GENETIC, ACCLIMATIZATION, AND ONTOGENETIC EFFECTS ON HABITAT SELECTION BEHAVIOR IN DAPHNIA PULICARIA.

Daphnia pulicaria from three different populations were observed to express within-population variation in habitat-choice behavior in field assays. Individuals from different habitats (i.e., lake depths) were isolated and cultured as clonal lines under standard conditions. Habitat choices by clonal descendants were then estimated in the field, using replicate experimental columns. There was significant heritable and ontogenetic variation in habitat choice, but the heritable effect was small relative to the phenotypic variation of the original isolates. In a second set of experiments, D. pulicaria that were acclimatized to different habitats showed a strong tendency to choose the habitat to which they had been acclimatized. These data suggest that a given genotype can use a wide range of habitats, given appropriate acclimatization. Although genetic variation is significant, we hypothesize that natural selection on correlated ecological traits is more likely to maintain patterns of genotypic segregation among habitats in Daphnia.