Field observations and laboratory experiments on the general food requirements of two species of freshwater snail, planorbis contortus (Linn.) and anctlus. fluviatilis mull.

A combination of field observation and laboratory experimentation has allowed a categorical definition of the dietary modes of P. contortus and A. fluviatilis. P. contortus is a detritivore which ingests the detrital material attached to the undersides of submerged stones. It may also live in weed beds where it continues its detrital habits. There is some evidence to suggest that this species is able to digest the bacterial fraction of its food. A. fluviatilis is a microherbivore which usually ingests epilithic algae. In their absence, however, lichen may be used although it results in less rapid growth and must be considered as a less rich energy source. The experimental evidence suggested that A. fluvialitis seeks food by random movement and contact chemoreception. The response of P. contortus is more prompt and probably involves distance chemoreception. The latter may be related to the utilization of bacteria.