The food of Ancylus fluviatilis (Müll.), a littoral stone-dwelling, herbivore.

A. fluviatilis is a herbivore which usually ingests periphyton. Evidence derived from both field observation and laboratory experiments suggests that the limpet prefers diatoms and that within this group Gomphonema is apparently the most attractive genus. These food preferences were most clearly defined in satiated individuals.Snails had no apparent effect on algal community structures in the field situation and it is suggested that this may either result from adaptability of the periphyton with respect to grazing disturbances, or from low snail density and thus low grazing intensity.The food preferences of A. fluviatilis in particular, and other algal exploiters in general, are discussed in terms of plant defense mechanisms. It is argued that an animal's ability to cope with algal cell walls (either in a mechanical or chemical sense) may be one of the major determinants in the food preference behaviour shown by herbivores feeding on algae.