Food level modifies the life-history response of Daphnia under chemically-induced predation stress

In a full-factorial design experiment, we tested the effect of predator presence (simulated by fish kairomones) under six different food levels (from below-threshold to over-optimal food concentration) on key lifehistory parameters of Daphnia magna. Life-history parameters were measured in high temporal resolution (4h intervals). Additionally, as an indicator of environmental stress, the expression of heat shock proteins (HSPs) of families 60, 70 and 90 were tested. The presence of predators resulted in decreased growth rate, decreased age and size at reproduction, and increased allocation of resources into reproduction. Dietary limitation resulted in decreased growth rate, delayed reproduction, decreased size at reproduction and a decrease in number but an increase in the size of neonates. The largest differences between the fish and no-fish treatments were found with the low concentrations of food: 0.1 and 0.5 mg C l-1. In Daphnia exposed to kairomones at the two lowest food levels, the expression of HSP90 was higher than in their conspecifics cultured without fish cues. Expression of HSP60 was elevated in the control as compared with individuals exposed to kairomones. These results show that food level can alter the responses of Daphnia to the presence of predators and that the amount of energy allocated into growth and reproduction can be phenotypically adjusted.