Growing large and bulky in the presence of the enemy: Daphnia magna gradually switches the mode of inducible morphological defences

1. 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.