Morphological defences and defence–cost trade‐offs in Daphnia in response to two co-occurring invertebrate predators

Abstract

Inducible morphological defences are crucial for understanding predator–prey interactions. Such defences have been mostly studied in a single-predator context, ignoring the fact that prey organisms are often exposed to multiple predators. In deep peri-Alpine European lakes, the keystone grazer Daphnia coexists with two cladoceran predators, Bythotrephes longimanus and Leptodora kindtii. Up to now, life history and morphological responses of Daphnia to these two predators have not been analysed systematically. We studied the responses of two life history (age at first reproduction, and offspring production) and five morphological traits (body size, body width, head size, spina size, and eye diameter) of eight Daphnia galeata clones to the presence of Bythotrephes and Leptodora in a common garden experiment. We compared each response trait between treatments using linear mixed models, and investigated the covariation between defence traits and demographic costs (neonate production) for the two predators. Our results show that the responses of Daphnia are predator- and trait-specific. Daphnia developed a typical helmet and a larger eye only in the presence of Bythotrephes, not in the presence of Leptodora. In contrast, both predators induced larger body sizes and longer spines. Age at first reproduction was latest and demographic costs were highest in the Bythotrephes treatment, suggesting that the development of a helmet exceeds the costs of spina elongation. The responses of Daphnia clones revealed a defence-cost trade-off for helmet formation in the Bythotrephes, and for spina elongation in the Leptodora treatment. Hence, despite Bythotrephes and Leptodora being closely related co-occurring predators, Daphnia responds with a unique combination of trait changes and defence-cost trade-offs to the two predators. The presence of predator-specific clonal defence-cost trade-offs suggests that the presence of these invertebrate predators can drive different evolutionary processes in natural zooplankton communities. Disentangling the evolutionary ecology of phenotypic responses of prey species to co-occurring predators will require multi-trait, multi-clone studies of induced antipredator defences.