The Influence of Non-Host Predators on Parasite-Induced Behavioral Changes in a Freshwater Snail

Abstract

Parasites can have striking influences on their host's behavior that serve to increase the probability of parasite transmission. The type of behavior induced often increases the probability of an encounter with the next host in the life cycle. However non-host predators could also serve as selective forces on the manipulative strategy of the parasite. In this study, I examined the daily foraging behavior of a New Zealand fish (Gobiomorphus cotidianus), and related it to the foraging habits of a snail (Potamopyrgus antipodarum) infected with the trematode Microphallus sp. Previous results have demonstrated that Microphallus alters the snails' behavior in a time-specific manner: infected individuals forage on the top of rocks late into the morning, when the final host (waterfowl) is feeding most, and then move to the bottom of the rocks. Since waterfowl continue to forage (albeit at a lower rate) throughout the day, the infected snails' movement to the bottom of the rocks is perplexing. Here I examined the gut contents of fish collected at different times of the day. I found that fish feed on snails predominantly in the late morning to early afternoon hours, which corresponds to the time that infected snails move to the bottom of the rocks. I also found that snails taken from fish guts are less likely to be infected by Microphallus than snails from the overall population. Thus it would appear that the behavior induced by Microphallus decreases the probability that an infected snail will be eaten by an unsuitable host.