04 Patella pellucida. Length 4.6mm. Growth Stage 3 (GS3). September 2016. Yorkshire, England. © P.Lightfoot.
Juvenile (GS3) in pit eaten into Laminaria stipe. A dark chestnut band on the distal face of the horseshoe-shaped pedal-retractor muscle can be seen through the translucent shell.
Concise Key id. features: 1Pp flic.kr/p/Z4FxDe
Part 1, SHELL FORMS: 2Pp flic.kr/p/Z4FxAi
Part 2, BODY & ANATOMY: 3Pp flic.kr/p/Z4FxtK
Part, 3 HABITS & ECOLOGY, BELOW.
Sets of OTHER SPECIES: www.flickr.com/photos/56388191@N08/collections/
GLOSSARY below.
Habits and ecology
Occurs on rocky shores, usually where Laminaria is present. Some wave action or current is preferred; it is often scarce or absent on extremely sheltered shores. It may occur in holdfasts on exposed shores where few, if any, can be found on fronds in some months. A current of 1.3 m/s (2.9 km/h) was found to be optimal for those on Saccorhiza fronds at Loch Hine, Eire, with a marked decline in numbers at lower and higher speeds (Ebling, 1948, in Kain & Svendsen, 1969), but it is difficult to be precise as other factors such as depth and competition for space by encrusting epibiota on the fronds can affect numbers. Depth limits are LWST to, presumably, the depth where Laminaria growth ceases, but presence/frequency within that zone varies. In the Isle of Man, the percentage of occupied holdfasts of large L. hyperborea was about 50% at 1m below LWST [where wave action is strongest], less than 10% at 11m and 0% at 20m (Kain & Svendsen, 1969), but abundance is affected by the interplay of other factors, including whether the local population utilizes holdfasts or, as in Norway, not.
Occurs and feeds primarily on Laminaria spp. and Saccorhiza and, when young, on diatoms etc caught on the surface of Laminaria 3Pp flic.kr/p/Z4FxtK . Newly settled juveniles, less than 4mm long, feed on small red algae, such as Rhodymenia palmata, and on calcareous red algae encrusting rocks 2Pp flic.kr/p/Z4FxAi . Other plants sometimes eaten, mainly by juveniles, include Alaria, Himanthalia 90Pp flic.kr/p/Z3ib4q and Fucus serratus 91Pp flic.kr/p/ZmLpYy (Wigham & Graham, 2017). It is uncertain whether these foodplants are adequate for development as adults are only occasionally found on them. They may be those that have failed to reach Laminaria or have been swept off it by storms 95Pp flic.kr/p/CYbTbS . The hard iron-mineralized teeth of the radula 86Pp flic.kr/p/ZjfvYC , about 75% length of shell 80Pp flic.kr/p/ZjfxcQ , enables larger specimens to eat, and excavate caves in, the base of a Laminaria stipe 92Pp flic.kr/p/ZmLoEm .
Defence: When on Laminaria fronds or stipes, the translucent juvenile shells show the colour of the underlying alga and the dark chestnut band on the muscle. This, and the brown shell-colour of adults, is cryptic when on brown algae, except for the iridescent blue/green rays 3Pp flic.kr/p/Z4FxtK . The colour of the rays is created by the light interference of zig-zag lamellae of calcite selectively reflecting blue/green in water. Saturation of the blue/green is intensified, and contrast with the visible white body within the shell is increased, by black, non-crystalline, colloidal particles underlying parts of the rays (Li et al., 2015) 17Pp flic.kr/p/YNK8Wo & 19Pp flic.kr/p/Z4Y4Cv . If the shell is rotated, different lines appear and disappear as the light's angle of incidence on them changes (Li et al., 2015, “supplementary movie1” at www.nature.com/articles/ncomms7322#supplementary-information ). Flashing on and off of the rays as the Laminaria waves about and changes the angle of light incidence, may distract from the animal's outline, or it may camouflage by resembling neighbouring iridescent weeds such as Chondrus crispus 93Pp flic.kr/p/ZmLnQA and en.wikipedia.org/wiki/Chondrus_crispus#/media/File:Chondr... .
