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 adaptatio ns of the parasite. However behavioral effects of parasites may be a genera lized response to parasitism and only coincidentally beneficial for parasit e transmission. For this reason, alternatives to the manipulation hypothesi s should be tested. Previous work demonstrated that the trematode parasite Microphallus SD influences the behavior of the snail Potamopyrgus antipodar um in a way that may increase the probability of transmission. Here I repor t 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 infec ted 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, Microphal lus-infected snails were found on top of rocks during the day less often th an the two other parasite groups. This evidence suggests that the behaviora l change caused by Microphallus is specific to Microphallus-infected snails . Second, Microphallus-infected snails responded to the lack of food differ ently from uninfected snails. Uninfected snails retreated to safer position s under rocks when the food source was removed from the top of the rocks, w hile Microphallus-infected snails remained on top of the rocks where the ri sk of consumption by the final host is greater. Taken together with previou s studies, these results suggest that infection by Microphallus results in behavior that enhances parasite transmission.