Parasites as prey: Daphnia reduce transmission success of an oomycete brood parasite in the calanoid copepod Boeckella

Environmental conditions can impact host-parasite dynamics in aquatic ecosystems through their influence on the survival and infectivity of free-living transmission stages. The challenge in predicting which conditions will lead to which outcomes (e.g. development of epidemics) is exacerbated by the fact that results are often specific to the waterbody, host and/or parasite. This study investigated the extent to which abiotic and biotic factors affected the transmission success of Aphanomyces, a freshwater oomycete brood parasite. During routine sampling of the copepod Boeckella hamata from a drinking water reservoir, we observed that this population was heavily infected by Aphanomyces during the winter and spring seasons, with prevalence exceeding 80%. We tested the effects of temperature, light and the presence of Daphnia on infection probability in experimental populations of B. hamata. Changes in temperature or light levels did not alter parasite prevalence. However, the presence of Daphnia significantly reduced the proportion of infected copepods in experimental treatments. This study confirms previous work in other host-parasite systems that Daphnia can reduce prevalence through feeding on infectious spores. This 'friendly competition' may offset any negative impacts Daphnia may have on copepods due to competition for shared resources. This study demonstrates the importance of understanding how community composition may impair parasite transmission.