One of the major research goals in ecology is to understand predator–prey interactions. However, our understanding of how prey express phenotypic plasticity in response to co-occurring multiple predators is limited in many systems. Here, we use the rotifer Brachionus calyciflorus as model organism to test how prey responds to co-occurring predators through a series of related experiments. Firstly, we examined the effects of the density of a fish predator Carassius auratus on morphological traits in B. calyciflorus. Secondly, we examined the effects of larval C. auratus and Asplanchna brightwellii (a rotifer predator) on morphological defensive traits (body length, body width, anterior spine length, and posterolateral spine length) of B. calyciflorus. We show that B. calyciflorus develops smaller body size and shorter spine length when exposed to media conditioned using larval fish, and that high larval fish densities induce more pronounced responses than low densities. We also show that B. calyciflorus is able to discriminate between the two predators by producing opposing morphological adaptations (i.e., larger body size and longer posterolateral spines against A. brightwellii, but smaller body size and shorter spine length when exposed to the visual predator [larval fish]). However, B. calyciflorus develops intermediate, trade-off responses in all morphological traits when exposed to conflicting threats from a combination of predators with different size and feeding strategies. These results show how prey are not necessarily passive victims, but rather, use countermeasures against predation – they actively develop responses to counteract their vulnerability to different predators by rapidly adjusting their morphology and life-history traits to the existing predator regimes.
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