The effects of a pesticide on inducible phenotypic plasticity in Daphnia

Many species of zooplankton can detect chemical signals (kairomones) releas ed by their predators and respond with defensive morphological and life-his tory changes. Daphnia longicephala were exposed to five different concentra tions of the carbamate insecticide, carbaryl, in the presence and absence o f kairomones released by Anisops gratus (Hemiptera: Notonectidae). In the a bsence of Anisops kairomone, there was a negative correlation between carba ryl concentration and length and brood size at maturity. Growth and reprodu ction were significantly impaired at carbaryl concentrations greater than o r equal to 0.32 mu g/litre. There was no effect of carbaryl on age at matur ity 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 lo nger to reach maturity than control animals. A concentration of 3.2 mu g/li tre carbaryl decreased relative crest size of 3-day-old daphnids but did no t affect crest size of adults. The Anisops kairomone dramatically altered t he effects of carbaryl on the life-history responses of D. longicephala at maturity. There was a positive correlation between the concentration of car baryl and age, length, and brood size at maturity. The effects of kairomone on life-history parameters are explained using a size threshold for matura tion hypothesis. (C) 1999 Elsevier Science Ltd. All rights reserved.