Ontogenetic Changes in Daphnia Responsiveness to Fish Kairomone

Exposing Daphnia to the presence of a predator at different stages of ontogenesis leads to different reactions. Predator-induced changes of life history parameters related to the first reproduction schedule, as age and size at first reproduction, take place in the stage preceding that one in which vitellogenesis occurs. Determination of the amount of energy allocated to this first reproduction, including offspring number, occurs in the first 24-h of life. In case of parameters related to reproduction schedule, time since induction until the release of the first clutch of offspring is tens of hours; in case of allocation patterns – more than 100. This suggests that Daphnia do not “remember” the information of a potential threat but, instead, the decisions on future development are undertaken once they have estimated the reigning predation regime. Shortening of the period during which eggs are carried in the brood chamber, thus shortening the time when Daphnia are more easily visible, may be another adaptation to reduce danger from fish predation and so increasing the probability of survival until the first successful reproduction.