How important is light in the aggregation behaviour of Daphnia pulex (Cladocera : Crustacea)?

The influence of light on the aggregation behaviour of Daphnia pulex was te sted in a laboratory experiment. The animals were kept in a circular tank c ontaining 16 l of water and the degree of aggregation was estimated by Lloy d's Patch Index. By varying the degree of competition for food and light co nditions (homogeneous light intensity at the water surface; a gradient in l ight intensity at the water surface; total darkness), it was possible to te st 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 da rkness, the hypothesis was rejected. However, under a gradient in light int ensity at the water surface, the behaviour of daphnids differed from behavi our under the other light regimes by aggregating irrespective of food level , indicating that light is important for aggregation.