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Life-history theory predicts adaptive shifts in response to size-selective predation, namely earlier reproduction, smaller age/size at maturity, and higher relative investment into reproduction. Such shifts should bring about reduced lifespan of potential prey. We tested this prediction in life-table experiments with clones of Daphnia hyalina and Diaphanosoma brachyurum, two species of contrasting anti-predatory strategies. The clones were derived from seven lakes of different trophy and held in water with and without fish kairomone, under standard laboratory conditions. Exposure to the kairomone caused a decrease in age of first reproduction and an increase in early-life reproductive effort but also an about 20% decrease of longevity in both species. Although shortened lifespan did not result in significant decrease in fitness of the tested species (in terms of lifetime reproductive output) it should be taken into account in considerations of costs and benefits of inducible defenses in cladocerans. [ABSTRACT FROM AUTHOR]

The life history of Daphnia exposed to fish kairomone at different developmental stages was examined in a laboratory experiment. The strongest life history response to the applied predation threat was observed in females exposed during the 4th instar. Compared to Daphnia experiencing the presence of fish at earlier or later instars, these individuals reached maturity at a smaller size and released fewer neonates. Moreover, their offspring also demonstrated the strongest reaction to predation threat, exhibiting the broadest phenotypic plasticity in the life history response to predation. The breadth of their reaction norm was, on average, two times larger comparing with individuals from other treatments. Broader phenotypic plasticity may offer clear selective advantages under the unpredictable predation regime. This finding highlights the adaptive role of maternal effect in shaping life history of cladocerans. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Large-sized Daphnia hyalina × galeata hybrid and small-sized Ceriodaphnia reticulata were raised together for 3 weeks in thermally stratified flow-through tubes (‘plankton organ’), in the presence and the absence of fish kairomone, and their relative performance was monitored, as indicated by the populations' growth rates (r). In the kairomone-free medium, D. hyalina grew faster than C. reticulata; the presence of kairomone reversed this ratio, producing relatively higher r in C. reticulata. The simulated fish presence reduced growth in D. hyalina population also when it was separated from the competitor, while growth in C. reticulata was altered by the kairomone only in the presence of Daphnia. It is suggested that costs of surface avoidance in Daphnia can reduce its competitive superiority to small-sized cladocerans, permanently dwelling at the surface. [ABSTRACT FROM AUTHOR]

Life history traits of Daphnia galeata clones were affected differently by kairomones exuded by two species of fish. The exposure to Gasterosteus kairomone resulted in an increase in population intrinsic rate of increase and body growth rate relative to the exposure to Perca kairomone. These differences between fish treatments have not been reported earlier. The results suggest differences between fish kairomones that are not due to concentration effects alone. This raises the possibility that kairomones are cocktails of chemicals rather than a single `fish substance'. It also supports the view the effects of size-selective predation vary depending on the local predator–prey assemblage examined. This finding leads to an additional, previously underestimated but potentially complex aspect in zooplankton ecology. [ABSTRACT FROM AUTHOR]