Crowding-induced changes in growth, reproduction and morphology of Daphnia

1. Daphnia may reach high population densities seasonally, or in patches, in lakes. To test the effects of chemicals released by high daphniid densities on their life-history traits, nine species of Daphnia, D. magna, D. pulicaria, D. pulex, D. hyalina, D. galeata, D. laevis, D. lumholtzi, D. ambigua and D. cucullata, were grown in water from crowded Daphnia cultures in a flow-through system in the presence of abundant food. 2. Water from Daphnia at greater than or equal to 85 L-1 depressed growth rate, and lowered body size and clutch at first reproduction of six species of small-bodied Daphnia (adult body length < 1.8 mm), but had no significant effects on larger species. Two clones of D. pulex differed in their growth rate in response to crowding, indicating that response patterns may vary within species. 3. Chemicals released by crowded D. magna reduced tail spine length in D. lumholtzi and D. cucullata by 37% and 11%, respectively, and induced changes in carapace morphology in D, lumholtzi and D, ambigua. 4. Chemicals released by crowded conspecifics may provide an additional, density-dependent mechanism of population regulation; when large species of Daphnia coexist at a high population density with small species, these chemicals may reinforce the competitive advantage of large species.