Sulphide-toxic habitats are not refuges from parasite infections in an extremophile fish

We still have a limited understanding of why animals colonize extreme environments, but one hypothesis (the Pathogen Refuge Hypothesis, PRH) posits that they might represent a safe haven from parasites and other pathogens. We tested this idea by quantifying endoparasite infections in five populations of the neotropical freshwater fish Poecilia mexicana, three populations from toxic, H2S-rich habitats and two control populations from nearby rivers and creeks. Counter to predictions derived from the PRH, only P. mexicana from toxic habitats harboured heterophyid metacercariae in their hearts. We further uncovered differences in parasite prevalences and abundances between the three toxic habitats. Our results provide evidence against the PRH and add depth to our understanding of life in (and adaptation to) extreme environments.