Differential infection of exotic and native freshwater amphipods by a parasitic water mold in the St. Lawrence River

A parasitic water mold (oomycete) of unknown origin was discovered infecting live native and exotic freshwater gammarid amphipods (Gammarus fasciatus and Echinogammarus ischnus, respectively) in the upper St. Lawrence River. Infections were associated with rapid die-offs of natural populations of amphipods, especially the exotic E. ischnus. Analysis of sequences of three different segments of the rRNA gene cluster indicated that the parasite was in the Saprolegniaceae family, and is related to other crustacean-associated Saprolegniaceae. Specific primers were designed based on the SSU rRNA gene and utilized for semi-quantitative analysis of parasite presence in live and dead amphipods. In laboratory experiments, infection prevalence was higher in E. ischnus individuals than in native amphipods. In addition, dead E. ischnus individuals exhibited more intense infections than G. fasciatus individuals. In contrast to the Great Lakes where E. ischnus has replaced G. fasciatus at many locations, the native species remains abundant in the St. Lawrence River more than a decade after invasion by E. ischnus in the late 1990s. We hypothesize that the parasite is facilitating the co-existence of the two amphipod species by reducing the abundance of E. ischnus in environments in which it might otherwise be dominant.