Summary1. Freshwater snails often lack visible growths of algae on their shells. We tested three possible mechanisms that may account for this (grazing, snail‐derived nutrients and chemical defences), using the ramshorn snail Helisoma trivolvis.2. The experiments were carried out in floating plastic enclosures in a pond and comprised seven treatments. Grazing treatments were: a lone snail (ungrazed, as self‐grazing does not occur), Helisoma with conspecifics, Helisoma with the co‐occurring pond snail Physa sp., empty shells with Helisoma, and ungrazed empty shells. Nutrient effects were possible in all treatments with occupied shells (lone snail; Helisoma with conspecifics, and with Physa) versus absent in other treatments. Testing for chemical defences compared algae on fresh empty shells, weathered shells (outer organic periostracum layer absent) and boiled fresh shells (with denaturization of susceptible proteins).3. Diatoms dominated algal assemblages on snail shells. Although the upright diatom Gomphonema gracile was abundant on all shells, it was dominant on the shells of snails housed with other snail grazers (either Helisoma or Physa).4. Only the lone snail (nutrients but no grazing) showed higher algal biomass, so presumably any nutrient effect in the treatments with grazers was masked. Both Helisoma and Physa were observed apparently grazing on Helisoma shells, and consequently algal biomass in multi‐snail treatments was similar to that on empty shells. Scanning electron microscopy revealed that algal density was highest near the aperture of live snails, but not empty shells; this is consistent with a nutrient addition effect. There was no evidence of chemical defences against algal growth.5. In soft‐bottomed freshwater habitats with abundant snails, shells of living snails provide nutrient‐augmented substrata that may indirectly boost overall snail production.