The effects of size, reproductive condition, and parasitism on foraging behaviour in a freshwater snail, Potamopyrgus antipodarum

Abstract

Predation risk is expected to affect the foraging behaviour of individual prey according to their size, sex and reproductive condition. Parasites may also affect the foraging behaviour of their hosts, due to increased energetic demands imposed by the parasite, or due to a parasite strategy to increase the likelihood of its transmission to the next host. Field studies were conducted to determine the effects of size, reproductive condition and trematode infection on foraging behaviour in a freshwater snail, Potamopyrgus antipodarum. Among uninfected snails, small individuals (<3.8 mm in length) foraged more than large individuals during daylight hours when the risk of predation is greatest; non-brooding adult females foraged more during the day than females that were brooding embryos. These results suggest that juveniles and non-brooding females trade off the risk of predation with the energy required for growth and reproduction, respectively. Infection by a digenetic trematode ( Microphallussp.) also caused significant changes in snail behaviour. This trematode has a two-host life cycle. Snails are the intermediate host, and ducks are the final host. Encysted parasite larvae are transmitted to ducks when they ingest snails infected with mature larval cysts. Results showed that snails infected by encysted, transmissible larvae foraged in the early morning hours, and retreated under rocks during the late morning. In contrast, snails infected by unencysted non-transmissible larvae retreated to positions under rocks in the early morning, and mirrored the risk-averse strategy of brooding females. These results suggest that, when the parasite larvae encyst and become transmissible to the final host, they induce the snails to forage later into the morning. Because foraging by waterfowl is greatest in the early morning, encysted larvae may manipulate snail behaviour to increase the likelihood of their transmission to the final host.