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.

Parasites are generally thought of as being intimately tied to their hosts, yet many parasites produce free-living stages. This raises the question: what are parasites doing when they are not being parasitic? We studied the spatiotemporal dynamics of free-living infectious stages and asked whether these dynamics were correlated with infections in 2 focal host species. We used a common and virulent bacterial parasite, Spirobacillus cienkowskii, which infects Daphnia spp. Densities of free-living infective stages were high in a stratified, eutrophic lake (up to ~10 5 to 10 6 cells l -1 ), but also spatiotemporally variable. There was a positive correlation between the density of these freeliving stages and the prevalence of infected Daphnia on the subsequent sampling date. This suggests that free-living stages increase in environmental reservoirs prior to the start of epidemics in Daphnia. We also studied the ability of free-living stages to persist outside their hosts for long periods of time.In laboratory microcosms, we found that S. cienkowskii persisted under simulated environmental conditions for more than 2 mo, before declining to below our method's detection limit after approximately 3 mo. Overall, our study of a common parasite of Daphnia reveals the potential importance of free-living stages to epidemic dynamics, and suggests that it is important to consider environmental reservoirs when studying disease dynamics.KEY WORDS: Spores · Daphnia dentifera · Daphnia pulicaria · Curse of the pharaoh · qPCR · Environmental transmission · Pathogen Resale or republication not permitted without written consent of the publisher Editorial responsibility: Urania Christaki,