Three's a crowd: The relationship among endoparasites, an epibiont and their Daphnia host

In freshwater communities, organisms interact in a variety of ways, including predation, competition and parasitism. Parasites are ubiquitous, playing an important role in shaping freshwater communities. Endoparasites live within internal organs of their host, while ectoparasites, also known as epibionts, are confined to the external part of the host's body. We conducted a series of experiments to examine the relationship between endoparasites and epibionts using the crustacean Daphnia magna as host, the rotifer Brachionus rubens as epibiont and three species of endoparasites. First, we tested host preference of the epibiont between Daphnia infected by endoparasites and uninfected Daphnia. Epibionts were found to attach more to uninfected Daphnia than to Daphnia infected by the yeast Metschnikowia bicuspidata. On the other hand, epibionts attached more to Daphnia infected by the microsporidium Hamiltosporidium tvaerminnensis than to uninfected Daphnia. Second, we examined the effects of epibionts on the infection of Daphnia by endoparasites. Infection prevalence tended to be higher, though not significantly, in the presence of epibionts. For two of the endoparasites, M. bicuspidata and H. tvaerminnensis, infection intensity (i.e. parasite spore production) was higher in the presence of epibionts. The infection intensity of M. bicuspidata and the bacterium Pasteuria ramosa was affected by the time of death of the Daphnia (i.e. virulence). Finally, we examined the effects of endoparasites and epibionts on the survival and offspring production of the Daphnia. Both host survival and offspring production were negatively affected by the endoparasites, while epibionts did not seem to affect the fitness of their host.