Diagnostic characters
A very variable shell which may be thin and transparent or thick and opaque; have a tall, slender shape or be short and broad, but which nearly always shows an aperture with flared lips and sometimes partly blocked by internal thickenings. Costae and labial varix present or absent. One metapodial tentacle. In brackish or fully marine habitats.
Other characters
The appearance of the shell depends on the habitat: shells from brackish water tend to be tall (8-9 whorls), slender (apical angle 35-45°), semitransparent, smooth or with up to ten costae on the last whorl, a thin varix or none on the outer lip, an oval aperture, and slight or no thickenings within the outer lip and on the columella. Shells from marine waters tend to be stubbier (5-7 whorls, apical angle 45-55°), opaque, usually with costae (up to 18 on the last whorl), a marked varix, an angulated aperture with the throat often partially blocked by thickenings within the outer lip and on the columella. In all, the most apical whorls are smooth. Solid shells may be white, yellowish or greenish; semitransparent ones are horn-coloured, the tint in all due to the periostracum. Longitudinal brown marks may lie between costae, and a brown spiral band may encircle the shell base. The varix is pale, sometimes with a brown band alongside it. Up to 9 mm high, 3 mm broad. In slender shells the last whorl occupies 40-45% of shell height, the aperture one third; in more squat ones the last whorl occupies two thirds and the aperture one half of the height.
The animal is cream, sometimes light brown; cephalic and metapodial tentacles are white, and there is white round the eyes; the foot is dark laterally and on the opercular lobes.
R. membranacea is typically associated with Zostera but also lives on other weeds at L.W.S.T. and below. It tolerates salinities down to 7%o. It is moderately frequent throughout the British Isles, except on the east coast of Scotland. Abroad it ranges from the Canary Islands to Norway and extends some way into the Baltic. In the southern parts of the range the animals may breed throughout the year, but in the north they are summer breeders. The egg capsules are hemispherical, clear, and commonly about 1 mm in diameter: they are fastened to weeds. The thickness of the walls varies with locality, thin in marine, thicker in brackish localities. The number of eggs per capsule also varies from 40-400, dependent upon the habitat. In some capsules all the eggs develop to free-swimming veligers, in others some eggs are eaten by the more precocious embryos, the larval stage is omitted and the young are born as juveniles (Rehfeldt, 1968; Rasmussen, 1973). The significance of this is not known.
In view of the great variation in the shell characters and reproductive behaviour it is not clear whether we are dealing with a single species which happens to be very sensitive to slight environmental changes, or whether we have an aggregate of sibling species susceptible of division when our know¬ledge of these is more complete.