Nutritional deficiencies are a very common phenomenon, and consumers generally face food that is not optimally suited for their needs. Especially herbivores are habitually confronted with food of inferior quality, usually a result of too‐low nutrient concentrations in plant material. Waterfleas of the genus Daphnia are good model organisms to study the effect of inferior quality food, and how animals deal with this. We tested the effect of algae to which we had given different phosphorus contents on both life history and feeding parameters of Daphnia magna. Phosphorus content of the algae strongly affected both the growth rate and the feeding activity of the daphniids. Feeding activity increased with declining food quality (increase in C:P ratio of the algae), whereas growth rates were maximal at intermediate C:P levels. We conclude that the direct limitation of phosphorus is a very important factor determining food quality for zooplankters. Daphniids counterbalanced lower P content of their food by spending more C (energy) on acquiring this limiting resource. This implies that when Daphnia are given phosphorus‐limited food both the addition of phosphorus and the addition of carbon (energy) should increase the growth rate of the animals (co‐limitation). The influence of the phosphorus content of the food on the feeding activity of Daphnia offers a mechanistic explanation for the observed homeostasis in daphniids.

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We investigated the mechanisms behind the negative effects of cyanobacteria on zooplankton by comparing the effects of Microcystis aeruginosa single cells, colonies, and toxins on the feeding behavior of Daphnia pulicaria in three independent experiments. The animals were fed a mixture of Scenedesmus supplemented by increasing proportions (0, 20, 50, 80, 100%) of Microcystis or concentrations (0, 50, 500, 5,000 ng ml Ϫ1 ) of purified microcystin-LR. The changes in feeding behavior, as indicated by the appendage beat, mandible or labrum movement rates were evaluated by a direct observation method that coupled video recording and computerized image analysis. Daphnia responded in a different manner to the presence of single cells and colonies. In the case of the single cells, the mandibular movement rate (MMR) declined more than appendage beat rate (ABR), suggesting that Daphnia have the ability to discriminate between Microcystis and Scenedesmus. Colonies, on the other hand, produced a typical feeding interference response: the animals increased their labral rejection rate (LRR) and showed starvation signs. LRR increased in the presence of both unicellular and colonial Microcystis. In both cases, the changes in MMR and ABR were rapidly reversible and hence unlikely to be caused by intoxication from the presence of cellbound microcystins. In contrast, the addition of purified microcystin-LR at the concentration of 5,000 ng ml Ϫ1 produced a nonreversible impairment of Daphnia feeding behavior.

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, 1 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.