River Nerite
Theodoxus fluviatilis
The River Nerite is a small, thick-shelled freshwater snail with a characteristic hemispherical shape and variable reticulate or zigzag patterns. It primarily inhabits clean, oxygen-rich running waters and the wave-swept zones of large lakes, adhering to hard substrates like stones or submerged wood. As a grazer, it feeds on biofilms and diatoms and is highly adapted to strong currents. The species is dioecious and lays egg capsules from which only a single juvenile eventually emerges.
Details
Identification
Hemispherical, very thick shell; aperture with a crescent-shaped lid (operculum); variable patterning (mostly dark with light spots); flat underside.
Social behavior
Often occurs gregariously in high densities on suitable stones, though it does not exhibit complex social behavior.
Diet
Uses its radula to graze on algal growth, especially diatoms, and organic biofilms from stones.
Spawning substrate
Hard surfaces such as stones, submerged wood, or the shells of conspecifics.
Overwintering
Remains at the bottom of the water body and reduces metabolic activity during low temperatures.
Ecology
Ecological role
Important primary consumer that regulates algal growth on hard substrates and serves as a food source for fish.
Natural predators
Fish (e.g., carp, tench, gobies), water birds, and predatory insect larvae.
Competitor species
Other grazing snails such as the New Zealand mud snail (Potamopyrgus antipodarum).
Ecosystem service
Contributes to water purification by controlling algal films and cycling nutrients within the food web.
Threats
River engineering, siltation of the riverbed, eutrophication, and the spread of invasive species like the Round Goby.
Scientific profile
Profile
Distinguishing features
The most important feature is the operculum, which has a strong internal process (apophysis). Unlike many other freshwater snails, it lacks an umbilicus. The shell surface is smooth and glossy. The shape is significantly flatter than that of Viviparidae or Lymnaeidae.
Diet
Primarily diatoms, but also green algae, cyanobacteria, and detritus found in the benthic biofilm.
Reproduction
Dioecious. Females lay approx. 1 mm large, egg-shaped capsules on hard substrate (often on the shells of conspecifics). One capsule contains 30-70 eggs, but only a single embryo develops, while the remaining eggs serve as nurse eggs.
Role in food web
Important primary consumer in running waters; controls algal growth on hard substrates and serves as prey for specialized predators.
Protection & threats
Main threats
River engineering (bank stabilization, damming), siltation of the riverbed (clogging), chemical pollution (insecticides, herbicides), and competition from invasive species such as Potamopyrgus antipodarum.
Conservation measures
Restoration of natural flow dynamics, protection and restoration of gravel spawning grounds, reduction of fine sediment input from agriculture, improvement of longitudinal connectivity.