02 Skeneopsis planorbis

Start and end point of body whorl (1) which makes up over 75% of the shell.

Apex of very depressed, almost involute spire protrudes very slightly (2) beyond the body whorl. Deformed shell with body whorl displaced shows apex clearly (3).

Diameter about 1.4 mm, Croatia, Adriatic Sea, August 2021. © N. Lete.

Full SPECIES DESCRIPTION BELOW

PDF available at www.researchgate.net/publication/389705401_Skeneopsis_pla...

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

 

Skeneopsis planorbis (O. Fabricius, 1780)

 

Synonyms: Turbo planorbis O. Fabricius, 1780; Skenea planorbis (O. Fabricius, 1780); Skeneia trochiformis Locard, 1886.

Current taxonomy: World Register of Marine Species (WoRMS) www.marinespecies.org/aphia.php?p=taxdetails&id=141539

Meaning of name

Skeneopsis: appearance similar to genus Skenea.

planorbis: a flat ring or coil.

 

GLOSSARY below.

 

Shell description

Skeneopsis planorbis has a small discoid shell with a maximum diameter of 1.55 mm (Fretter, 1948). Unlike most snails, its four loosely coiled, tumid whorls leave a wide funnel-shaped umbilicus exposing the whole extent of the proximal sides of the whorls 01Sp flic.kr/p/2qQdJLy and form a very depressed, almost involute spire with a blunt apex and apical angle of about 150 to 160º (Fretter & Graham, 1978). A carefully positioned edge-on view of the disc is needed for the apex to be seen protruding slightly beyond the body whorl 02Sp flic.kr/p/2qQcLZS & 16Sp flic.kr/p/2qQbEEs . There is a deep suture between the convex whorls 01Sp flic.kr/p/2qQdJLy . The body whorl makes up over 75% of the shell 02Sp flic.kr/p/2qQcLZS .

The circular aperture has a complete, thin, often slightly flexuose, peristome which is almost completely unattached to the body whorl 01Sp flic.kr/p/2qQdJLy . On young specimens especially, the edge of the peristome is sometimes uncalcified periostracum. Apart from many fine growth lines, and occasionally slight spiral lines adapically on the body whorl, there is no surface sculpture.

The colour of an occupied shell results from shell colour and an age and environment related combination of translucency, colours of body within, periostracum development, erosion and mucous coating. When young, the shell may be glossy and semitransparent revealing colours of the body within 03Sp flic.kr/p/2qQcJTd . The glossiness is soon lost but the body colours may still show 04Sp flic.kr/p/2qQdJsT . The most recently created part of the body whorl by the aperture, where calcification is incomplete, is usually paler than the rest of the shell 03Sp flic.kr/p/2qQcJTd . Other parts of the shell may be eroded when water quality is suboptimal 05Sp flic.kr/p/2qQ7cXZ . Sometimes the shell surface has a row of reddish brown marks on either side of the periphery 06Sp flic.kr/p/2qQcJtf . This form was called variety maculata by Jeffreys (1867) who said it was more common in the Mediterranean than in Britain. He also named a clear white and transparent shell form as variety hyalina 03Sp flic.kr/p/2qQcJTd . Eroded specimens often completely encase the shell in a thick untidy coat of mucus with embedded algal filaments and diatoms 07Sp flic.kr/p/2qQcS2W . Sometimes the algae give the shell an olive green hue.

The circular operculum is concave, with 7 or 8 concentric turns and a central nucleus (Jeffreys, 1867). It is clear-white or like grey smoked glass 08Sp flic.kr/p/2qQbFCe .

Body description

The flesh is translucent white, sometimes with varying amounts and intensity of grey tint 05Sp flic.kr/p/2qQ7cXZ & 09Sp flic.kr/p/2qQ7cFr . Patches of yellow on various parts of the body may be visible through the shell.

The head has two widely diverging, unpigmented, cylindrical cephalic tentacles, each with a conspicuous black eye laterally on the swollen base 10Sp flic.kr/p/2qQcT9R . Each eye usually has a large, opaque, bright yellow patch behind it 11Sp flic.kr/p/2qQbEYi but it is sometimes white. There is a broad snout with a dorsomedial groove dividing it into two lobes 12Sp flic.kr/p/2qQ7cDh . The longitudinal slit mouth lies ventrally between and near the ends of the lobes.

Some internal organs can be seen through the clear transparent shell of some specimens. The buccal mass between the tentacles is usually covered by bright yellow pigment, but occasionally some pale specimens have all yellow replaced with opaque white. A right and a left salivary gland 12Sp flic.kr/p/2qQ7cDh run forwards alongside the oral tube from the buccal mass to two cuticular jaws each of which has six transverse rows of about 20 teeth. From the posterior of the buccal mass the oesophagus traverses the length of the body whorl.

