THE ECOLOGICAL INTERACTIONS BETWEEN A MICROSPORIDIAN PARASITE AND ITS HOST DAPHNIA-MAGNA

1. Freshwater plankton populations suffer frequent epidemics of microparasitic diseases. The mechanisms which lead to outbreak and spread of these parasites are poorly understood. A set of experiments was carried out to distinguish between hypotheses explaining the introduction, spread and persistence of a microparasite in Daphnia magna. 2. Transmission of the microsporidian parasite Pleistophora a intestinalis is horizontal through ingestion of free-floating parasite spores, or from spores taken up from pond sediments by the host. At 4 degrees C parasite spores remained infectious after 3 months, explaining how the parasite persists through periods of host diapause. 3. Parasite transmission probability was inversely related to the water volume in which infected and uninfected hosts were kept. The same density effect was found for the intensity of infections. 4. Host nutritional conditions did not influence parasite multiplication inside the host. However, well-fed hosts became infected more often than poorly fed daphnids of the same age, which can best be explained by their larger size, and consequently their higher filtering rates. 5. Both sexes and all life stages tested of the host were susceptible to infection. Growth and transmission of the parasite was greatly impaired at 6 degrees C, but no differences in parasite growth were found between 12, 16, 20 and 23 degrees C. 6. The impact of P. intestinalis on host fecundity was inversely correlated with initial spore dose. 7. Long spore survival outside the host, reduced transmission at low temperatures and density-dependent transmission were the main factors in the interaction of P. intestinalis and its host D. magna. The results are consistent with field studies of other horizontally transmitted microsporidian parasites in cladoceran and rotifer populations.