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We reared clones of the waterflea Daphnia galeata, a common grazer in many types of lakes, under several food regimes to study adaptations to feeding conditions in filter screen morphology and life history. As food regimes, we used low and high concentrations of the green alga Scenedesmus, a high concentration of the filamentous cyanobacterium Oscillatoria, and a mixture of Scenedesmus and clay. Natural seston from Lake IJsselmeer was also tested. Clones from two contrasting habitats (mesotrophic versus hypertrophic lake) did not differ in either population growth rate, r, or filtering structures. However, all the clones exhibited phenotypic plasticity when reared under low and high food concentrations. In relation to their body length, daphnids grew larger filter screens with longer setae in response to low food concentrations, thus enhancing filtering efficiency. Supplementing the low food concentration with inert clay particles had no effect on either the growth performance or filter screen morphology. In both cases, the growth and reproduction were low and filtering areas large. Filter screens were of intermediate size when daphnids were supplied with food supporting intermediate growth and reproduction. From these laboratory results, we concluded that the nutritional status of Daphnia is a more important cue for the phenotypic response of the filter screen morphology than particle concentration. [KEYWORDS: Life-history consequences; phenotypic plasticity; resource depression; feeding-behavior; eutrophic lake; suspended clay; reaction norms; zooplankton; reproduction; cladocerans

Morphological adaptations in filtering screens of Daphnia galeata to food quantity and food quality