The mantle cavity extends the whole length of the body whorl. It contains a monopectinate ctenidium with only nine large filaments, a bipectinate osphradium and, in males, a long penis which arises from behind the right tentacle and extends the full length of the cavity 12Sp flic.kr/p/2qQ7cDh .

The foot is relatively short, truncated at the anterior and the posterior third tapers to a blunt point 13Sp flic.kr/p/2qQ7cgi . The anterior edge is bilaminate and contains the opening of the anterior pedal mucous gland 14Sp flic.kr/p/2qQ7c3h . The translucent white sole 13Sp flic.kr/p/2qQ7cgi shows the opaque white anterior pedal gland and posterior pedal mucous gland with a translucent gap between them across which the foot can fold 14Sp flic.kr/p/2qQ7c3h . The opening to the posterior gland is faintly visible at about 66% of the length back from the anterior edge 13Sp flic.kr/p/2qQ7cgi .

Dorsally on the metapodium, the opercular disc does not project beyond the edge of the circular operculum resting on it, though it does have a covered posterior lobe 15Sp flic.kr/p/2qQcRu8 . The operculum is translucent, often mottled with grey or black like smoked glass, so the disc beneath it, varying in colour from black or grey to whitish with an opaque yellow mark, can often be discerned 15Sp flic.kr/p/2qQcRu8 & 08Sp flic.kr/p/2qQbFCe .

Key identification features

Skeneopsis planorbis

1) Discoid shell, maximum diameter 1.55 mm.

2) Wide open umbilicus shows proximal side of all whorls.

3) Depressed spire, apex protrudes slightly above body whorl 02Sp flic.kr/p/2qQcLZS .

4) Translucent shells show body colours including yellow blotches. Worn shells coated with thick mucus and algal filaments 07Sp flic.kr/p/2qQcS2W .

5) Operculum and opercular disc often grey or blackish, with yellow mark on disc;

6) Head has cylindrical cephalic tentacles and yellow patch behind each eye 11Sp flic.kr/p/2qQbEYi .

7) Bifid snout does not flare into oral lobes.

8) Shell carried erect but tilted to left and right as it travels 16Sp flic.kr/p/2qQbEEs .

9) Lives among fine and filamentous algae such as Corallina officinalis 19Sp flic.kr/p/2qQdLnj . and Cladophora spp.

Similar species

Omalogyra atomus (R. A. Philippi, 1841)

1) Discoid, biconcave shell, spirally coiled in a single plane, maximum diameter 1 mm 17Sp flic.kr/p/2qQcHv8 .

2) Wide open umbilicus shows all whorls.

3) Wide open concave spire shows all whorls, apex does not protrude.

4) Reddish brown shell, body uniformly translucent white 18Sp flic.kr/p/2qQdHjk .

5) Operculum white.

6) Head has no cephalic tentacles and no yellow patch behind eyes.

7) Bifid snout flares into two semicircular lobes.

8) Shell carried vertically without tilting to either side as it travels.

9) Lives on delicate algae such as Ulva lactuca and Ulva intestinalis (Fretter & Graham, 1978).

Species in the genus Skenea

1) Low helical spiral (in 3 dimensions), maximum height 2 mm.

2) Distinct umbilicus.

3) Spire protrudes well above body whorl.

4) Shell white or colourless.

5) Round operculum with many turns.

6) Head has cephalic tentacles and neck lobes. Epipodial tentacles on body.

7) Slightly bifid snout.

Habits and ecology

S. planorbis lives among fine and filamentous algae such as Cladophora, Corallina 19Sp flic.kr/p/2qQdLnj , Pylaiella, Ectocarpus, Polysiphonia and Plocamium (Fretter & Graham, 1962) in rock pools and moist situations on shores at MTL and below, and sublittorally to 70 m. It feeds on diatoms living on the algae, detritus caught on the algae and on algal fragments (Fretter & Graham, 1978). The radula rasps up diatoms and algal cells while the jaws grip the substrate. By the jaws, the salivary glands 12Sp flic.kr/p/2qQ7cDh produce secretions to lubricate the radula and cement together food particles rasped by it (Fretter & Graham, 1962).

S. planorbis can live in both sheltered pools and on exposed coast if suitable algae are available. At a site in North Wales a 0.5 litre sample of Rhodothamniella alga with sediment it had bound to a rock in a large sheltered pool had 29 S. planorbis and a 0.5 litre sample of Corallina from wave exposed rocks 75 metres away had 62 S. planorbis (pers. obs. October 2009) 20Sp flic.kr/p/2qQdJUV . In the pool 20 rocks lacking fine algae which were brushed in water produced only 2 S. planorbis.

