Shortened lifespan: another cost of fish-predator avoidance in cladocerans?

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.