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Daphnia magna; phenotypic plasticity; growth rate; predation; reproductive synchrony
Abstract:
Patterns of growth and reproduction (first, second and third broods) of a Daphnia magna clone were studied in a life-table experiment with and without chemical signals from fish predators, under three different food regimes (0.2, 0.5 and 2.0 mg C l(-1)). At high resource levels, predation played a role as a factor synchronizing reproductive events. Somatic growth rates increased with increasing food concentration and were lower under the presence of fish than in the control treatments. Smaller female size in the fish treatments was accompanied by reduced number of eggs released in the second and third (but not the first) clutch. The effect of decreased cumulative number of offspring was counterbalanced by the gain in the second fitness component decreased age in releasing successive broods, which eventually resulted in equivalent intrinsic rates of increase (r) in both, the fish and fish-free environments. Consequently, Daphnia life-history performance under the presence of predator cues, without accompanying mortality, did not incur fitness costs, across the studied range of food concentrations.
