Feeding behavior of Daphnia has been intensely studied, yet the genera
lly observed behavior of maximal feeding at low food concentrations co
ntradicts the predictions of optimal foraging theory. To explore this
inconsistency, I investigated the behavioral feeding response of Daphn
ia magna through direct observation of thoracic filtering appendage be
at rates at low food concentrations. I observed animals that were subj
ected to either varying starvation periods or to different food concen
trations prior to the experiments. Starvation led to a behaviorally me
diated decrease in appendage beat rate, which depended on the food con
centration at which the daphnids were cultured. Starved daphnids consi
stently showed an almost immediate increase in their appendage beat ra
te once food was added, irrespective of the length of the starvation p
eriod (from 1 to 3 d). Therefore, the initial decrease in appendage be
at rate displayed by animals during starvation could not have been cau
sed by the deprivation of energy (exhaustion) alone. Furthermore, the
food conditions under which the animals were cultured influenced the b
ehavioral response. After 1 h of starvation, animals cultured at high
food level showed no behavioral response to the addition of food, whil
e animals cultured at low food level increased their appendage beat ra
te significantly. The results of this study contradict the maximal fee
ding strategy and highlight problems of the optimal foraging strategy.