Effects of infochemicals released by gape-limited fish on life history traits of Daphnia: a maladaptive response?

Life history shifts in daphnids in response to fish infochemicals are generally interpreted as an adaptive response to positive size-selective predation. This interpretation does, however, not hold for larval and small juvenile planktivorous fish, which due to gape limitation, feed on small and medium sized prey. In a life table experiment we show that daphnids exposed to infochemicals excreted by small gape-limited perch and larger perch changed their life history in the same direction, irrespective of the contrasting size-selection of the fish. However, responses to fish infochemicals were strongly influenced by food conditions for daphnids. In the high food treatments size at maturity was decreased in the presence of fish infochemicals, whereas age at maturity remained unchanged. Under low food conditions, size at maturity was generally smaller compared with the high food situation, but unaffected by fish infochemicals. By contrast, age at maturity, which was increased at low food levels, was significantly lower in fish treatments compared with the control. We conclude that life history responses of daphnids to gape-limited fish can indeed be maladaptive, but only in situations of high food availability. This combination of factors is, however, rather unlikely because gapelimited fish usually occur in late spring during the clear water phase when daphnids are severely food limited. We thus hypothesize that the costs of this maladaptive response under negative size-selective predation will be low under field conditions and the selective advantage under positive size-selective predation later in the season will outweigh these costs.