Herbivory versus omnivory: Linking homeostasis and elemental imbalance in copepod development

Abstract

Organisms usually face and acquire food that is elementally imbalanced, and although they are affected by its quality, they do not reflect the elemental composition of their resources. We analysed whether elemental homeostasis during the ontogeny of freshwater calanoid copepods varied with herbivorous or omnivorous diet. Boeckella gracilis and B. brevicaudata are herbivorous species, whereas Parabroteas sarsi changes its feeding strategy throughout its ontogeny. We hypothesized that homeostatic investment would decrease when food matches consumer requirements in copepods that change their diet from herbivorous microphagy towards a predatory regime. Elemental analyses of each cohort and its food resource revealed a relation between trophic strategy and imbalance. A high level of elemental imbalance was found in the herbivorous species, which were strictly homeostatic during their whole life cycle. The shift in P. sarsi trophic strategy resulted in changes in both imbalance and homeostasis. When nutrient conditions are imbalanced, energy needs to be allocated to stoichiometric regulation in order to maintain a relatively tight homeostasis. Our findings provide an example of how variable feeding strategies can counter the imbalance in C:nutrient ratios that organism typically face. © The Author 2010.