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

1. Daphnia may reach high population densities seasonally, or in patches, i n lakes. To test the effects of chemicals released by high daphniid densiti es on their life-history traits, nine species of Daphnia, D. magna, D. puli caria, D. pulex, D. hyalina, D. galeata, D. laevis, D. lumholtzi, D. ambigu a 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 r ate, 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 significan t effects on larger species. Two clones of D. pulex differed in their growt h 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. l umholtzi 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, de nsity-dependent mechanism of population regulation; when large species of D aphnia coexist at a high population density with small species, these chemi cals may reinforce the competitive advantage of large species.