cilia forwards along the pallial groove to exit the gill-less anterior on the right of the animal 76Pp flic.kr/p/ZjfytY & 77Pp flic.kr/p/YhjaDJ . It is unlikely that the red features referred to as the osphradia 53Pp flic.kr/p/Z6REoG have the usually attributed function of water-quality testing as there are no associated ctenidia to forwarn, and they are located in the path of the exhalant water after it has flowed over the pallial gills and shortly before it exits the anterior of the shell. There are anatomical signs that they may be vestiges of a lost double osphradium-ctenidium complex like that still found in Diodora graeca flic.kr/p/neENxW (Spengel, 1881, in Fretter & Graham, 1962, p314). Graham & Fretter (1947) had a 'suspicion' that the leaflets on frond-dwellers are smaller than those on holdfast-dwellers, and that this can be correlated with the greater exposure to water movement of frond dwellers. This seems to be supported by some images 78Pp flic.kr/p/Zjfyby that show large leaflets projecting from the shell of holdfast dwellers, but no leaflets projecting from the shell of frond dwellers. But, measurement of live, expanded, gill area is difficult, and individuals can extend or retract the mantle and gills.

 

Alimentary and excretory features .

The inner lips of the mouth, described above, open into the buccal cavity. The anterior wall of the cavity is reinforced by a pliable, white, chitinous, antero-dorsal plate, called the “jaw” though it is not articulated and does not bite 60Pp flic.kr/p/Y8tJDM . It serves as an attachment for several muscles and has two lateral wings that meet dorsally at an angle and form an anterior shield for the inner lips when they are open. Within the buccal cavity there is a pink odontophore 61Pp flic.kr/p/Z6RCAJ & 79Pp flic.kr/p/Zjfxp3 consisting of a right and left bolster which is covered in thick cuticle. Much of a bolster is made of strong cartilage-like material. The odontophore occupies a large amount of the body, 25% of its length. There is a black mark within its anterior half that is visible through the dorsal surfaces of it 80Pp flic.kr/p/ZjfxcQ , the epithelium of the head and the overlying mantle 81Pp flic.kr/p/ZjfwMb , and, on many translucent shells, it underlies and adds to the intensity of a patch of black pigment on the anterior of the shell's interior 82Pp flic.kr/p/ZjfwtL .

The radula is created in a semi-translucent sac 83Pp flic.kr/p/ZjfwgS . Its exposed anterior, widened into a hyaline shield 84Pp flic.kr/p/Zjfwaj , rests on the dorsum of the odontophore 85Pp flic.kr/p/Zjfw5u . The strong, iron-impregnated, unarticulated teeth are firmly fixed in a backwardly inclined position on the radula 86Pp flic.kr/p/ZjfvYC , but the anterior tip of the radula bends over the front of the odontophore so that the front rows of four teeth are inclined forwards like a chisel 85Pp flic.kr/p/Zjfw5u . To feed, the strong muscles of the large odontophore thrust it forwards against the front of the buccal cavity which, reinforced by the jaw, restrains the odontophore but allows the front teeth to project strongly from the narrow vertex of the gap between the wings of the jaw 60Pp flic.kr/p/Y8tJDM . When applied to Laminaria, the teeth effectively chisel into the tough frond or stipe, and the curve of the withdrawing teeth acts as a scoop to lift particles back to the oesophagus in the buccal cavity. The action is surrounded by the outer lips which prevent the escape of loosened food fragments 60Pp flic.kr/p/Y8tJDM . Like other limpets and some sea snails that graze rock surfaces, P. pellucida has a long, powerful radula, but it is shorter than most, being about 75% the length of the shell 37Pp flic.kr/p/XQh5nv , requiring only a single fold to fit it inside the body 87Pp flic.kr/p/ZjfvNs which is about the same length as it 80Pp flic.kr/p/ZjfxcQ . Other British patellids have radulae between about 100% and 250% of shell length (Fretter & Graham, 1962). The cause of the correlation between length and rock grazing is uncertain. It may be that a lengthy process is needed for the teeth to acquire the required hardening mineralization, so less length/time would be needed to form a radula for grazing alga that is tough, but not as hard as rock. Tooth creation starts at the slightly bifid, white, inner end of the radular sac 83Pp flic.kr/p/ZjfwgS with secretion of colourless transparent cuticular material by odontoblast cells. As each new tooth commences, the previous one is pushed forwards along the sac. Cells along the roof of the sac make incremental additions, including the hardening salts of iron and silicon, to the teeth as they travel along the sac, with a progressive change from colourless through darkening shades of yellow/orange visible through the translucent sac walls. Creation is complete by the time the tooth reaches the buccal cavity, where the radula emerges from the sac onto the odontophore 85Pp flic.kr/p/Zjfw5u .

