Does inducible defense mitigate physiological stress responses of prey to predation risk?

Exposure of prey to predation risk usually leads to increased metabolism and decreased nutrient deposition as physiological stress responses (PSRs) of prey to their predators. Many studies using freshwater animals as models failed to find evidences as predicted from PSRs. One possible reason is that energetic costs during development of inducible defenses may lower the quantity of resources, which can be allocated to production of costly PSRs. Results obtained from the classic Brachionus-Asplanchna model indicated that inducible defenses did not alleviate PSRs in B. calyciflorus, which were manifested as increased respiration and enhanced body molar ratios of carbon (C) to nitrogen (N) and C to phosphorus (P). Moreover, activities of antioxidant systems also increased in defended B. calyciflorus. There were no alterations in somatic growth and swimming activity in B. calyciflorus when they were exposed to Asplanchna kairomones; however, we observed predator-induced -reduction of food intake in Brachionus. Our results suggest that enhanced metabolisms of defended prey in support of fight-or-flight responses are likely to be the physiological adaptation of prey to diminish hydrodynamic costs accompanied with morphological alterations (e.g., decreased swimming speed) and maintain their fitness under predation risk.