Plasticity in filtering screens of Daphnia cucullata x galeata hybrids andparental species at two food concentrations

The coexistence of Daphnia cucullata x galeata hybrids with the parental sp ecies D. galeata and D. cucullata was investigated by measuring areas and m esh sizes of filtering structures of these herbivorous zooplankton taxa cul tivated at low and high food concentrations. The clearance rates and somati c growth rates were also determined. When reared at low food concentration, all taxa had larger filtering areas. Larger filtering areas also resulted in higher clearance rates. Differences between taxa in both filtering area and clearance rate were caused mainly by interspecific size differences. Hy brids had the largest absolute mesh sizes, and the parental species had sma ller mesh sizes. Hybrids also showed heterosis in somatic growth rate at hi gh food concentration. The observed taxon-specific differences in mesh size and somatic growth rate contribute to resource partitioning between the ta xa and thus to their successful coexistence in lakes.