The effects of a pesticide on inducible phenotypic plasticity in Daphnia

Many species of zooplankton can detect chemical signals (kairomones) released by their predators and respond with defensive morphological and life-history changes. Daphnia longicephala were exposed to five different concentrations of the carbamate insecticide, carbaryl, in the presence and absence of kairomones released by Anisops gratus (Hemiptera: Notonectidae). In the absence of Anisops kairomone, there was a negative correlation between carbaryl concentration and length and brood size at maturity. Growth and reproduction were significantly impaired at carbaryl concentrations greater than or equal to 0.32 mu g/litre. There was no effect of carbaryl on age at maturity and it did not induce crest growth in the absence of kairomone. In the presence of kairomone, D. longicephala developed a large crest, but took longer to reach maturity than control animals. A concentration of 3.2 mu g/litre carbaryl decreased relative crest size of 3-day-old daphnids but did not affect crest size of adults. The Anisops kairomone dramatically altered the effects of carbaryl on the life-history responses of D. longicephala at maturity. There was a positive correlation between the concentration of carbaryl and age, length, and brood size at maturity. The effects of kairomone on life-history parameters are explained using a size threshold for maturation hypothesis. (C) 1999 Elsevier Science Ltd. All rights reserved.