Perceived predation risk, parasitism, and the foraging behavior of a freshwater snail (Potamopyrgus antipodarum)

Foraging behavior can be influenced by such factors as predation risk, indi vidual size, and parasite infection. Snails (Potamopyrgus antipodarum) plac ed in tanks with large rocks were exposed to four types of water: (I) water with crushed snails, (2) water from a tank in which fish (Gobiomorphus cot idianus) were fed only trout chow, (3) water from a tank where the fish wer e also fed snails, and (4) plain water. Snails could respond by moving to t he top of rocks (where algal food was present) or to the bottom of rocks (w here the predation risk was lower). The snails responded to fish chemicals by moving to the bottom of rocks. The response was dependent on snail size and fish diet. Smaller snails moved to the bottom of rocks more than larger snails did. Trematode-infected snails were found on top of the rocks more than other classes of snails, but infected snails still moved to the bottom of rocks in response to the fish predator. Snails eaten by fish in the fie ld tend to be smaller than snails in the overall available population. Thus , snails that are more vulnerable to predation respond more intensely to th e odor of fish by moving to the bottom of racks. This size-dependent respon se to fish appears to be independent of the occurrence of trematode infecti on.