Abstract
The influence of light on the aggregation behaviour of Daphnia pulex was tested in a laboratory experiment. The animals were kept in a circular tank containing 16 l of water and the degree of aggregation was estimated by Lloyd's Patch Index. By varying the degree of competition for food and light conditions (homogeneous light intensity at the water surface; a gradient in light intensity at the water surface; total darkness), it was possible to test the hypothesis that light has to be present as a proximate factor in the formation of swarms in D. pulex. As the daphnids formed swarms in total darkness, the hypothesis was rejected. However, under a gradient in light intensity at the water surface, the behaviour of daphnids differed from behaviour under the other light regimes by aggregating irrespective of food level, indicating that light is important for aggregation.
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