03 Velutina velutina

Live shell has a substantial light brown periostracum which is raised into many slightly waved, spiral ridges.

The periostracum reduces translucency of live specimens so the viscera are only faintly discernible (See also fig. 04 flic.kr/p/2gdwYwJ ) . The white, apical protoconch lacks a periostracum.

Shell height 10 mm. ELWS, Gairloch, north-west Scotland, May 2019. Leg. D. McKay & S. Taylor.

 

Full SPECIES DESCRIPTION below

PDF available at www.researchgate.net/publication/377437235_Velutina_velut...

Sets of OTHER SPECIES:

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Velutina velutina (O. F. Müller, 1776)

Meaning of scientific name:

Velutina (Latin) = velvety (referring to the texture of the live, periostracum-coated shell surface).

Synonyms: Bulla velutina Müller O.F., 1776; Velutina laevigata (O. F. Müller, 1777); Velutina laevigata var. alba Jeffreys, 1867.

Current taxonomy: World Register of Marine Species (WoRMS) www.marinespecies.org/aphia.php?p=taxdetails&id=141905

Vernacular: Velvet snail (English); Cragen felfed (Welsh); Gewone fluweelhoren (Dutch); Fløjlsskæl (Danish); Sjöpungsnäcka (Swedish): Glatte Samtschnecke (German); Veloutée lisse (French).

 

GLOSSARY below.

 

Shell description

In Britain, the shell height is commonly about 10 mm 1Vv flic.kr/p/2gdx1kd , with a usual maximum of 18 mm (McMillan, 1968) and extreme of 22 mm (D.W. McKay, in litt. 2019). Specimens from Greenland are larger and thicker than in Britain (Jeffreys, 1867). The shell depth (perpendicular distance from the summit of a living specimen to substrate), is about 50% of shell height (see 1Vv flic.kr/p/2gdx1kd for illustration of dimension terms). The shell has thin walls and only two and a half rapidly-expanding, tumid whorls with the body whorl constituting the great majority of the shell. The sutures between the whorls are deep but obscured by the thick velvety periostracum on living specimens. The apical angle is wide with the apex often recessed into a depression making it almost involute 2Vv flic.kr/p/2gdwZh1 .

The slightly glossy, translucent, white or pinkish-white calcareous shell has a sculpture of many very fine growth lines crossed by many very fine, low spiral ridges, forming a faint reticulated pattern 2Vv flic.kr/p/2gdwZh1 . On live shells, these lines are concealed beneath the substantial light brown periostracum which is raised into many slightly waved, spiral ridges 3Vv flic.kr/p/2gdwHru . Occasionally, the periostracum is white (Telnes, 2019). The brown periostracum reduces the translucency of live specimens so the viscera are only faintly discernible 3Vv flic.kr/p/2gdwHru & 4Vv flic.kr/p/2gdwYwJ . The white, apical protoconch lacks a periostracum. Within 24 hours of drying, the periostracum loses the spiral ridges and shreds into bristle-like fragments 5Vv flic.kr/p/2gdwYds , and it readily flakes off. Consequently, strandline shells 2Vv flic.kr/p/2gdwZh1 usually have little or no periostracum and are white or pinkish white.

The aperture 1Vv flic.kr/p/2gdx1kd is almost circular, about 90% of the shell-height, and about three times as large as visible whorls in apertural view. The palatal and columellar lips run into each other in a smooth curve; the parietal lip is very short but broader than the columellar lip. The adapical angle of the aperture is rounded. There is no umbilicus but on large older specimens the apex may protrude slightly, and the whorls are more loosely coiled; sometimes with the body whorl separating slightly from the columellar lip to give the false impression of an umbilical slit.

The external colour of a live shell is brown when the periostracum is present and translucent white or pinkish white when the periostracum is lost 5Vv flic.kr/p/2gdwYds ; pink being more frequent and intense in the south of its range. Internally, the shell is white or pinkish white, but appears brown when the external periostracum shows through it 1Vv flic.kr/p/2gdx1kd . The large round aperture resembles that of limpets and, like them, V. velutina has no operculum to seal it.

