Biotic interactions and the maintenance of a Daphnia hybrid species complex

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Hybrids of the water flea Daphnia occur in many lakes. However, little is known about the factors that cause the success of Daphnia hybrids. In a joint project of two laboratories (University Konstanz D, EAWAG Dübendorf CH), we want to study the possible role of biotic interactions in the maintenance of a Daphnia hybrid complex. Daphnia hyalina, D. galeata and their hybrids occur in Bodensee (German-Swiss-Austrian Border) and in Greifensee (Switzerland). The parent species are more abundant in Lake Constance, while hybrids dominate in Greifensee. Both lakes differ in important aspects (morphometry, trophic state), which is reflected by different biotic influences on Daphnia. Compared to Lake Constance, Daphnia in Greifensee are more often exposed to low quality food (toxic blue-greens) and have less of a refuge from fish predation (due the anoxic hypolimnion). Differences between both lakes in the invertebrate predation regime and in parasite load of Daphnia are very probable. Besides field sampling programmes, we want to establish a collection of about 50 Daphnia clones from both lakes (parent species and hybrids, recent clones and old clones hatched from sediment cores). These clones will be used for life history experiments in the laboratory which test the influences of low quality food. of fish kairomones, of invertebrate predator kairomones and of a protozoan parasite. Food quality and invertebrate predator experiments will be done in Konstanz; parasite and fish experiments will be done in Dübendorf. (p)