SHORT COMMUNICATION How Daphnia cope with algae selected for inedibility in long-

Abstract. Macro video records of restrained feeding Daphnia enabled us to measure simultaneously carapace gape, claw rejection rate, filter limb beat rate, and mandible movement rate. We compared the effects of high and low concentrations of highly edible algae and of inedible algae, the latter selected by long-term Daphnia grazing in oligotrophic microcosms. Inedible algae slowed the filtering process and influenced the carapace gape (widening at low concentration and narrowing at high), but did not affect the rejection rate. Herbivores have relatively fixed nutritional needs, which need to be met from plants whose composition varies widely (Sterner, 1993). Thus, ability to select nutritious food will be strongly selected for in herbivores, while optimal herbi-vores will put selective pressures on plants to reduce their edibility (Leibold, 1989; Grover and Holt, 1996). Daphnia is in these respects a typical herbivore, faced with food particles ranging from the highly nutritious (Chlorella, Scenedesmus spp.) to very poor food sources such as cyanobacteria and gelatinous green algae (Lampert, 1987; Sterner, 1989). Moreover, recent studies have shown that the physiological state of alga