Effects of neonate size and food concentration on the life history responses of a clone of the hybrid Daphnia hyalina x galeata to fish kairomones

1. The influence of fish kairomones on the life history parameters of a clone of the hybrid Daphnia galeata x hyalina was measured in laboratory experiments with small and large neonates at one food concentration. The influence of fish kairomones on neonates of the same size at two food concentrations was also investigated. 2. Neonate size (maternal investment per egg) changed the influence of fish kairomones on life history parameters. Daphnids that were small at birth showed greater differences between fish (kairomones present) and no-fish (kairomones absent) treatments for age at maturity and number of eggs in the first clutch than animals that were large at birth. On the other hand, animals that were large at birth differed more in size at maturity between fish and no-fish treatments than small neonates. 3. The food concentrations used in the experiments (0.25-0.75 mg Cl-1) hardly affected the influence of fish kairomones on the life history characteristics studied. 4. The effect of fish kairomones on the size of neonates from the first clutch was similar for large and small daphnids. In the no-fish treatment significant differences were found between small and large neonates for age at maturity and number of eggs in the first clutch. In the fish treatment size at maturity differed significantly between small and large neonates. 5. In life history experiments, neonate size has to be taken into account when measuring the effects of fish kairomones because size at birth influences the results.