Freshwater snails often lack visible growths of algae on their shells. We tested three possible mechanisms (grazing, snail-derived nutrients, and chemical defence of shells) that might affect algal biomass on shells of the ramshorn snail Helisoma trivolvis. The experiment was housed in floating plastic enclosures in a pond and included seven treatments. Grazing treatments were: a lone snail (ungrazed shell), Helisoma with conspecifics, Helisoma with the pond snail Physa sp., empty shells with Helisoma, and ungrazed empty shells. Nutrient effects were possible in treatments with occupied shells versus absent in treatments with empty shells. Testing for chemical defence compared algae on weathered shells, and fresh and boiled shells. Diatoms dominated the shell algal assemblages. Localized nutrients increased algal biomass on occupied but ungrazed shells (the lone snail treatment). Shell grazing by other snails was common and reduced algal biomass, masking nutrient effects. There was no evidence of chemical defence of the shell. In soft-bottomed freshwater habitats with abundant snails, shells of living snails provide nutrient-augmented substrates that increase grazing area and may boost overall snail production.