Response of Daphnia to substances released from crowded congeners and conspecifics

The effects of chemicals released from crowded congeners and conspecifics on life history parameters of the freshwater zooplankters Daphnia cucullata and Daphnia pulex were examined. Length and age at maturity of D. pulex were affected by crowding chemicals. Reproduction was lower in crowded medium, and ephippia were produced. Newborn D. pulex in crowded medium were significantly longer than the controls. The intrinsic rate of population increase of D. pulex was 14 and 25% lower than the control when exposed to crowded medium from D. cucullata and D. pulex, respectively. Neither urea nor ammonia (at 1 mg l(-1)) seemed to be responsible for these effects in D. pulex. In D. cucullata, no significant effect of crowding infochemicals on length and age at maturity was found. However, crowding chemicals reduced reproduction. No ephippia were produced in crowded medium, but up to 83% non-developing eggs were observed in D. cucullata. Newborns were similarly sized in crowded and standard medium. The intrinsic rate of population increase of D. cucullata was 44 and 96% lower than the control when exposed to crowded medium from D. cucullata and D. pulex, respectively. Clearance rates of D. pulex were significantly reduced in crowded media compared with standard medium, which could partly explain why the animals exposed to crowding chemicals reacted as if they were food limited.