Other species, such as Lacuna vincta and Steromphala cineraria take advantage of pits made by P. pellucida to feed on the exposed, nutritious core of Laminaria stipes 25Pp flic.kr/p/YNN5QC .
Li et al. (2015) suggested that P. pellucida may have Batesian mimicry of nudibranchs with iridescence assumed to warn of repugnant secretions or stinging sequestered cnidocytes. But Batesian mimics are usually less numerous than their models as predators learn from the more numerous example and apply their learning to all. Iridescent nudibranchs are usually much less numerous than P. pellucida, so are unsuitable as models to mimic. Adults concealed in holdfasts have little need for camouflage or warning colouration, and are often coated with epizoic growths 92Pp flic.kr/p/ZmLoEm . The use of the rays for sexual attraction in a non-copulating species is unlikely, especially as the rays become much less obvious at maturity, and their primitive eyes, almost certainly, cannot distinguish the rays.
Breeds most months, with maximum in winter and spring; November to February in Plymouth. No copulation; female releases thousands of individual, planktonic ova with gelatinous coverings that swell on contact with water. Ova are fertilized by sperm shed into water by males. Eggs soon hatch into shell-less, rotating, planktonic trochophore larvae (stage passed within egg by most less “primitive” spp.)
Development: shell type numbers below refer to those in Part 1 - Shell Description.
After a few days, the trochophore larvae metamorphose into planktonic veligers with a weakly developed velum for swimming and food capture, and a slightly coiled shell (GS 1) 1Pp flic.kr/p/Z4FxDe .
After a short pelagic life, the veligers settle on the substrate and metamorphose into the juvenile limpet form (GS2) . Initially, they feed on small red weeds and can be found on rock surfaces 2Pp flic.kr/p/Z4FxAi . By the time they attain shell length 4mm the radula is strong enough to rasp Laminaria as well as scrape up diatoms and other microscopic organisms that have settled on the fronds 3Pp flic.kr/p/Z4FxtK . To reach the Laminaria, the animals secrete mucus to form a 'sail' many times the size of the shell. The sails bring the animals to near neutral buoyancy and catch any water movement so they readily lift off from the substrate. As 1m² substrate carrying a Laminaria bed has about 8 to 12m² of Laminaria surface (Vahl, 1983), there is a good chance that the sails will strike a Laminaria plant and stick to it, as the mucus is a strong instant underwater adhesive. (While manipulating a supine specimen in water for photography, the 0.5mm wide tip of my forceps struck the invisible mucus being exuded. When I lifted the forceps, the animal followed with a piece of adhering Laminaria four times its weight, and was detatched only with difficulty). Any that fail to contact Laminaria, or a sufficient substitute, will die; the scarcity of the species in very sheltered conditions may be because of the lack of water movement sufficient for lift off. The great majority of the survivors (juveniles GS3) take up residence on the frond where their position is maintained by negative geotaxy (Vahl,1983) and their orientation may be prow-like anterior towards on coming wave action or current 3Pp flic.kr/p/Z4FxtK , but the evidence for the cause of orientation is not clear as the movement of waves over waving fronds is difficult to ascertain. A few of the settling juveniles land on the stipe or get into a holdfast where they initially feed on a finger of the holdfast 6Pp flic.kr/p/Z4FqqB . All, regardless of where they live, have similar juvenile shells, except that those in holdfasts may start to develop a rougher surface 9Pp flic.kr/p/Yss7so . The extremely low profile of a smooth, juvenile shell on a frond or stipe, orientated with narrow anterior facing the current, has minimal drag 3Pp flic.kr/p/Z4FxtK , and the broad foot with strongly adhesive mucus has a very firm grip, so wave backwash up to 20m/s (c.70km/h) can be withstood at this stage (Denny, 1984, in Fretter & Graham, 1994). The low, streamlined shell profile is maintained by loss of shell material at the anterior as the posterior grows. The loss may be caused by physical wear, but as the substrate is plant surface it seems unlikely. Resorption locally by the anterior of the mantle is a possibility.
