Food quantity and quality modulates inducible defences in a common predator-prey system

Zooplankton display different inducible defences against invertebrate and vertebrate predators. The response pattern to gape-limited invertebrate predators involves increased somatic growth and offspring body size but delayed maturity and reduced offspring numbers. In contrast to this general pattern, the freshwater model organism Daphnia magna has been reported to exhibit a different response when encountering the gape-limited tadpole shrimp Triops cancriformis. Under laboratory conditions, D. magna showed increased somatic growth, earlier maturation, and an increase in both offspring number and size. We propose here that the discrepancy between the previously observed and the theory-based response patterns against invertebrate predators is due to differences in food availability in the applied laboratory settings and assessed whether the defensive response of D. magna against T. cancriformis is modulated differently by food quantity and quality. We found a strong impact of food quantity and quality on the defence response of D. magna to T. cancriformis kairomones. The prey seem to be able to overcome trade-offs between morphological defence traits and reproductive traits, but distinctly between high food quantity and high food quality. Thereby, reproductive traits were preferred over morphological defences. Furthermore, removal of particles from the T. cancriformis-conditioned water caused a defence pattern in D. magna that was consistent with the general response pattern known from other invertebrate predators, thus explaining the described discrepancy to previous studies with T. cancriformis.  Our study highlights the importance of assessing food-related effects on predator-prey interactions to understand trophic relationships and food web processes.