Rissoa membranacea

(Adams, 1800)

Description (shell):
Shell form is extremely variable in this species and several varieties have been described. Some variability relates in part to environmental conditions, though it is likely that R. membranacea is an aggregate species of which some of the major varieties are segregates.
Shell thin, with eight to nine flat-sided, or very slightly tumid whorls. Last whorl occupies about 60 % of total height; apex slightly pointed. Ornament of fine growth lines, microscopic spiral striae, and sinuous costae with shallow profile, generally confined to last whorl (up to ten costae) and extending as far as periphery. Aperture somewhat triangular, with thin, everted peristome and broad, white labial costa; columella often with slight tooth-like prominence opposite umbilical region; inner lip reflected over umbilical groove.

var. labiosa (Montagu):
More solid with five to seven flattened whorls. Last whorl 66-70 % of shell height, with 15-18 costae; tooth on columella pronounced, throat thickened (R. membranacea var. labiosa). Southern Britain, in more marine situations (salinity 20-35 ä).

var. octona (Nilsson):
Slender shell with long spire of to 12 tumid whorls; last whorl with ten costae, or none, comprising 40 % of shell height. Costae, when present, may be found on all but most apical whorls. Aperture oval without thickening of throat; columella tooth poorly developed. Northern Britain, in brackish waters (salinity 12-16 ä).

var. cornea (Lovén):
Shell small, thin, with five or six slightly tumid whorls; last whorl approx. 60 % of shell height, lightly costate. Aperture oval; columella tooth clear; throat thickened. Northern Britain in brackish waters (salinity 6-7 ä).

Up to 9 x 3 mm.

Light buff, greenish, or pale horn coloured with longitudinal yellow-brown markings. In costate shells darker markings lie between costae; apical region often tinged with violet.

One metabolical tentacle; colour cream, sometimes light brown. Cephalic and metabolical tentacles are white and there is white around the eyes. Foot is dark laterally and on the opercular lobes.

All varieties are associated with Zostera , or weeds of similar form, at LWST and below, and are tolerant of a wide range of salinities (down to 7 ä).

Distributed from Shetland to Canary Islands (Distr. R. membranacea).