Parasite-induced change in host behavior of a freshwater snail: parasitic manipulation or byproduct of infection?

Host behavioral changes due to parasitism are often assumed to be adaptations of the parasite. However, behavioral effects of parasites may be a generalized response to parasitism and only coincidentally beneficial for parasite transmission. For this reason, alternatives to the manipulation hypothesis should be tested. Previous work demonstrated that the trematode parasite Microphallus sp. influences the behavior of the snail Potamopyrgus antipodarum in a way that may increase the probability of transmission. Here I report work conducted to test alternatives to the manipulation hypothesis. In a field study, the effect of Microphallus on behavior was compared to that of two other castrating parasite groups to determine if the behavioral change is simply a byproduct of parasitism. Also, the foraging behaviors of infected and uninfected snails were examined in the presence and absence of food resources to determine if the hunger level of Microphallus-infected snails could account for the parasite-induced behavioral change. First, Microphallus-infected snails were found on top of rocks during the day less often than the two other parasite groups. This evidence suggests that the behavioral change caused by Microphallus is specific to Microphallus-infected snails. Second, Microphallusinfected snails responded to the lack of food differently from uninfected snails. Uninfected snails retreated to safer positions under rocks when the food source was removed from the top of the rocks, while Microphallus-infected snails remained on top of the rocks where the risk of consumption by the final host is greater. Taken together with previous studies, these results suggest that infection by Microphallus results in behavior that enhances parasite transmission.