MCCURDY, DG*; FORBES, MR; MAUTNER, SI; COOK, K; LOGAN, SL; LANCASTER, D; Albion College; Carleton University; Carleton University; Carleton University; Albion College; Albion College; ; : Are female-biased sex ratios in a key amphipod caused by male-biased predation or sex-distorting parasites?

The burrowing amphipod Corophium volutator is a key prey species for migratory shorebirds and benthic fish in north-Atlantic soft-bottom communities. At sites in the Bay of Fundy, Canada, Corophium populations are known to exhibit extreme female-biased sex ratios which may lead to male-limitation and impact recruitment in this species. We investigated two causes for these female-biased sex ratios: (1) the possibility that benthic fish feed disproportionately on male Corophium (predator hypothesis), and (2) the possibility that microsporidian parasites distort sex ratios of amphipods to increase parasite transmission rates (parasite hypothesis). From gut-content analysis of benthic fish, we found that fish fed more on male amphipods than expected based on samples of amphipods collected from within the substrate. However, using an empirically-derived model we also found that daily foraging pressure by fish was not sufficient to account for sex-ratio biases observed in natural populations of Corophium. Using PCR, we detected and characterized a novel microsporidian parasite found in female, but not in male gonads, consistent with the presence of sex-distorting parasites that are transmitted only through female hosts. Further, prevalence of this microsporidian was higher in Corophium populations that were more female-biased and infected females housed in the laboratory produced broods that were more female-biased than those produced by uninfected females. Both fish predators and microsporidian parasites have the potential to impact Corophium populations and demography by contributing to extreme female-biased sex-ratios that occur in this species.