Spatial variation in sex-biased parasitism of an androdioecious mangrove fish

Divergent selection between the sexes may enable parasites to better infect one host sex over the other, potentially leading to sex-biased parasitism. Hosts also typically harbor a component community of parasite species that can vary across a spatial scale. We explored spatial variation in sex-biased parasitism in mangrove rivulus (Kryptolebias marmoratus), an androdioecious fish in which simultaneous hermaphrodites can facultatively transition to male. Using cup traps deployed in the mangroves, we sampled populations that varied in sex ratio; three with relatively high and low percentages of males in Belize and the Florida Keys, respectively. Fish were sexed, and the external body and all organs examined for parasites using microscopy. Hermaphrodites had significantly higher parasite species diversity overall (Simpson's index). Sex differences existed for only one parasite species (heart-specific Ascocotyle sp. digenean metacercariae), with significantly male-biased parasitism in one Belizean population and significantly hermaphrodite-biased parasitism in another. Host mass also predicted the abundance of some parasite species independent of sex: larger individuals had higher abundances of gill-specific and heart-specific Ascocotyle sp. digenean metacercariae. Parasite species diversity (Simpson's index) and richness were higher in Belize than the Florida Keys. While males are often the more heavily parasitized sex, our study did not support this, and our results indicate that host-parasite interactions are driven by a complex relationship between host sex, body mass, geography, and parasite species.