Crawling is lubricated by mucus from the anterior pedal gland 13Sp flic.kr/p/2qQ7cgi & 14Sp flic.kr/p/2qQ7c3h . It can crawl on the substrate or inverted under the surface film of pools. As it travels the shell is held up on edge at a tilt and it often rocks to left and right 16Sp flic.kr/p/2qQbEEs . Often, only the hinder part of the foot is applied to the substrate. Movement among algae is with the aid of mucus from the posterior pedal gland which is shaped into a thread in the groove running back from the gland opening 13Sp flic.kr/p/2qQ7cgi . It hardens into a strong thread on contact with sea water. It can also be suspended from the surface film of pools and used to move between the surface and the bed of the pool.

Some other minute species lack a ctenidium and rely on pallial respiration, but S. planorbis has a bipectinate osphradium to test water quality and a ctenidium for oxygenating its haemolymph, though it is only monopectinate with 9 filaments. When most specimens in a population are eroded it may be because of poor water quality. They often produce a thick coat of mucus over the shell, perhaps as protective response to the erosion 21Sp flic.kr/p/2qQcR7z .

It has separate sexes which breed in all months in Britain (Lebour, 1937) so all stages can be found together, but the majority spawn in spring at Plymouth (Fretter, 1948). The main breeding season starts earlier and ends later when weather is suitable.

The male fertilizes the female internally with its long penis 12Sp flic.kr/p/2qQ7cDh . The female attaches spherical or ovoid capsules individually to an algal filament. The large, relative to snail size, capsules are about 0.45 mm in diameter and have a thick protective wall 22Sp flic.kr/p/2qQcHou . Each capsule has one, or occasionally two, heavily yolked eggs surrounded by white albumen (Fretter & Graham, 1962). The veliger stage is passed within the capsule and development takes 3 to 4 weeks at 12 to 15ºC, or 2.5 weeks at 14.5 to 17.5ºC, before crawling snails with brown shells of about 0.32 mm diameter rasp their exit with their radulae (Fretter, 1948).

By early June, rock pools at Plymouth have innumerable young with an average shell diameter of 0.48 mm. Numbers are at a maximum in the summer when ample food promotes rapid growth (Fretter & Graham 1962). By January they have grown to an average diameter of 0.85 mm, but are not yet sexually mature. By this time probably all the adults which produced them are dead (Fretter, 1948).

Distribution and status

S. planorbis is found from Greenland to Florida, USA and from Iceland and Northern Norway to the Canary Islands and western half of Mediterranean. It is rare or absent from the eastern half of the Mediterranean, the Black Sea, the Baltic beyond the Kattegat and both sides of the southern half of the North Sea, GBIF map www.gbif.org/species/2301716 .

The GBIF map shows many records in the Netherlands and Belgium, but they are fossils or on strandings of floating material from elsewhere. There is no record of live in situ S. planorbis in these two countries in De Bruyne et al. (1994) and no live in situ specimens have been found in years of sampling locations with habitats suitable for S. planorbis (M. Faasse, M. van Her and S. van Leeuwen, 2025 pers. comm. 2 March).

In Britain and Ireland, though often overlooked because of its minute size, it occurs, often abundantly, practically everywhere where the shores are suitable though it is absent between the Humber Estuary and Dover (Wigham & Graham, 2017), and NBN Atlas shows no record from the north-east coast of the Irish Sea. U.K map NBN species.nbnatlas.org/species/NHMSYS0021055468 (accessed February 2025).

Acknowledgements

I gratefully thank Neven Lete for images of shells, and Chris Isaacs for images of pale live specimens showing internal organs. I thank Marco Faasse, Maarten van Her and Sylvia van Leeuwen for information about the status of the species in the Netherlands and Belgium.

Links and references

Caballer, M., Ortea, J. & Narciso S. 2011. Description of two new species of Rissoella Gray, 1847 (Mollusca, Gastropoda, Heterobranchia) from Venezuela, with a key to the Caribbean species known for the genus. zookeys.pensoft.net/articles.php?id=3048

 

De Bruyne, R.H., Bank, R.A., Adema, J.P.H.M. and Perk, F.A. 1994, Nederlandse naamlijst van de weekdieren (Mollusca) van Nederland en Belgie. Nederlandse Malacologische Vereniging.

 

Forbes, E. & Hanley S. 1853. A history of the British mollusca and their shells. vol. 3 , London, van Voorst. (As Skenea planorbis) p.156 to 158 . archive.org/details/historyofbritish03forb/page/156/mode/...