Sexual maturity is reached at shell lengths of about 5 mm and upwards (Graham & Fretter, 1947). A change from shell loss to growth at the anterior commences shortly after maturation, at shell length 7 or 8mm. By 10mm length there is a considerable band of new shell around the whole shell with encircling growth lines, parallel to the substrate, (early adult GS4) that have an angular unconformity where they meet the earlier tilted growth lines 15Pp flic.kr/p/YNKaz3. Blue lines are usually fewer, less distinct or absent on the new shell. The profile, viewed from the side, becomes a higher dome with a nick on the anterior profile where the convex early surface meets a nearly straight new surface 15Pp flic.kr/p/YNKaz3 . Drag in water currents becomes much greater than on the low profiles of GS3 shells, and most on the exposed frond are swept off before they reach 11mm length to die or, possibly, reach other Laminaria in deeper, calmer water with the aid of a mucus sail.
In very sheltered waters with small amounts of wave action, some on fronds survive to lengths of 14mm or 15mm and form impressive, smooth, high, conoid shells resembling a Phrygian cap (GS5) 20Pp flic.kr/p/Z4Y3uZ . But this form is relatively scarce, though easily seen when present, as the sheltered conditions required for its survival are not conducive to large populations of the species. On both sheltered shores and more exposed one, a few individuals find less turbulent conditions on the stipe 24Pp flic.kr/p/YsLJPW which has a much shorter arc of movement, diminishing to zero at its base, than the fronds that thrash back and forth in storms through a curve with about 2m diameter. The shell may resemble GS5, but if a wide pit is eaten into the stipe the newly deposited shell may flare out more widely so that the conoid has a lower profile with a break in slope at the junction of new and earlier shell 26Pp flic.kr/p/YMdFeA .
By far the largest number of those who survive into (GS5) are those who found their way into holdfasts at an early stage and have grown secure from wave impact, unless the plant is torn away. On parts of some shores, including exposed ones, about 50% of Laminaria holdfasts may be inhabited. Occupation is usually restricted to a single adult (GS4 or 5h) per holdfast . Occasionally, when several holdasts are intermingled, one pit make break through into another 94Pp flic.kr/p/ZmLn1u . For those on fronds, lifespan is about a year or less. Those in holdfasts may live for two or more years, if the plant survives. Occasional stranded specimens, perhaps from holdfasts cast up from deep calm water, have exceptionally large shells with growth rings suggesting an age of four years 25.1Pp flic.kr/p/YXMQGC .
The absence from Norway of P. pellucida living in holdfasts, and of the associated distorted shell form lacking blue rays on most of the adult growth, suggests a lack of genetic interchange between Britain and Norway. It appears that the short planktonic larval stage is insufficient to survive the time needed for it to drift from Britain to Norway.
Distribution and status
Iceland and Murman Coast (N. Russia) to Straits of Gibraltar. Not resident further into Baltic than Øresund, south-west Sweden or continental coast of North Sea apart from on occasional strandings of washed in Laminaria. GBIF map www.gbif.org/species/5728509
Frequent around Britain and Ireland on hard substrate where Laminaria is present. Both P. pellucida and Laminaria are absent or rare in Liverpool Bay and Flamborough Head to Kent apart from occasional individuals washed in from other areas. Often overlooked at times when only those hidden in holdfasts are present. U.K. distribution map NBN species.nbnatlas.org/species/NHMSYS0021056396
Acknowledgements
I (I.F.S) should like to thank Debbie Evans, Dr Marco Faasse, David Fenwick, Allan Rowat and Neil Ward for information and use of their images, and Dr Ivan Nekhaev and Dr Julia Sigwart for advice with the text and interpretation of the images. I am extremely grateful to my co-authors Erling Svensen and Paula Lightfoot for their invaluable contributions of images, information and constructive criticism. Any errors or omissions are my responsibility.
Links and references
Forbes, E. & Hanley S. 1849-53. A history of the British mollusca and their shells. vol. 2 (1849), van Voorst, London. (As Acmaea virginea; Free PDF at archive.org/stream/historyofbritish02forb#page/428/mode/2up Use slide at base of page to select pp.429-433.)
Fretter, V. and Graham, A. 1962. British prosobranch molluscs. Ray Society, London.
