Dupont, 1979
Described form occurring in the area (Van der Spoel et al., 1997): Diacria trispinosa (De Blainville, 1821) forma atlantica Dupont, 1979
Description
A medium sized, pelagic snail with typical colour patterns, shell 1 cm broad. The slender shell is uncoiled but bilateral symmetrical with a long, sometimes broken off, caudal spine. The rim of the aperture and the middle of the dorsal ribs is brown. It is dark brown anteriorly, the posterior part is light brown, the spines are white.
Relatively large shell, the lateral spines are straight, the caudal spine [D.t.atlantica-caud] is proportionally short, the shell aperture is wide. It is dark brown anteriorly, the posterior part is light brown, the spines are white [Diacria t. atlantica D.t.atlantica-dr]. The caudal membrane is placed far posteriorly which makes the caudal spine short in relation to the teleoconch.
Juveniles have a long dorso-lateral flattened needle-like shell with lateral sides diverging only anteriorly. The protoconch I is globular. There is no sculpture on protoconch I or II. The lateral sides of protoconch II are thickened.
Size
Shell length up to 9 mm, width up to 10.5 mm.
Ecology
This animals are phytophagous and epipelagic in cold water. It feeds on small plankton mainly phytoplankton in the epi- and mesopelagic zone [D.t.atlantica-sw]. It swims with two wings and sinks with retracted wings. The shell is frequently covered by hydropolyps. [D.t.atlantica-hyd].
Depth range
Epi- to mesopelagic depths.
Distribution in the North Sea
Might be found in the northern North Sea as an occasional guest from the NE Atlantic.
World distribution
This forma is restricted to the N-Atlantic Ocean between 40° and 70°N.
[After Van der Spoel et al., 1997]