Plate LXXIV figs. 1,2,3. in Vol 4. archive.org/details/historyofbritish04forbe/page/n464/mod...

Plate GG fig. 1 & 1a (animal) in Vol. 1

archive.org/details/historyofbritish01forbe/page/n618/mod...

 

Fretter, V. 1948. The structure and life history of some minute prosobranchs of rock pools: Skeneopsis planorbis (Fabricius), Omalogyra atomus (Philippi), Rissoella diaphana (Alder) and Rissoella opalina (Jefffreys). J. mar. biol. Ass. U. K. , 27 (3): 597-632.

pdf at plymsea.ac.uk/1294/ or www.mba.ac.uk/nmbl/old_jmba/vol27/vol27no3.htm

 

Fretter, V. and Graham, A. 1962. British prosobranch molluscs. London, Ray Society.

 

Fretter, V. and Graham, A. 1978. The prosobranch molluscs of Britain and Denmark. Part 4 – Marine Rissoacea. Suppl. 6, J. Moll. Stud.

 

Graham, A. 1988. Molluscs: prosobranch and pyramidellid gastropods. Synopses of the British Fauna (New Series) no.2 (Second edition). Leiden, E.J.Brill/Dr. W. Backhuys. pp.662.

 

Jeffreys, J.G. 1867.British conchology. vol. 4. London, van Voorst. archive.org/details/britishconcholog04jeffr/page/64/mode/... (p. 64 as Skenea planorbis)

 

Lebour, M.V. 1937. The eggs and larvae of the British prosobranchs with special reference to those living in the plankton. J. mar. biol. Ass. U.K., 22: 105 – 166. plymsea.ac.uk/953/

 

Waarneming.nl Zeeposthorentje Skeneopsis planorbis (Fabricius, 1780).

waarneming.nl/species/240836/observations/?date_after=190...

(accessed March 2025).

 

Wigham, G.D. & Graham, A. 2017. Marine gastropods 2: Littorinimorpha and other, unassigned, Caenogastropoda. Synopses of the British Fauna (New Series) no.61. (344 pages). Telford, England. Field Studies Council.

 

Glossary

adapical = towards the apex of the shell.

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

biconcave shell = discoid shell with both apical and basal sides concave, e.g. Omalogyra atomus.

 

bifid = divided into two parts by a cleft.

bipectinate = like a feather with a central axis or rachis, and a series of filaments or lamellae on either side.

buccal mass = anterior of digestive system including a radula, odontophore and muscles.

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

cilia = (pl.) vibrating linear extensions of membrane used in feeding or locomotion.

cuticle = non-cellular outer layer of an organism that comes in contact with the environment; in mollusca it often contains chitin.

 

cuticular = made of cuticle.

diatom = microscopic aquatic alga with siliceous cell-walls.

epipodial = (adj.) of the epipodium.

epipodium = collar or circlet running round sides of foot of some gastropods, bearing bosses, lobes and/or tentacles.

 

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

dorsomedial = on or close to the midline of the dorsal surface.

helical = of a spire which turns in three dimensions like a spiral staircase.

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.

 

jaws = thickened cuticle formed into 2 jaws of 6 rows of 20 teeth in the oral tube.

mantle = sheet of tissue that secretes the shell and forms a cavity for the gill, genital and renal openings, anus etc. in most marine molluscs.

 

monopectinate = axis with a row of filaments/lamellae on one side only.

MTL = mean tide level.

mucous = (adj.) of mucus or its production.

mucus = (noun) slimy substance secreted by mucous glands for lubrication, protection, etc.

oesophagus = tube from buccal mass to stomach.

opercular = (adj.) of the operculum.

opercular disc = part of foot which operculum rests on and is attached to as it grows.

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

osphradium = organ for testing water quality (chemical and/or for particles) usually near ctenidium (gill).

 

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

pallial respiration = oxygen absorbed from water by the mantle.

pedal mucous gland = gland in foot secreting mucus to lubricate crawling, form mucous tethering lines etc.

 

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

periphery = (of snail shell) perimeter of body whorl at its widest point. Sometimes marked by a keel or coloured band.

 

peristome = rim round shell-aperture, mouth or other opening.

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

proximal = towards the centre of the body or point of attachment.

salivary gland = produces secretions to lubricate the radula and cement together food particles.

 

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

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

veliger = shelled larva of marine gastropod or bivalve mollusc which swims by beating cilia of a velum (bilobed flap). Stage may be passed in plankton or within liquid-filled egg-capsule.

 

 

Done