Parasites play a central role in ecology and evolutionary biology. They can alter gene frequencies, population and meta-population dynamics, and community structure of their hosts, but the magnitudes of these effects often vary among host-parasite interactions. Daphnia pulicaria (Crustacea: cladoceran), a dominant zooplankter in permanent lakes of the Midwestern United States, is host to a multitude of micro-parasites including fungi, bacteria and microsporidia. In Michigan and Illinois D. pulicaria is commonly infected with the bacterium Spirobacillus cienkowskii. Here we examine the effects of this parasite on D. pulicaria life history and ask how host genotypes differ in their susceptibility to S. cienkowskii. Both hosts and parasites were isolated from a permanent, stratified, eutrophic lake at Kickapoo State Park, Vermillion County, IL. Five replicates of nine host genotypes were either exposed to S. cienkowskii spores or left unexposed. Age at maturity, age-specific fecundity, and time until death were observed. We then asked how risk of infection differed with age. To do so, we used all genotypes that showed susceptibility to infection in the first experiment and infected healthy animals at day 1, 5 or 10 of life.

Results/Conclusions

Animals exposed to S. cienkowskii became heavily infected. Infection caused complete castration, a finding that is supported by field observations that all infected females were barren. There was also a significant decline in the lifespan of infected hosts compared to healthy animals of the same genotype. Although both experiments are ongoing, the preliminary findings demonstrate significant effects on D. pulicaria life history. The results of the age-specific infection experiment will help to quantify the role of population age structure on the spread of disease.