Predator-induced shifts in Daphnia life-histories under different food regimes

Abstract: Patterns of growth and reproduction (first, second and third broods) of aDaphnia magna clone were studied in a life-table experiment with and without chemi-cal signals from fish predators, under three different food regimes (0.2, 0.5 and 2.0mgCl –1 ).At high resource levels, predation played a role as a factor synchronizing repro-ductive events. Somatic growth rates increased with increasing food concentration andwere lower under the presence of fish than in the control treatments. Smaller femalesize in the fish treatments was accompanied by reduced number of eggs released in thesecond and third (but not the first) clutch. The effect of decreased cumulative numberof offspring was counterbalanced by the gain in the second fitness component –decreased age in releasing successive broods, which eventually resulted in equivalentintrinsic rates of increase (r) in both, the fish and fish-free environments. Conse-quently, Daphnia life-history performance under the presence of predator cues, with-out accompanying mortality, did not incur fitness costs, across the studied range offood concentrations.Key words: Daphnia magna , phenotypic plasticity, growth rate, predation, reproduc-tive synchrony.