DIFFERENTIAL BEHAVIOR AND SHIFTS IN GENOTYPE COMPOSITION DURING THE BEGINNING OF A SEASONAL PERIOD OF DIEL VERTICAL MIGRATION

During the first few weeks of a recurring seasonal period of diel vert ical migration in Lake Maarsseveen (The Netherlands), part of the hybr id Daphnia galeata x hyalina population migrated, while another part r emained in the epilimnion. In the epilimnion, 0+ perch prey upon daphn ids during daytime. Gradually, the number of adult Daphnia in the epil imnion decrease until the epilimnion is nearly devoid of daphnids. The population as a whole may decrease, as in 1991, or may increase as in 1992. Genotype composition, as determined by allozyme analysis, chang ed substantially within a fortnight in 1992, and one genotype became d ominant. Our data are in agreement with the hypothesis that predation on different genotypes (clones) occurs during the beginning of a seaso nal period of diel vertical migration, though our data do not allow to exclude alternative explanations.