Daphnia respond to kairomones from a variety of predators by altering their morphology, behavior, and life history. I use the statistical technique of meta-analysis to quantitatively evaluate the results of 27 independent studies that examine various life history responses of Daphnia to kairomones released from three common predators: Chaoborus, Notonecta, and planktivorous fish. Daphnia exhibit a fundamentally different set of life history responses to Chaoborus (delayed reproduction, decreased size of first clutch, slight tendency toward increased size at maturity) than they do to Notonecta and fish (earlier reproduction, increased clutch size, smaller size at maturity). These opposite responses appear to be related to different patterns of prey selection exhibited by the predators. Food level does not affect the life history responses of Daphnia to Chaoborus kairomones; however, the length of exposure to these kairomones does influence the degree of delay in reproduction. In addition, changes in size at maturity in response to predator kairomones are positively correlated with changes in both age at maturity and size of the first clutch. While the life history shifts exhibited by Daphnia in the presence of Notonecta and fish appear to be clearly adaptive, the fitness consequences of Chaoborus-induced responses are unclear.

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The effectiveness of antipredator defenses is greatly influenced by the environment in which an organism lives. In aquatic ecosystems, the chemical composition of the water itself may play an important role in the outcome of predator–prey interactions by altering the ability of prey to detect predators or to implement defensive responses once the predator’s presence is perceived. Here, we demonstrate that low calcium concentrations (<1.5 mg/L) that are found in many softwater lakes and ponds disable the ability of the water flea, Daphnia pulex to respond effectively to its predator, larvae of the phantom midge, Chaoborus americanus. This low-calcium environment prevents development of the prey’s normal array of induced defenses, which include an increase in body size, formation of neck spines, and strengthening of the carapace. We estimate that this inability to access these otherwise effective defenses results in a 50–186% increase in the vulnerability of the smaller juvenile instars of Daphnia, the stages most susceptible to Chaoborus predation. Such a change likely contributes to the observed lack of success of daphniids in most low-calcium freshwater environments, and will speed the loss of these important zooplankton in lakes where calcium levels are in decline.

The use of inducible defenses is a common strategy to reduce predation while minimizing associated costs for prey. The most effective use of these defenses, however, may involve turning them on and off at different stages of ontogenetic development, with the timing dependent on prey body size and the nature of the predation environment. We develop a model based on the strike efficiency of a size-selective predator that examines the interaction between induced morphological defenses and prey body size, including the consequences of this interaction for the optimal development of the defenses during the prey's ontogeny. We then examine this model with respect to a model system of inducible defenses: neck spine induction in the water flea Daphnia in response to predatory larvae of the phantom midge Chaoborus. In accordance with predictions of the model, the body size and timing of neck spine acquisition during Daphnia development are related to the relative sizes of the Daphnia and Chaoborus species interacting in a pond or lake. The Daphnia species examined first acquire neck spines in either the first, second, or third juvenile instar, at body lengths that range from 0.58 to 0.83 mm. Neck spine formation is initiated at larger Daphnia body sizes when these prey are subject to predation by a larger Chaoborus species (C. trivittatus) and at smaller sizes when exposed only to a smaller predator (C. americanus). Induction of these morphological defenses in Daphnia occurs later in juvenile development in the smaller of the two species we examined (D. minnehaha) than in the larger (D. pulex). Delayed acquisition of neck spines also occurs when Daphnia are exposed to predation by larger Chaoborus. The close match between model predictions and the patterns observed in nature suggests that these patterns are adaptive developmental responses to different predator environments.

The production of neck spines by Daphnia pulex in response to the presence of predatory Chaoborus larvae entails a demographic cost as well as a benefit in reducing predation. I develop a model that quantitatively analyzes the costs and benefits of defensive spine formation in D. pulex by modifying life tables of both the spined (SM) and typical (TM) morphs of this prey to account for the effects of different levels of Chaoborus predation on population growth rate. At low Chaoborus densities the population growth rate (and thus fitness) of TM exceeds that of SM and spine formation is therefore disadvantageous in the population. Above a critical Chaoborus density, however, the opposite is true and spine formation is favored. The exact value for this critical Chaoborus density is influenced by both food availability for Daphnia and the degree of spatial overlap between predator and prey. The model predicts that spine induction is more advantageous under relatively poor food conditions, which suggests that the cost of this antipredator defense may not be an energy loss, but merely a lengthening of the developmental process in spined instars. The model also predicts that any predator or prey behavior that reduces spatial overlap between the two species in nature will make the formation of defensive spines less advantageous.

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Juvenile Daphnai pulex develop neck spines in response to a chemical agent released by predatory Chaoborus larvae. While these defensive structures reduce the vulnerability of Daphnia to the insect predator, they also entail a demographic cost. We investigated the nature and degree of this cost through an analysis of cohort life tables involving both the typical morph (TM), which lacks neck spines, and spined morph (SM) at 22°C in two different food regimes: "natural" food conditions (53—@m filtered pond water) and "ideal" food conditions (1 x 105 cells/mL Chlamydomonas sp.). No consistent pattern of differences between TM and SM occurred with respect to survivorship, clutch sizes, body sizes, mean egg volume, or number of juvenile instars. Development rates of both juvenile and adult instars, however, were significantly slower in SM. The presence of neck spines increased the age maturity for D. pulex by 8.4—14.6%, and the duration of adult instars exposed to Chaoborus—factor, whose eggs will develop into SM, was 2.8% longer than for those not exposed. This caused delayed reproduction in SM and resulted in a population growth rate (°) that was °8—9% lower than in TM. This relatively large demographic cost of spine formation in D. pulex produces a strong selection pressure to forgo the formation of these spines when Chaoborus predators are absent.