Microsporidian parasites are ubiquitous in aquatic ecosystems and infect a broad range of animal phyla. We recently discovered a microsporidian that infects mottled sculpin (Cottus bairdi) and is characterized by multiple xenomas (parasite filled cysts) in the body cavity of infected fish. Mottled sculpin are important top predators in trout streams and have strong effects on invertebrate community structure. We examined parasite prevalence and fitness effects to determine if this disease has the potential to have indirect effects on benthic community structure. 
Results/Conclusions

We used PCR and DNA sequencing to identify the parasite. It is likely a member of the genus Glugea, but appears to be a previously unreported species. We sampled seven southwest Michigan trout streams to determine the prevalence and fitness effects of Glugea in mottled sculpin populations. Infected sculpin were present in five of the sampled populations and prevalence exceeded 90% in three populations. Xenomas ranged in size from minute to 1 cm in diameter and were frequently associated with the body wall, fat body, kidneys, or gonads. Infected individuals had visibly less fat body than uninfected fish. Xenoma mass was up to 5% of the host mass in heavy infections. In both males and females xenomas sometimes filled the gonads to exclusion of eggs or sperm. Among infected females, 37% had xenomas filling space within the ovary. The testes were affected in 17% of infected males.  Preliminary results suggest that infection has negative effects on female fitness. Given the high prevalence of this parasite and negative fitness effects, it is possible that the disease mediates the impact of sculpin on the benthic community in some systems.