Graham, A. 1988. Molluscs: prosobranch and pyramidellid gastropods: keys and notes for the identification of the species. Brill & Backhuys, for Linn. Soc. Lond. & Estuarine and Brackish-water Sciences Assoc. Synopses of the British Fauna (New Series) no.2. Edition 2 (662pages). Leiden. (Edition 1 of series, 1971, 112 pages, is no substitute.)
Graham, A. & Fretter, V. 1947. The life history of Patina pellucida (L.) J. mar. biol. Ass. U.K. 26: 590 to 601.
plymsea.ac.uk/1257/1/The_life_history_of_Patina_pellucida...
Jeffreys, J.G. 1862-69. British conchology. vol. 3 (1865). London, van Voorst. (As Tectura virginea) Free PDF at archive.org/stream/britishconcholog03jeff#page/248/mode/2up . Use slide at base of page to select pp.248- 250.
Kain, J.M. & Svendsen, P.1969. A note on the behaviour of Patina pellucida in Britain and Norway. Sarsia, 38:1, 25-30. dx.doi.org/10.1080/00364827.1969.10411147
Lebour, M.V., 1937. The eggs and larvae of the British Prosobranchs with special reference to those living in the plankton. Journal of the Marine Biological Association of the United Kingdom, 22: 105 to166.
Li, L. et al. 2015. A highly conspicuous mineralized composite photonic architechture in the translucent shell of the blue-rayed limpet. Nat. commun. 6:6322 doi:10,1038/ncomms7322(2015). Free pdf (open access) at www.nature.com/articles/ncomms7322
and supplementary images and video at www.nature.com/articles/ncomms7322#supplementary-information
Wigham, G.D. & Graham, A. 2017. Marine gastropods 1: Patellogastropoda and Vetigastropoda. Synopses of the British Fauna (New Series) no.60. (172pages). Field Studies Council, Telford, England.
Yonge, C.M. and Thompson, T.E. 1976. Living marine molluscs. Collins, London.
Current taxonomy: World Register of Marine Species (WoRMS) www.marinespecies.org/aphia.php?p=taxdetails&id=147459
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).
04 Patella pellucida. Length 4.6mm. Growth Stage 3 (GS3). September 2016. Yorkshire, England. © P.Lightfoot.
Juvenile (GS3) in pit eaten into Laminaria stipe. A dark chestnut band on the distal face of the horseshoe-shaped pedal-retractor muscle can be seen through the translucent shell.
Concise Key id. features: 1Pp flic.kr/p/Z4FxDe
Part 1, SHELL FORMS: 2Pp flic.kr/p/Z4FxAi
Part 2, BODY & ANATOMY: 3Pp flic.kr/p/Z4FxtK
Part, 3 HABITS & ECOLOGY, BELOW.
Sets of OTHER SPECIES: www.flickr.com/photos/56388191@N08/collections/
GLOSSARY below.
Habits and ecology
Occurs on rocky shores, usually where Laminaria is present. Some wave action or current is preferred; it is often scarce or absent on extremely sheltered shores. It may occur in holdfasts on exposed shores where few, if any, can be found on fronds in some months. A current of 1.3 m/s (2.9 km/h) was found to be optimal for those on Saccorhiza fronds at Loch Hine, Eire, with a marked decline in numbers at lower and higher speeds (Ebling, 1948, in Kain & Svendsen, 1969), but it is difficult to be precise as other factors such as depth and competition for space by encrusting epibiota on the fronds can affect numbers. Depth limits are LWST to, presumably, the depth where Laminaria growth ceases, but presence/frequency within that zone varies. In the Isle of Man, the percentage of occupied holdfasts of large L. hyperborea was about 50% at 1m below LWST [where wave action is strongest], less than 10% at 11m and 0% at 20m (Kain & Svendsen, 1969), but abundance is affected by the interplay of other factors, including whether the local population utilizes holdfasts or, as in Norway, not.
