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

The coexistence of Daphnia cucullata x galeata hybrids with the parental species D. galeata and D. cucullata was investigated by measuring areas and mesh sizes of filtering structures of these herbivorous zooplankton taxa cultivated at low and high food concentrations. The clearance rates and somatic 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. Hybrids had the largest absolute mesh sizes, and the parental species had smaller mesh sizes. Hybrids also showed heterosis in somatic growth rate at high food concentration. The observed taxon-specific differences in mesh size and somatic growth rate contribute to resource partitioning between the taxa and thus to their successful coexistence in lakes. [KEYWORDS: Daphnia; hybrids; feeding; plasticity; developmental instability Life-history variation; phenotypic plasticity; interspecific hybridization; developmental stability; mesh-sizes; heterozygosity; populations; adaptation; consequences; variability