Diversity of Daphnia galeata life history traits in a vertically structured environment

Abstract

We used laboratory experiments to investigate the life history traits of clonal lineages of Daphnia galeata isolated from two vertically segregated subpopulations in a deep dimictic reservoir: one from the epilimnion and the other from the deep hypolimnion. We collected clones twice for the experiments, first at the beginning of thermal stratification in May and then towards the end of the season in September. The results for May showed longer postembryonic development (PED), bigger eggs in the first clutch and slightly higher somatic increments in the hypolimnetic clones. In addition, a remarkable tendency to produce ephippia was recorded in the hypolimnetic clones, but not in the epilimnetic clones. In September, the results were reversed—hypolimnetic clones had shorter PED, lower somatic increments and a slightly higher number of eggs in the first clutch. Our results suggest a differentiation of life history traits in the two D. galeata populations inhabiting contrasting microhabitats of a reservoir as a result of (i) different depth preferences of certain clones in the beginning of thermal stratification in spring and (ii) divergent selection processes due to segregation in different environmental conditions. The ability of Daphnia to live in diverse environments, causing divergent selection processes, enhances the overall phenotypic and genotypic diversity of the whole reservoir population.