 

Body description

The body is white 6Vv flic.kr/p/2gdwGBP , sometimes with a faint yellowish or pinkish hue (Jeffreys, 1867), varying in translucency in its different parts, with a very fine opaque white stipple most noticeable where translucency is strongest 7Vv flic.kr/p/2gdwXyX . The mantle varies from white or cream to yellowish white with a fine, opaque, white stipple 7Vv flic.kr/p/2gdwXyX (“frosted with white points” Fretter & Graham, 1978). The mantle edge is thickened by large, inflated, puckered, connective tissue cells 8Vv flic.kr/p/2gdwXsz blocking the entrance to the mantle cavity, except for the inhalant and exhalant passages. The connective tissue may protrude beyond, but not reflect onto, the palatal lip (anterior of aperture of live specimen) 9Vv flic.kr/p/2gdwX4U . Usually, the mantle is reflected slightly onto only the columellar and parietal lips (posterior of aperture of live specimen). The flat head and its smooth, tapering cephalic tentacles are coloured as the body. The anterior of the head is widely truncated as it has no exterior snout, but the short yellow, inverted proboscis is sometimes visible within the semi-transparent head 10Vv flic.kr/p/2gdwFM2 . There is an eye on a basal swelling at the distal side of each cephalic tentacle. A white or yellowish penis 6Vv flic.kr/p/2gdwGBP, and 7Vv flic.kr/p/2gdwXyX is located behind the right tentacle of all adults as, unusually for Caenogastropoda, the species is hermaphrodite.

The dorsal surface of the foot is coloured as the body, but the opaque stipple may be difficult to discern as the foot has weak translucency 11Vv flic.kr/p/2gdwWUa . When in motion on a flat surface, the unstippled white sole is ovate with a constriction close to the broadly curved anterior shaped like an axehead with flexible corners 8Vv flic.kr/p/2gdwXsz . The foot readily folds transversely at the constriction. Through the inhalant and exhalant passages in the pallial connective tissue it is sometimes possible to partially see within the mantle cavity 12Vv flic.kr/p/2gdwrrG the ctenidium; the smaller, but similarly comb-shaped osphradium (misinterpreted as a second ctenidium in early works [Forbes & Hanley, 1853 and Jeffreys,1867]); the hypobranchial gland; and rectum. If the relaxed and killed animal is removed from the shell and the black pigment 13Vv flic.kr/p/2gdwrg6 rubbed off the translucent mantle, it should be possible to see 14Vv flic.kr/p/2gdwqXR & 15Vv flic.kr/p/2gdwVUp the ctenidium and osphradium, the right columellar muscle; the smaller left columellar muscle; the very extensive white hypobranchial gland lining the entire roof of the mantle cavity to the right of the ctenidium; the genital ducts; the rectum; the auricle and ventricle of the heart; and the ovotestes.

 

Key identification features

Velutina velutina

1) When live, white, or pinkish white, calcareous shell is covered, except apical protoconch, by substantial brown periostracum that is raised into spiral ribs 1Vv flic.kr/p/2gdx1kd . Periostracum distorts when dry 5Vv flic.kr/p/2gdwYds .

2) When live, shell is weakly translucent and only indistinctly shows the viscera 4Vv flic.kr/p/2gdwYwJ & 16Vv flic.kr/p/2gdwVHc .

3) Aperture large and almost circular; about three times as large as visible whorls in apertural view 1Vv flic.kr/p/2gdx1kd .

4) Columellar lip has no umbilicus or groove 1Vv flic.kr/p/2gdx1kd .(Gap between body whorl and aperture sometimes occurs on large old specimens and might be mistaken for umbilical groove).

5) Spire does not usually protrude beyond body whorl; apex often recessed into a depression 2Vv flic.kr/p/2gdwZh1 . Apex on large old shells may protrude slightly

6) When live and active, mantle may extend beyond the aperture rim, but only reflected onto the shell at the rear of the aperture (columellar/ parietal lips) 9Vv flic.kr/p/2gdwX4U .

7) Flesh translucent white with fine, opaque white stipple 6Vv flic.kr/p/2gdwGBP . Mantle white, cream and/or yellowish white 7Vv flic.kr/p/2gdwXyX .

 

Similar species

Velutina plicatilis (O. F. Müller, 1776)

1) Shell is horn coloured, made principally of thick, slightly hispid periostracum; not raised into spiral ribs. Flimsy, thin, white, interior calcareous layer readily flakes off when dry, leaving just the periostracum 20Vv flic.kr/p/2gdwUPJ .

2) When live, shell is translucent often showing form and colour of viscera within; mainly yellow and orange 16Vv flic.kr/p/2gdwVHc .

3) Large, elongate oval aperture; about three times as large as visible whorls in apertural view 20Vv flic.kr/p/2gdwUPJ .

4) Columellar lip has no umbilicus or groove 20Vv flic.kr/p/2gdwUPJ .

5) Spire does not protrude beyond body whorl; usually recessed into a depression 20Vv flic.kr/p/2gdwUPJ .

6) When live and active, mantle is obviously reflected a short way onto the shell all around the aperture 16Vv flic.kr/p/2gdwVHc .

7) Flesh and mantle bright yellow or orange 16Vv flic.kr/p/2gdwVHc .

