Predator‐induced phenotypic plasticity in Daphnia pulex: Life history and morphological responses to Notonecta and Chaoborus

Results from a laboratory life-table study show Daphnia pulex has a unique set of rapidly induced responses to waterborne chemicals from each of two predator species. Additionally, Daphnia exhibits a unique set of induced responses when simultaneously exposed to both predators. Daphnids possessed neck teeth and experienced delayed maturity when exposed to waterborne chemicals released from larvae of the phantom midge Chaoborus americanus. Possessing the Chaoborus-induced phenotypic plasticity was not associated with a lower population growth rate relative to that in the control treatment. When exposed to waterborne chemicals released from the backswimmer Notonecta undulata, D. pulex exhibited an unexpected assemblage of responses. The Notonecta-induced phenotypic plasticity included rapid juvenile growth to a large size at first reproduction, little growth beyond maturity, and high reproductive output. Simultaneous exposure to chemical cues from Notonecta and Chaoborus resulted in a life history and morphologies that agreed with predicted life history and morphological responses of Daphnia that had been simultaneously exposed to largeand small-size selective predators. Predators are an important evolutionary force shaping the life histories, morphologies, and behaviors of their prey. Several aquatic taxa possess phenotypically plastic life histories and morphologies that may be induced by the presence of specific predators (Have1 1987; Dodson 19893; Harvell 1990). Predator-induced phenotypic plasticities can be viewed as being particularly adaptive because any disadvantage incurred by having a predator-resistant form is borne only when the predator is present. Among zooplankton, several species exhibit seasonal variation in morphologies to which the term cyclomorphosis has been applied (Hutchinson 1967; Black and Slobodkin 1987). Dodson (1974a) suggested zooplankton change shape as an adaptive strategy to foil the efforts of specific size-selective predators. An abundance of recent evidence supports his hypothI Present address: Water Resources Laboratory, University of Louisville, Louisville, Kentucky 40292. Acknowledgments I thank Stanley I. Dodson, John E. Havel, Nina Hemphill, and an anonymous reviewer for comments on the interpretation of the results presented and on the preparation of this manuscript. Daniel D. Wiegmann provided statistical advice. I received financial support through a Henry Freeman Vilas Graduate Fellowship from the University of Wisconsin-Madison Graduate School and an NSF grant (BSR 88-05805) to S. I. Dodson. esis. Several of the morphological changes seen in common cyclomorphic species of zooplankton can be induced by waterborne chemicals released from invertebrate predators (see Havel 1987; Stemberger and Gilbert 1987). Also, several predators are each capable of inducing a different morphological response in a prey species (Dodson 1989a). In practically every reported case of predator-induced morphological shifts, possession of the induced morphology is coupled with shifts in life history traits (for Daphnia, Black and Dodson 1990; Riessen and Sprules 1990; for rotifers, Stemberger 1988; for Bryozoa, Harvell 1986). When life history traits of the induced form result in lower fitness relative to the uninduced form, in the absence of the predator, investigators have interpreted the life history shifts as the cost of the induced morphology (Stemberger 1988; Walls and Matti 1989; Black and Dodson 1990; Riessen and Sprules 1990). In this study, life-table experiments were used to collect survivorship, fecundity, and morphological data from Daphnia pulex individuals that were exposed to chemical cues from two invertebrate predators, the backswimmer Notonecta undulata (Insecta, Hemiptera) and larvae of the phantom midge Chaoborus americanus (Insecta, Diptera). Both are common predators of Daphnia and often co-occur (Dodson and Have1 1988), although