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.

Summary1. Phenotypic plasticity in defensive traits has been proven to be effective in ecosystems with frequently changing predator regimes. However, if a single dominant predator exerts predation pressure on each ontogenetic stage, prey should adapt by developing defensive traits for each life stage within a cost-benefit framework. This may require a change of defensive mechanisms between juvenile and adult life stages. 2. In this study, we examined the morphological defences of the cladoceran Daphnia magna Strauss induced by the tadpole shrimp Triops cancriformis Bosc. We tested for the induction of morphological defences by directly exposing daphnids to the predator and conducted a life span experiment to determine if the expression of the induced morphological defences varies throughout the life span of D. magna. In addition, we studied the adaptive value, i.e. the effectiveness, of the Triops-induced morphological defences in D. magna by conducting predation trials. 3. We found that, in D. magna, the expression of an array of inducible morphological defences, which act synergistically to provide effective protection, changes during the daphnids lifetime in response to the tadpole shrimp T. cancriformis. This gradual switch in the protective function of single traits between juvenile and adult stages represents a novel functionality and complexity of inducible defences. Both direct contact with the predator and chemical cues (kairomones) released by T. cancriformis induce an increased body length, body width and an elongation of the tail spine in D. magna. This study is the first to show that kairomones released by a predator can induce 'bulkiness' as a defensive mechanism in Daphnia. Finally, we demonstrate the effectiveness of the Triops-induced morphological defences (i.e. an elongated tail spine and increased bulkiness) by conducting predation trials. 4. Our study provides rare evidence for morphological defences in D. magna, and in addition shows that prey species gradually switch between plastic traits to maintain effective defences throughout their entire lifetime. Hence, our results help to shed light on the mechanisms governing phenotypic plasticity within natural populations.

The development of structural defences, such as the fortification of shells or exoskeletons, is a widespread strategy to reduce predator attack efficiency. In unpredictable environments these defences may be more pronounced in the presence of a predator. The cladoceran Daphnia magna (Crustacea: Branchiopoda: Cladocera) has been shown to develop a bulky morphotype as an effective inducible morphological defence against the predatory tadpole shrimp Triopscancriformis (Crustacea: Branchiopoda: Notostraca). Mediated by kairomones, the daphnids express an increased body length, width and an elongated tail spine. Here we examined whether these large scale morphological defences are accompanied by additional ultrastructural defences, i.e. a fortification of the exoskeleton. We employed atomic force microscopy (AFM) based nanoindentation experiments to assess the cuticle hardness along with tapping mode AFM imaging to visualise the surface morphology for predator exposed and non-predator exposed daphnids. We used semi-thin sections of the carapace to measure the cuticle thickness, and finally, we used fluorescence microscopy to analyse the diameter of the pillars connecting the two carapace layers. We found that D . magna indeed expresses ultrastructural defences against Triops predation. The cuticle in predator exposed individuals is approximately five times harder and two times thicker than in control daphnids. Moreover, the pillar diameter is significantly increased in predator exposed daphnids. These predator-cue induced changes in the carapace architecture should provide effective protection against being crushed by the predator’s mouthparts and may add to the protective effect of bulkiness. This study highlights the potential of interdisciplinary studies to uncover new and relevant aspects even in extensively studied fields of research.

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