 

Lacuna pallidula (da Costa, 1778)

1) When live, calcareous, white shell is covered by substantial periostracum that is smooth, apart from numerous growth lines; brown or olive-brown on adults 21Vv flic.kr/p/2gdwpr4 . Periostracum readily flakes off when dry 22Vv flic.kr/p/2gdwCSQ .

2) When live, adult shell is opaque 21Vv flic.kr/p/2gdwpr4 .

3) Large, D shaped aperture about same size as visible whorls in apertural view 22Vv flic.kr/p/2gdwCSQ .

4) Columellar lip has large groove (chink) and umbilicus 22Vv flic.kr/p/2gdwCSQ .

5) Low spire protrudes, not recessed into a depression 22Vv flic.kr/p/2gdwCSQ .

6) When live and active, mantle may protrude very slightly, but is not reflected onto the shell 21Vv flic.kr/p/2gdwpr4 .

7) Flesh usually white, often with some yellow on snout, penis and parts of foot. Sometimes flesh is pink or whitish grey 21Vv flic.kr/p/2gdwpr4 .

 

Capulus ungaricus

1) When live, white, calcareous shell is covered, including apical protoconch, by substantial pale, straw-coloured periostracum which is raised into spiral ribs 23Vv flic.kr/p/2gepyX3 . Periostracum extends beyond aperture as fragments which form a fringe on adults.

2) When live, shell is weakly translucent.

3) Aperture large and almost circular with only the small curled spire visible in apertural view 25Vv flic.kr/p/2i3gDm7

4) Columellar lip has no umbilicus or groove.

5) Small, downward spirally coiled spire protrudes almost centrally from beyond body whorl. Apex not recessed into a depression, and turned slightly to the right of the animal 23Vv flic.kr/p/2gepyX3 & 25Vv flic.kr/p/2i3gDm7 .

7) Flesh yellowish with white speckles . Proboscis permanently extended (Graham, 1988).

8) Juveniles often found near open end of Pomatoceros tubes 24Vv flic.kr/p/2gepjfF ; adults near bivalves and Turritella, purloining food particles brought in by the hosts’ inhalant currents, or their pseudofaeces.

Habits and ecology

V. velutina lives mainly sublittorally to depths of 1000 metres Hayward & Ryland (1995) on hard substrate, especially shelly ground (Forbes & Hanley, 1853), where its unitary ascidian prey, Ascidia, Phallusia and Styela coriacea occur (Fretter & Graham, 1962). Rarely, live specimens are found at ELWS in northern Scotland.

Respiration is with inhalant water brought into the mantle cavity via a passage through the connective tissue by the left of the head 17Vv flic.kr/p/2gdwE6m . In the cavity, it first passes through the reddish osphradium 12Vv flic.kr/p/2gdwrrG which tests the water quality and probably removes large particles of detritus. It then passes through the dull yellow ctenidium which acquires oxygen from the water for the haemolymph (‘blood’) passing through it and releases carbon dioxide which passes with the water out of the animal via an exhalant passage through the connective tissue on the right side of the body. Locomotion is on the large foot lacking a medial dividing groove on the sole. When live, the foot usually remains in contact, limpet-like, with the substrate. V. velutina feeds by biting open the tough test of unitary ascidians to access the soft inner parts. The rectum and anus 12Vv flic.kr/p/2gdwrrG are near the exhalant passage where faeces exit the animal with the exhalant water. Clark in Jeffreys (1867) reported that it "discharges a very copious and tenacious clear white slime" which may have protective qualities. It may breed all year as veliger larvae have been seen in summer and autumn off Plymouth, S. England (Fretter and Graham, 1962), and mating at 3ºC and ova have been photographed in Norway in December and January . If breeding is triggered by temperature it is likely that it takes place at different dates at different levels of its great depth range in different geographical areas. Fretter and Graham (1962) considered it to be a simultaneous hermaphrodite. A photo of mating 18Vv flic.kr/p/2gdwVcs shows a couple lying head-on, foot to foot, on their left sides so their right-side genitalia are adjacent. In the photo, the massively engorged penis of only one of them can be seen to be penetrating the other. For oviposition, the radula cuts round holes a few mm from each other in the ascidian. The foot positions large, spherical pot-shaped egg capsules in the holes 19Vv flic.kr/p/2gdwDNh and 19.1Vv flic.kr/p/2oX6A8w. Only the transparent plug in the opening of the capsules is visible on the surface of the ascidian. Each capsule contains hundreds of unshelled ova in an albuminous fluid. The plug is freed from the opening for the planktonic bilobed veliger larvae to escape.

