Ecological interactions of the microparasite Caullerya mesnili and its host Daphnia galeata

It has been repeatedly suggested that parasites play an important role in the ecology and evolution of Daphnia populations: however, little is known about Daphnia-parasite interactions in lakes with vertebrate predation on Daphnia. Between September 1997 and April 1998, an epidemic of the protist parasite Caullerya mesnili in Daphnia galeata and Daphnia hyalina occurred in Lake Constance (Germany), infecting up to 50% of all individuals. Using laboratory experiments, we investigated the epidemiological interactions between this parasite and its host D. galeata at the individual and the population level. C. mesnili was found to be transmitted directly and horizontally through waterborne infection stages. Transmission of the parasite was dependent on the host density, and all life stages of female D. galeata were susceptible to infection. In a life table experiment at low and high food levels, the life expectancy and fecundity of infected D. galeata were dramatically reduced at both food levels as compared to the uninfected controls. Additionally, we found a significant interaction between infection and food level, indicating a stronger parasitic effect in well-fed hosts. To test the effects of the parasite at the population level, we compared the size of D. galeata populations infected with C. mesnili with the size of parasite-free microcosm populations. The population size of infected D. galeata was significantly lower than that of uninfected populations after 4 weeks. In all four infected replicate populations, the parasite drove the host population to extinction.