Occurs and feeds primarily on Laminaria spp. and Saccorhiza and, when young, on diatoms etc caught on the surface of Laminaria 3Pp flic.kr/p/Z4FxtK . Newly settled juveniles, less than 4mm long, feed on small red algae, such as Rhodymenia palmata, and on calcareous red algae encrusting rocks 2Pp flic.kr/p/Z4FxAi . Other plants sometimes eaten, mainly by juveniles, include Alaria, Himanthalia 90Pp flic.kr/p/Z3ib4q and Fucus serratus 91Pp flic.kr/p/ZmLpYy (Wigham & Graham, 2017). It is uncertain whether these foodplants are adequate for development as adults are only occasionally found on them. They may be those that have failed to reach Laminaria or have been swept off it by storms 95Pp flic.kr/p/CYbTbS . The hard iron-mineralized teeth of the radula 86Pp flic.kr/p/ZjfvYC , about 75% length of shell 80Pp flic.kr/p/ZjfxcQ , enables larger specimens to eat, and excavate caves in, the base of a Laminaria stipe 92Pp flic.kr/p/ZmLoEm .
Defence: When on Laminaria fronds or stipes, the translucent juvenile shells show the colour of the underlying alga and the dark chestnut band on the muscle. This, and the brown shell-colour of adults, is cryptic when on brown algae, except for the iridescent blue/green rays 3Pp flic.kr/p/Z4FxtK . The colour of the rays is created by the light interference of zig-zag lamellae of calcite selectively reflecting blue/green in water. Saturation of the blue/green is intensified, and contrast with the visible white body within the shell is increased, by black, non-crystalline, colloidal particles underlying parts of the rays (Li et al., 2015) 17Pp flic.kr/p/YNK8Wo & 19Pp flic.kr/p/Z4Y4Cv . If the shell is rotated, different lines appear and disappear as the light's angle of incidence on them changes (Li et al., 2015, “supplementary movie1” at www.nature.com/articles/ncomms7322#supplementary-information ). Flashing on and off of the rays as the Laminaria waves about and changes the angle of light incidence, may distract from the animal's outline, or it may camouflage by resembling neighbouring iridescent weeds such as Chondrus crispus 93Pp flic.kr/p/ZmLnQA and en.wikipedia.org/wiki/Chondrus_crispus#/media/File:Chondr... .
Other species, such as Lacuna vincta and Steromphala cineraria take advantage of pits made by P. pellucida to feed on the exposed, nutritious core of Laminaria stipes 25Pp flic.kr/p/YNN5QC .
Li et al. (2015) suggested that P. pellucida may have Batesian mimicry of nudibranchs with iridescence assumed to warn of repugnant secretions or stinging sequestered cnidocytes. But Batesian mimics are usually less numerous than their models as predators learn from the more numerous example and apply their learning to all. Iridescent nudibranchs are usually much less numerous than P. pellucida, so are unsuitable as models to mimic. Adults concealed in holdfasts have little need for camouflage or warning colouration, and are often coated with epizoic growths 92Pp flic.kr/p/ZmLoEm . The use of the rays for sexual attraction in a non-copulating species is unlikely, especially as the rays become much less obvious at maturity, and their primitive eyes, almost certainly, cannot distinguish the rays.
Breeds most months, with maximum in winter and spring; November to February in Plymouth. No copulation; female releases thousands of individual, planktonic ova with gelatinous coverings that swell on contact with water. Ova are fertilized by sperm shed into water by males. Eggs soon hatch into shell-less, rotating, planktonic trochophore larvae (stage passed within egg by most less “primitive” spp.)
Development: shell type numbers below refer to those in Part 1 - Shell Description.
After a few days, the trochophore larvae metamorphose into planktonic veligers with a weakly developed velum for swimming and food capture, and a slightly coiled shell (GS 1) 1Pp flic.kr/p/Z4FxDe .
