BIERBOWER, SM*; SPARKES, TC; DePaul University; DePaul University: Parasite-mediated sexual selection in an intermediate host, Caecidotea intermedius (Isopoda): effects of sperm production and male-male competition

The acanthocephalan parasite Acanthocephalus dirus infects the aquatic isopod Caecidotea intermedius (intermediate host) and one of several freshwater fishes (e.g., sunfish) as a definitive host. Inside C. intermedius, the parasite develops through non-infective stages before reaching the cystacanth stage, which is infective to definitive hosts. Previous work has shown that development into the cystacanth stage correlates with a decrease in male pairing success and that this effect can be explained by cystacanth-related changes in male mating behavior. Specifically, infected males are less likely to initiate mating attempts with females than uninfected males. Here, we examined whether this effect could be explained by either cystacanth-related variation in male sperm production or cystacanth-related changes in the dynamics of male-male competition. Using a laboratory-based experiment, we showed that variation in sperm production could not explain the effect because cystacanth-infected males contained viable sperm and could fertilize a female�s entire clutch. Using a field-based experiment, we showed that male-male competition was also unlikely to explain the effect because cystacanth-infected males showed the same behavioral response to females in non-competitive situations that they had shown previously in competitive situations. We conclude that variation in both sperm production and male-male competition were unlikely to explain the cystacanth-related decrease in male mating success observed in nature and propose alternative mechanisms to explain parasite-mediated sexual selection in C. intermedius.