Theodoxus fluviatilis

(Linné, 1758)

Description (shell):
Shell solid and glossy, hemispherical, with low spire. There are three whorls, the last whorl forming most of the shell (95 % of shell height). The sutures are of variable depth but distinct. The only ornament is fine growth lines. Aperture D-shaped, half blocked by an expansion of the columellar region. Aperture occupies 90 % of shell height. There is no umbilicus.

Up to 8 x 8 mm.

The shell is usually dark in colour with a pattern of yellow-white streaks, and there are often three darker areas separated by two lighter ones on the last whorl. The dark areas are subsutural, peripheral and basal in position. The columellar plate is white or yellow with a darker edge along the inner lip.

There is a broad snout, its edge lobulate. At its base lie the cephalic tentacles, capable of great extension and retraction. An eye on a separate eye stalk lies alongside each. The mantle edge is thick and papillated. In males a flattened penis with an open seminal groove lies behind the right tentacle. The foot has an epipodial fold on each side but this carries no tentacles; that on the left is joined to the eye stalk. The operculum, partly calcified, is D-shaped and has an internal peg. The flesh is white-yellow, with many black flecks, particularly on the snout, the mantle edge and the foot (T. fluviatilis-animal).

Lives mainly in rivers, hiding under stones or wood by day, foraging by night on plants and detritus. It also occurs in canals and the littoral region of lakes where there is movement in the water. The animals are limited to hard waters. They tolerate a moderately high salinity.

Occurs generally throughout Europe except for the most northern parts, the Iberian peninsula and the central Danube basin.