Direct distributional response in Daphnia pulex to a predator kairomone

The hypothesis that zooplankton are capable of direct and differentiated behavioural responses to different biotic factors was tested. A multichamber flow-through technique enabled predetermined food levels of the green alga Scenedesmus acutus to be maintained constant throughout the experiments. By adding water from a culture of fourth instar Chaoborus flavicans larvae to one of the end chambers, a concentration gradient of the chemical substance was established across the parallel chambers. Evidence is presented for a direct avoidance response in the water flea Daphnia pulex to a substance released by the predatory midge C. flavicans. This response occurs in the absence of visual cues and is independent of food availability. Finally, the results indicate that swarming in D. pulex may occur in the absence of both light and predator scent, but is influenced by food concentrations.