Role of temperature in diapause response to fish kairomones in crustacean Daphnia.

The effect of non-lethal thermal conditions on the diapause response to a simulated threat of fish predation was tested in the freshwater planktonic crustacean Daphnia magna. From an early developmental stage, female Daphnia were either exposed or not exposed to fish kairomones that notified of the threat of fish predation at the benign growth temperatures of 18, 22 or 26°C. The proportion of females switching from the production of subitaneous to diapausing eggs and their rate of reproduction were recorded. Besides the faster development and more intense reproduction observed at higher temperatures, a smaller proportion of the females formed protective, diapausing eggs when exposed to fish kairomones than was the case in colder conditions. The production of diapausing eggs was not observed when the threat of fish predation was absent. These results indicate an interactive effect of fish kairomones and thermal conditions on diapause induction in D. magna. We interpret these findings in the context of strategies for the maximisation of reproductive success. The production of well protected diapausing eggs (which assures low yet stable gains under diverse thermal conditions) may be a more beneficial life history strategy at lower temperatures, where the chances of survival of active individuals until maturation and successful reproduction are low. Higher temperatures permit faster maturation and more intensive reproduction that may surpass numerical losses of active descendants due to predation, making diapause a less rewarding option.