BEHAVIORAL AGGREGATION OF DAPHNIA-PULEX IN RESPONSE TO FOOD GRADIENTS

1. Natural populations are heterogeneously distributed in space and often form dense local aggregations. 2. Here, we examine the ability of the freshwater zooplankter Daphnia pulex to modify its spatial position in response to food gradients that span the natural range encountered in lakes and ponds. 3. When presented with a food gradient, individuals chose to feed at the high-food end of gradients whose upper range is typical of field conditions, and they avoided the high-food end of very 'high-food' gradients that are atypical of the field. 4. In addition, the degree to which individuals aggregated along the gradient varied in a non-linear manner with overall food concentration. The strongest aggregative response was obtained at intermediate food levels typical of the field. 5. We also show that the behavioural response occurs on a relatively fast time-scale. Individuals were capable of tracking food gradients that were destroyed and recreated over a time-scale of 3-4 h.