(Fleming, 1823)
Full name (Van der Spoel et al., 1997): Limacina retroversa (Fleming, 1823) subsp. retroversa (Fleming, 1823) forma retroversa (Fleming, 1823)
Description
A small, shelled, pelagic thecosomatous pteropod with a left-coiled shell. The spire is moderately highly coiled. There is no umbilical keel. Transverse striae are present.
Radula formula 1-1-1 and it is composed of about nine rows [L.r.retroversa-r].
Shell small, transparent and spirally coiled. The number of whorls is 6-7. The last whorl is large, occupying 2/5 of the total shell, the suture is clear [Limacina spp.]. The shell aperture is not large and its border is irregularly rounded. The umbilicus is narrow but deep. A fine spiral striation, composed of small dots, is found especially on the last whorl. A wing protrusion is present [L.r.retroversa soft]. The operculum is sometimes lost in adults, it is oval spoon-shaped [L.r.retroversa-op].
Juveniles have a small left coiled shell.
Colour
The spire has more than seven colourless or brown-purple transparent whorls.
Size
Shell height about 2.5 mm, maximum diameter 3 mm, average diameter 2.6 mm.
Depth range
The forma retroversa occupies only a small area in the N Atlantic, over deep, as well as, over shallow water.
Remarks
Limacina retroversa retroversa is commonly found in colder waters with a temperature range: 2-15°C and salinity range: 30-36äS. It feeds on phytoplankton and it is a mucus feeder.
The forma Limacina retroversa balea is described elsewhere: Limacina retroversa balea. This forma (balea) is most widely distributed over deep water and shows a great dependence on currents.
Distribution in the North Sea
N and S North Sea, Skagerrak.
World distribution
North Atlantic Ocean, between 40° and 70°N.
Remark
Two forms of this species can be recognised: the formae Limacina retroversa retroversa and Limacina retroversa balea (see alsoLimacina spp. ). Both forms likely have a distinct dispersal range.
For the forma retroversa the maximum temperature range is normally 15°C. As to the lower temperature limit, vertical migrations seem also to be limited by a 2°C layer, and the forma does not penetrate into this colder layer even when this layer lies near the surface.
The forma retroversa shows a rather symmetrical distribution pattern between 50°N and 65°N. In the W-Atlantic retroversa is found living more northward than balea, in the E-Atlantic more southward. It appears that retroversa is more influenced by currents than balea.
Forma balea is mostly caught in the northern part of the North Sea and over deep water west of Ireland, but large numbers were also collected over or near the continental shelf. The salinity range for balea is 31.8ä-35.00ä with an optimum of 32ä, the temperature range is 11°C-18°C, exceptionally 2.6°C. It may be that certain populations change from an oceanic to a neritic way of life. Currents may transport specimens from an oceanic stock population nearer to the coast. The distribution of the forma balea ranges to about 30°N south, due to the Labrador Current, and it is found as far north as 75°N, due to the Gulf stream in the E-Atlantic.
[After Van der Spoel et al., 1997]