Phenotypic plasticity in Daphnia magna Straus: Variable maturation instar as an adaptive response to predation pressure

Life history responses of four Daphnia magna clones at two food levels were studied to assess the importance of maturation instar on the plasticity of fitness responses under simulated mortality regimes. Females of the clones studied could vary offspring size with consequent effects on their maturat ion time. Significant genetic variability in life history and fitness respo nses, measured as the intrinsic rate of population increase, within and acr oss food levels was observed, but most of this variation could be attribute d to maturation instar differences among clones within and across environme nts. In the laboratory, without extrinsic mortality, females maturing earli er always had higher fitness than those maturing later, indicating a clear fitness cost of delaying maturity. Nevertheless using a model, we showed th at the observed maturation instar effects on life history responses can lea d to differences in fitness under different size-selective predation regime s, such that females with delayed maturity have higher fitness under invert ebrate predation while females maturing earlier have higher fitness under f ish predation regimes. These results suggest that intraclonal variation in offspring size and hence in the number of maturation instars can be an adap tation to living in habitats subject to temporal fluctuations in fish and i nvertebrate predation pressure.