Diel variation in feeding behavior of Daphnia pulex. Influences of food density and nutritional history on mandibular activity 1

Individuals of the planktonic cladoceran Daphnia pulex express well defined daily patterns in mandibular activity. The 24‐h cycles are characterized by high, uniform daytime rates and distinct nocturnal minima. Daytime rates are strongly dependent on animal size and ambient food concentration, the highest values being obtained with small animals feeding on dense cell suspensions. Starved D. pulex express diel patterns which differ from cycles of prefed animals, and those of prefed groups vary with food cell density. Typical 24‐h patterns of mandibular beat can be substantially altered by increases in food availability, especially when the changes occur shortly before light–dark transitions. The results implicate nutrition and feeding history as primary influences in the expression of feeding‐related periodicities in cladoceran zooplankton.