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要旨

Feeding behavior of Daphnia has been intensely studied, yet the generally observed behavior of maximal feeding at low food concentrations contradicts the predictions of optimal foraging theory. To explore this inconsistency, I investigated the behavioral feeding response of Daphnia magna through direct observation of thoracic filtering appendage beat rates at low food concentrations. I observed animals that were subjected to either varying starvation periods or to different food concentrations prior to the experiments. Starvation led to a behaviorally mediated decrease in appendage beat rate, which depended on the food concentration at which the daphnids were cultured. Starved daphnids consistently showed an almost immediate increase in their appendage beat rate once food was added, irrespective of the length of the starvation period (from 1 to 3 d). Therefore, the initial decrease in appendage beat rate displayed by animals during starvation could not have been caused by the deprivation of energy (exhaustion) alone. Furthermore, the food conditions under which the animals were cultured influenced the behavioral response. After 1 h of starvation, animals cultured at high food level showed no behavioral response to the addition of food, while animals cultured at low food level increased their appendage beat rate significantly. The results of this study contradict the maximal feeding strategy and highlight problems of the optimal foraging strategy.