Distribution and status

V. velutina has a circumpolar distribution from the high Arctic southwards to Galicia, Spain; Cape Cod, Atlantic USA; San Diego, Pacific USA; and Japan. There are very few records from thwe continental coast of the southern North Sea. GBIF map www.gbif.org/species/2299230 . It occurs widely around Britain and Ireland where there is suitable hard substrate with ascidians. Most live records are sublittoral. Lttoral finds are usually dead, but there are some live littoral records from northern Scotland. There are few records, dead or alive, from the north-east Irish Sea, mid Wales, south-west England and between Kent and the Humber; Estuary. U.K. interactive map NBN species.nbnatlas.org/species/NHMSYS0021055637

Acknowledgements

This account would not have been possible without the shore work of David McKay and Simon Taylor and their help in supplying me with a live specimen to photograph. Sincere thanks are also due to Erling Svensen and James Lynott for the use of images of mating and ova; and, for comparison purposes, to Chris Rickard for photos and specimens of V. plicatilis and to David Fenwick for images of Capulus ungaricus.

Links and references

Forbes, E. & Hanley S. 1849-53. A history of the British mollusca and their shells. vol. 3 (1853), London, van Voorst. (As Velutina laevigata archive.org/stream/historyofbritish03forbe#page/346/mode/2up p346)

 

Fretter, V. and Graham, A. 1962. British prosobranch molluscs: their functional anatomy and ecology. London, Ray Society.

 

Fretter, V. and Graham, A. 1978. The prosobranch molluscs of Britain and Denmark. Part 6 – Cerithiacea, Strombacea, Hipponicacea, Calyptraeacea, Lamellariacea, Cypraeacea, Naticacea, Tonnacea, Heteropoda. J. Moll. Stud. Suppl. 9.

 

Graham, A. 1988. Prosobranch and pyramidellid gastropods. Linnean Synopsis No.2 (Second Edition). Leiden, Brill & Backhuys.

 

Hayward, P.J. & Ryland, J.S. (eds.) 1995 and reprints to 2009. Handbook of the marine fauna of North-West Europe. Oxford University Press.

 

Jeffreys, J.G. 1862-69. British conchology. vol. 4 (1867). London, van Voorst. (As Velutina laevigata archive.org/details/britishconcholog04jeffr/page/240 . Use slide at base of page to select pp 240 to 244.

 

McMillan, N.F. 1968. British shells. London, F. Warn.

 

Müller, O.F. 1776. Zoologiae Danicae prodromus : seu Animalium Daniae et Norvegiae indigenarum ; characteres, nomina, et synonyma imprimis popularium Original description, as Bulla velutina, item 2922 on p. 242.

www.biodiversitylibrary.org/item/47550#page/278/mode/1up

 

Telnes, K. accessed June 2019. The marine flora and fauna of Norway

www.seawater.no/fauna/mollusca/velutina.html [Subaquatic images of live specimens, including one with a white periostracum.]

 

 

Glossary

abapical = away from the apex of the shell.

adapical = towards the apex of the shell.

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

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

columella = solid or hollow axial “little column” around which gastropod shell spirals.

columellar = (adj.) of or near central axis of spiral gastropod.

columellar lip = lower (abapical) part of inner lip of aperture.

convolute shell = body whorl envelopes and conceals all earlier whorls including apex e.g. Trivia.

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

distal = away from centre of body or from point of attachment.

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

height = (of gastropod shells) distance from apex of spire to base of aperture.

hispid = surface roughened by minute, stiff bristles.

hypobranchial gland = thickened, sometimes puckered, tissue on roof of mantle cavity of many gastropods. Secretes mucus to trap and consolidate particles from inhalant water. Often other biologically active compounds produced.

 

involute shell = body whorl envelopes and conceals all other whorls except earliest apical whorls which are recessed below the full height of the shell e.g. Retusa truncatula.

 

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

 

operculum = plate of horny conchiolin, rarely calcareous, used to close shell aperture.

osphradium (pl. osphradia) = organ for testing water quality (chemical and/or for particles) usually near approach of inhalant current to ctenidium.

 

palatal lip = outer lip of gastropod aperture.

pallium = (see mantle).

pallial = (adj.) of the pallium/mantle.

parietal lip/wall = upper part of inner side of gastropod aperture, often lacking clear lip structure with just a glaze on side of whorl adapically of columellar lip.

 

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

plankton = animals and plants that drift in pelagic zone (main body of water).

proboscis = internal feeding tube, containing the buccal mass with radula, that is only extended, often by inversion like a sock turned inside out, when feeding.

 

reflected = (of mantle edge) folded back over the rim of the shell’s aperture.

umbilicus = cavity up axis of some gastropods, open as a hole or chink on base of shell, often sealed over.

 

suture = groove or line where whorls of gastropod shell adjoin.

tumid = bulging, distended, distinctly convex.

veliger = shelled larva of marine gastropod or bivalve mollusc which swims by beating cilia of a velum (bilobed flap).

 

Done