Theodoxus fluviatilis

Theodoxus fluviatilis, common name the river nerite, is a species of small freshwater and brackish water snail with a gill and an operculum, an aquatic gastropod mollusk in the family Neritidae, the nerites.

This widely distributed neritid snail species occurs from Europe to Central Asia. It has a thick shell with a calcified operculum. The coloration pattern on the shell is very variable. Theodoxus fluviatilis lives in freshwater and in brackish water, in rivers and lakes on stones. It feeds mainly by grazing on biofilms and diatoms.

Some of the populations of this species are spreading, and these can reach densities up to thousands of snails per square meter. Females lay egg capsules, each of which contains a large number of eggs, but only one snail hatches from the capsule. The snails reach sexual maturity in a year, and the total lifespan is 2 or 3 years.