As Hydrobia ulvae:
Diagnostic characters
Shell small, spire moderately high, whorls nearly flat-sided in profile, with brown periostracum; prosocline growth lines the only clear ornament. Aperture pointed apically. Snout with dark transverse line immediately behind a pale tip and (usually) dark lateral lines and (sometimes) a mid-dorsal one. A black transverse one on each cephalic tentacle a little behind its tip.
Other characters
The spire has 6-7 whorls and is a little cyrtoconoid. There is an umbilical chink in most shells. Up to 6mm high, 2.5-3 mm broad; last whorl occupies 60-70% of shell height, aperture about 40%.
The head has a long snout, bifid at its tip. The left tentacle is thicker than the right and shows different ciliary currents. The mantle skirt has a single pallial tentacle on the right. In the mantle cavity of males, there is a large penis shaped like a question mark: its tip is blunt and it shows a smoothly curved profile on the right anteriorly. The flesh is yellowish grey with black and bright yellow speckles. In addition to the colour pattern on the snout mentioned above there is a backwardly projecting, black, triangular area with its base between the tentacles. The eyes lie in a pale patch usually bordered anteriorly by a black streak.
H. ulvae occurs, often in enormous numbers, on wet banks of sand or mud, preferring estuarine conditions but occasionally on open coasts and liking habitats with moving tidal waters (Cherrill & James, 1985). It extends upwards from sublittoral levels and is commonest on the uppermost third of the beach. It also occurs on weeds and in salt marshes where the snails are often large (Chatfield, 1972). It eats diatoms collected from the surface of sand grains (Fenchel, 1975; Fenchel, Kofoed & Lappalainen, 1976), some bacteria (Jensen & Siegismund, 1980), silt, and such algae as Ulva and Enteromorpha, preferring the latter (Nicol, 1935). It feeds best on particles 40-80 mu in size and cannot apparently take in pieces greater than 210 (Lopez & Kofoed. 1980). According to Newell (1962) the animals exhibit a behaviour cycle related to feeding: at ebb tide the snails migrate seawards, feeding as they go; when the tide starts to flow they float to the surface and are carried back to their starting level. Vader (1964) and Barnes (1981a,b) have not confirmed this as a regular cycle of activity. The animals can live and breed between salinities 5-40%o. The species occurs throughout the British Isles and elsewhere between the north of Norway, all of the Baltic except the most eastern parts, the Atlantic coasts of Europe and of Africa to Senegal, and in the Mediterranean.
Breeding occurs in spring and summer, sometimes with early and late peaks. The eggs are laid in jelly masses attached to sand grains or other objects, mainly to the shells of other adults. The mass is hemispherical, contains commonly 3-7 colourless eggs although up to 70 have been recorded and gets covered with sand grains. Veliger larvae hatch in 2-3 weeks, but seem to vary from place to place in their length of life before settling (Pilkington, 1971; Fish & Fish, 1977). Adults may live up to 5 years (Quick, 1924; Anderson, 1971).
Vernacular name: mud snail.