After a short pelagic life, the veligers settle on the substrate and metamorphose into the juvenile limpet form (GS2) . Initially, they feed on small red weeds and can be found on rock surfaces 2Pp flic.kr/p/Z4FxAi . By the time they attain shell length 4mm the radula is strong enough to rasp Laminaria as well as scrape up diatoms and other microscopic organisms that have settled on the fronds 3Pp flic.kr/p/Z4FxtK . To reach the Laminaria, the animals secrete mucus to form a 'sail' many times the size of the shell. The sails bring the animals to near neutral buoyancy and catch any water movement so they readily lift off from the substrate. As 1m² substrate carrying a Laminaria bed has about 8 to 12m² of Laminaria surface (Vahl, 1983), there is a good chance that the sails will strike a Laminaria plant and stick to it, as the mucus is a strong instant underwater adhesive. (While manipulating a supine specimen in water for photography, the 0.5mm wide tip of my forceps struck the invisible mucus being exuded. When I lifted the forceps, the animal followed with a piece of adhering Laminaria four times its weight, and was detatched only with difficulty). Any that fail to contact Laminaria, or a sufficient substitute, will die; the scarcity of the species in very sheltered conditions may be because of the lack of water movement sufficient for lift off. The great majority of the survivors (juveniles GS3) take up residence on the frond where their position is maintained by negative geotaxy (Vahl,1983) and their orientation may be prow-like anterior towards on coming wave action or current 3Pp flic.kr/p/Z4FxtK , but the evidence for the cause of orientation is not clear as the movement of waves over waving fronds is difficult to ascertain. A few of the settling juveniles land on the stipe or get into a holdfast where they initially feed on a finger of the holdfast 6Pp flic.kr/p/Z4FqqB . All, regardless of where they live, have similar juvenile shells, except that those in holdfasts may start to develop a rougher surface 9Pp flic.kr/p/Yss7so . The extremely low profile of a smooth, juvenile shell on a frond or stipe, orientated with narrow anterior facing the current, has minimal drag 3Pp flic.kr/p/Z4FxtK , and the broad foot with strongly adhesive mucus has a very firm grip, so wave backwash up to 20m/s (c.70km/h) can be withstood at this stage (Denny, 1984, in Fretter & Graham, 1994). The low, streamlined shell profile is maintained by loss of shell material at the anterior as the posterior grows. The loss may be caused by physical wear, but as the substrate is plant surface it seems unlikely. Resorption locally by the anterior of the mantle is a possibility.
Sexual maturity is reached at shell lengths of about 5 mm and upwards (Graham & Fretter, 1947). A change from shell loss to growth at the anterior commences shortly after maturation, at shell length 7 or 8mm. By 10mm length there is a considerable band of new shell around the whole shell with encircling growth lines, parallel to the substrate, (early adult GS4) that have an angular unconformity where they meet the earlier tilted growth lines 15Pp flic.kr/p/YNKaz3. Blue lines are usually fewer, less distinct or absent on the new shell. The profile, viewed from the side, becomes a higher dome with a nick on the anterior profile where the convex early surface meets a nearly straight new surface 15Pp flic.kr/p/YNKaz3 . Drag in water currents becomes much greater than on the low profiles of GS3 shells, and most on the exposed frond are swept off before they reach 11mm length to die or, possibly, reach other Laminaria in deeper, calmer water with the aid of a mucus sail.
In very sheltered waters with small amounts of wave action, some on fronds survive to lengths of 14mm or 15mm and form impressive, smooth, high, conoid shells resembling a Phrygian cap (GS5) 20Pp flic.kr/p/Z4Y3uZ . But this form is relatively scarce, though easily seen when present, as the sheltered conditions required for its survival are not conducive to large populations of the species. On both sheltered shores and more exposed one, a few individuals find less turbulent conditions on the stipe 24Pp flic.kr/p/YsLJPW which has a much shorter arc of movement, diminishing to zero at its base, than the fronds that thrash back and forth in storms through a curve with about 2m diameter. The shell may resemble GS5, but if a wide pit is eaten into the stipe the newly deposited shell may flare out more widely so that the conoid has a lower profile with a break in slope at the junction of new and earlier shell 26Pp flic.kr/p/YMdFeA .
By far the largest number of those who survive into (GS5) are those who found their way into holdfasts at an early stage and have grown secure from wave impact, unless the plant is torn away. On parts of some shores, including exposed ones, about 50% of Laminaria holdfasts may be inhabited. Occupation is usually restricted to a single adult (GS4 or 5h) per holdfast . Occasionally, when several holdasts are intermingled, one pit make break through into another 94Pp flic.kr/p/ZmLn1u . For those on fronds, lifespan is about a year or less. Those in holdfasts may live for two or more years, if the plant survives. Occasional stranded specimens, perhaps from holdfasts cast up from deep calm water, have exceptionally large shells with growth rings suggesting an age of four years 25.1Pp flic.kr/p/YXMQGC .
