Diel periodicity and selectivity in the feeding rate of the predatory copepod Mesocyclops edax

The diel periodicity and selectivity in the feeding behaviorof the predatory cyclopoid copepod Mesocyclops edax was investigatedat 3 h intervals over two 24 h sampling periods in nature. Gutcontent analyses revealed an increase in gut fullness at 11.30and 20.30 in July, and at 20.30 in August. The increase in gutfullness at these times could not be explained by an increasein prey density, changes in predator-prey overlap or differencesin prey vulnerability. We suggest that M.edax exhibits a truediel periodicity in the intensity of its predatory activities,although the alternative hypothesis of a diel periodicity ingut passage time cannot be ruled out. Vanderploeg and Scavia'sE^* selectivity index indicated a preference for rotifer andcladoceran prey over copepods.