Retusa umbilicata
Shell description
The thin shell is external and whitish transparent in colour, usually with an elongate cylindrical shape; however the shell shape is extremely variable. The opening is small, but extends the length of the shell. The top of the shell (apex) is sunken. The shell is often slightly umbilicated. The shell is largely smooth with faint growth lines; however faint spiral grooves may be present in the front and hind part of the shell. The length of the shell varies between 1–4.3 mm.
Animal description
The body is white in colour, and the head shield is lobed towards the back and there is no middle line. There are no side extensions of the foot (parapodial lobes) and no extension of the mantle under the snail (pallial lobe). The larval kidney is not visible trough the shell. The eyes of this species are visible.
Anatomy
The radula is absent. The gizzard is rounded and not surrounded by muscle fibres, and holds three unequally sized, light brown plates with dark brown knobs (tubercles) on dorsal surface. The largest knobs form a peak on the hind part of the plates. The underside of the plates has a netlike structure of depressions and holes under the knobs on the dorsal surface. Usually the paired plates are narrower than unpaired plate. The gizzard with plates is visible trough the shell. The foregut carries a large crop. The male reproductive system consists of a flat prostate emerging at a right angle from the penial sheath; the latter has folds.
Ecology
Occurs in depths from 8–2000 m on clay, mud, sand, mushy sand and rocks (Thompson 1988, Høisæter 2009).
Geographical distribution
Occurs in Norway from west Finnmark and southwards (Høisæter 2009). Further distribution from New England to the British Isles, the Baltic Sea, Mediterranean Sea, Madeira, West Africa and the Cape of Good Hope (Thompson 1988).
Dorsal and ventral views of shell (automontage pictures), scale bar = 0.5 mm.
Ventral view of shell (SEM), scale bar = 200 μm.
Dorsal view of live animal (H = 1.9 mm).
Ventral view of live animal (H = 1.9 mm).
Dorsal view of gizzard plates (SEM), scale bar = 20 μm.
Ventral view of gizzard plate (SEM), scale bar = 20 μm.
Male reproductive system, scale bar = 10 μm. Abbreviations: pc: penis chamber, pr: prostate.
References
Høisæter T (2009). Distribution of marine, benthic, shell bearing gastropods along the Norwegian coast. Fauna Norvegica 28: 5-106.
Thompson TE (1988). Molluscs: Benthic Opisthobranchs. In: Brill EJ og Backhuys W. Leiden. Mollusca, Gastropoda: keys and notes for the identification of the species. Vol. 8. New York, Copenhagen, Cologne. 356 sider.