Differential behaviour 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 vertical migration in Lake Maarsseveen (The Netherlands), part of the hybrid Daphnia galeata x hyalina population migrated, while another part remained in the epilimnion. In the epilimnion, 0+ perch prey upon daphnids during daytime. Gradually, the number of adult Daphnia in the epilimnion 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, changed substantially within a fortnight in 1992, and one genotype became dominant. Our data are in agreement with the hypothesis that predation on different genotypes (clones) occurs during the beginning of a seasonal period of diel vertical migration, though our data do not allow to exclude alternative explanations. [KEYWORDS: Daphnia; 0+ perch; genotypes; selective predation; allozyme analysis Daphnia-hyalina; lake maarsseveen; zooplankton; fish; evolution; magna; phototaxis; kairomones; mechanism]