Prevalence, Host Specificity and Impact on Host Fecundity of Microparasites and Epibionts in Three Sympatric Daphnia Species

Abstract

1. Recent reports indicate that microparasites might play an important role in the ecology and evolution of freshwater zooplankton. To quantify the impact of parasitism on Daphnia in natural populations we studied the microparasites of D. magna, D. pulex and D. longispina in three ponds in southern England for a 1-year period. Parasite prevalence and richness, epibiont prevalence and density of host food (planktonic algae) were quantified and their effect on host fecundity analysed. 2. Seventeen parasite species were detected, including eight microsporidia, one haplosporidium, one ameoba, three fungi, three bacteria and one unidentified parasite. The mean proportion of infected adults across the year and the three ponds was 84.7% for D. magna, 53.6% for D. pulex and 38.6% for D. longispina for the 11 most common parasites. Body size was generally positively correlated with parasite prevalence and richness. This relation was found both across and within host species. Multiple infections by different parasite species were common. 3. In all three Daphnia species, parasitism was associated with a reduction in fecundity. The small D. longispina was less affected per parasite compared to the two larger host species, D. magna and D. pulex. Clutch presence was more strongly reduced than clutch size, and the effect increased with parasite richness. The data suggest that parasites castrated their host completely, rather than reducing clutch size. 4. Host specificity varied between parasite species. Two parasite species infected only one or two of the three hosts investigated, and six parasite species showed differences in their specificity between ponds. 5. Four different epibiont groups were distinguished; algae, Protozoa, one fungus and one bacterium. Epibiont prevalence differed between ponds and species. Overall, epibionts were not host specific, but intensity of infection differed strongly between species and the three ponds. Epibiont infection was independent of parasite infection. The presence of epibiontic algae reduced clutch size in D. magna significantly. 6. Our results suggest that parasites can have important effects on Daphnia ecology and evolution. Differential susceptibility of the three host species to parasites suggests that parasites might play a role in determining competition and coexistence in our three study sites.