The yellow digestive gland 55Pp flic.kr/p/Z6RDUW & 81Pp flic.kr/p/ZjfwMb composed of a mass of tubules is usually the most obvious organ on the surface of the visceral mass when the shell is removed. Digestive cells in the tubules ingest particulate food to digest it intracellularly (Fretter & Graham, 1994, p. 219). The tubules extend into the blood filling the haemocoel, and their very thin covering of connective tissue allows the passage of nutrients into the blood. Food material passes along the long coiled intestine which is usually white like the consumed white inner parts of Laminaria 81Pp flic.kr/p/ZjfwMb & 87Pp flic.kr/p/ZjfvNs . Faecal boluses pass through the rectum 81Pp flic.kr/p/ZjfwMb and are compressed and bound with mucus to emerge as faecal rods 88Pp flic.kr/p/ZjfvsN from the anus at the rear right of the nuchal cavity 87Pp flic.kr/p/ZjfvNs and are conveyed to, and expelled from, the right anterior of the shell by cilia and the flow of exhalant water 77Pp flic.kr/p/YhjaDJ . Compression of faeces assists removal without fouling the gills. Excreta from the nephridipores by the anus are expelled by the same route. Particulate matter removed by cilia on the gill leaflets, and sediment caught in mucus from the lateral glandular streak, are conveyed along the pallial groove by cilia and exhalant water, to join the anterior expulsion.

 

Reproductive organs

The gonads are situated between the viscera and foot. During the breeding season, they may spread over much of the viscera 87Pp flic.kr/p/ZjfvNs & 89Pp flic.kr/p/Zjfvq3 , but on juveniles , and on adults out of breeding condition, they are usually hidden or nearly so 55Pp flic.kr/p/Z6RDUW & 82Pp flic.kr/p/ZjfwtL , on an animal removed from the shell. Female ovaries are granular, and male testes have numerous interconnected tubules. Fertilization is external, so the male has no penis. Gametes leave both sexes through the right nephridium (kidney) via its nephridipore at the rear right of the nuchal cavity, and carried from the shell by cilia and the forward flowing exhalant current.

 

GLOSSARY

 

afferent (adj. of vessel) = carrying blood etc. towards an organ.

aperture = mouth of gastropod shell; outlet for head and foot.

apex (definition for this account) = position of the larval protoconch (see summit).

branchial (adj.) = of or relating to gills (branchiae).

cephalic = (adj.) of or on the head.

cnidocytes = explosive stinging cells of hydroids, jellyfish, sea anemones etc. en.wikipedia.org/wiki/Cnidocyte

ctenidium = comb-like molluscan gill; usually an axis with a row of filaments either side.

 

efferent (adj. of vessel) = carrying blood etc. away from an organ.

ELWS = extreme low water spring tide (usually near March and September equinoxes).

 

 

epizoic (of a plant or animal) = growing or living non-parasitically on the exterior of a living animal.

 

GS1, GS2 etc. = Growth Stage 1 (planktonic veliger larva), Growth Stage 2 (newly settled juvenile) etc.

 

GS5f = Growth Stage 5 (late adult) living on Laminaria frond.

GS5h = Growth Stage 5 (late adult) living in Laminaria holdfast.

GS5s= Growth Stage 5 (late adult) living on Laminaria stipe.

 

haemocoel = system of interconnected spaces (sinuses) containing blood within body of a mollusc.

 

holdfast = rootlike in appearance, but not in function, tendrils attaching seaweed to the substrate. (a.k.a. hapteron).

mantle = sheet of tissue that secretes the shell and forms a cavity for the gill in most marine molluscs.

 

MLWS = mean low water spring tide level (mean level reached by lowest low tides for a few days every fortnight; Laminaria or Coralline zone on rocky coasts).

 

osphradium (pl. osphradia) = organ for testing water quality (chemical and/or for particles) usually near approach of inhalant current to ctenidium or pallial gills. Structure varied; including comblike, papillate or ribbing.

 

pallial (adj.) = of, relating to, or produced by the mantle (pallium).

periostracum = thin horny layer of chitinous material often coating shells.

phylogenetic (of development) = of change due to genetic make up.

resorb = absorb again that which was previously produced.

resorption = the process of absorbing again that which was previously produced.

 

stipe = stem of some brown seaweeds that supports the fronds and may contain a core of cells that transports sugars and nutrients within the alga.

 

summit (definition for this account) = highest point of the shell above the substrate (see apex).

 

trochophore = spherical or pear-shaped larva that moves with aid of girdle of cilia that beat to cause rotation. Stage preceding veliger, passed within gastropod egg in most spp. but free in plankton for patellid limpets, most Trochidae and Tricolia pullus, and, with no veliger, chitons.

 

veliger = shelled larva of marine gastropod or bivalve mollusc which feeds and swims by beating cilia of a velum (small on P. pellucida).

 

Done