Retusa truncatula
Shell description
The thick shell is external and white to whitish transparent in colour, usually with an elongate cylindrical shape. The opening is narrow, but extends the length of the shell and usually beyond the top of the shell. The top of the shell (apex) is sunken; in some specimens the apex may be truncate (shortened) or extend slightly out of the shell (mamilate). The shells are mostly umbilicated. The shell surface has a pattern consisting of prominent growth lines; however spiral grooves may be found in mamilate specimens. The length of the shell varies between 0.6–3.5 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 of the dorsal surface. Usually the paired plates are narrower than the unpaired plate. The foregut carries a large crop. The male reproductive system consists of a penis chamber that gradually widens into a flat prostate with some accessory glands. The system is surrounded by transparent tissue.
Ecology
Occurs on sand, muddy sand, Laminaria saccharina kelp and gravel from the intertidal zone down to depths of 300 m (Thompson 1988, Høisæter 2009).
Geographical distribution
Occurs along the entire Norwegian coast, the British Isles, the Canary Islands and the Mediterranean Sea (Thompson 1988, Brattegard & Holthe 2001, Høisæter 2009).
Ventral view of shell (SEM), scale bar = 200 μm.
Dorsal view of shell (automontage picture), scale bar = 200 μm.
Apex of shell (SEM), scale bar = 20 μm.
From left to right: dorsal view of unpaired and paired gizzard plate, ventral view of paired plate (SEM), scale bar = 20 μm.
Male reproductive system, Scale bar = 100 μm. Abbreviations: pc: penis chamber, pr: prostate.
References
Brattegard T og Holthe T (2001). Distribution of marine, benthic macroogranisms in Norway. Research report for DN-2001-3. Directorate for Nature Management.
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.