The absence from Norway of P. pellucida living in holdfasts, and of the associated distorted shell form lacking blue rays on most of the adult growth, suggests a lack of genetic interchange between Britain and Norway. It appears that the short planktonic larval stage is insufficient to survive the time needed for it to drift from Britain to Norway.
Distribution and status
Iceland and Murman Coast (N. Russia) to Straits of Gibraltar. Not resident further into Baltic than Øresund, south-west Sweden or continental coast of North Sea apart from on occasional strandings of washed in Laminaria. GBIF map www.gbif.org/species/5728509
Frequent around Britain and Ireland on hard substrate where Laminaria is present. Both P. pellucida and Laminaria are absent or rare in Liverpool Bay and Flamborough Head to Kent apart from occasional individuals washed in from other areas. Often overlooked at times when only those hidden in holdfasts are present. U.K. distribution map NBN species.nbnatlas.org/species/NHMSYS0021056396
Acknowledgements
I (I.F.S) should like to thank Debbie Evans, Dr Marco Faasse, David Fenwick, Allan Rowat and Neil Ward for information and use of their images, and Dr Ivan Nekhaev and Dr Julia Sigwart for advice with the text and interpretation of the images. I am extremely grateful to my co-authors Erling Svensen and Paula Lightfoot for their invaluable contributions of images, information and constructive criticism. Any errors or omissions are my responsibility.
Links and references
Forbes, E. & Hanley S. 1849-53. A history of the British mollusca and their shells. vol. 2 (1849), van Voorst, London. (As Acmaea virginea; Free PDF at archive.org/stream/historyofbritish02forb#page/428/mode/2up Use slide at base of page to select pp.429-433.)
Fretter, V. and Graham, A. 1962. British prosobranch molluscs. Ray Society, London.
Graham, A. 1988. Molluscs: prosobranch and pyramidellid gastropods: keys and notes for the identification of the species. Brill & Backhuys, for Linn. Soc. Lond. & Estuarine and Brackish-water Sciences Assoc. Synopses of the British Fauna (New Series) no.2. Edition 2 (662pages). Leiden. (Edition 1 of series, 1971, 112 pages, is no substitute.)
Graham, A. & Fretter, V. 1947. The life history of Patina pellucida (L.) J. mar. biol. Ass. U.K. 26: 590 to 601.
plymsea.ac.uk/1257/1/The_life_history_of_Patina_pellucida...
Jeffreys, J.G. 1862-69. British conchology. vol. 3 (1865). London, van Voorst. (As Tectura virginea) Free PDF at archive.org/stream/britishconcholog03jeff#page/248/mode/2up . Use slide at base of page to select pp.248- 250.
Kain, J.M. & Svendsen, P.1969. A note on the behaviour of Patina pellucida in Britain and Norway. Sarsia, 38:1, 25-30. dx.doi.org/10.1080/00364827.1969.10411147
Lebour, M.V., 1937. The eggs and larvae of the British Prosobranchs with special reference to those living in the plankton. Journal of the Marine Biological Association of the United Kingdom, 22: 105 to166.
Li, L. et al. 2015. A highly conspicuous mineralized composite photonic architechture in the translucent shell of the blue-rayed limpet. Nat. commun. 6:6322 doi:10,1038/ncomms7322(2015). Free pdf (open access) at www.nature.com/articles/ncomms7322
and supplementary images and video at www.nature.com/articles/ncomms7322#supplementary-information
Wigham, G.D. & Graham, A. 2017. Marine gastropods 1: Patellogastropoda and Vetigastropoda. Synopses of the British Fauna (New Series) no.60. (172pages). Field Studies Council, Telford, England.
Yonge, C.M. and Thompson, T.E. 1976. Living marine molluscs. Collins, London.
Current taxonomy: World Register of Marine Species (WoRMS) www.marinespecies.org/aphia.php?p=taxdetails&id=147459
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).