Quantifying directed behavioural stress response in two planktonic rotifer species

Abstract In a spatially heterogeneous world, it is key for animals to sense the quality of their immediate environment and perform a directed avoidance behaviour when the environment is harmful. In aquatic environments, steep chemical gradients can be found at the thermocline or at the sediment-water interface resulting in small scale changes of good and harmful patches. Zooplankton that can escape from harmful patches increases its fitness. In this study, the responses of two planktonic, cosmopolitan micro-invertebrates, Brachionus calyciflorus and Brachionus rubens, to the stressors pH, copper and salt, and to food, were tested with a straightforward experimental set-up. A clear avoidance behaviour of both species was observed when the salt concentration was increased by 1 and 2 g l−1 and at an increase in copper by 0.3 and 1 mg l−1 as compared with controls. The response to pH was species-specific: B. calyciflorus was unresponsive, whereas B. rubens avoided the environment with a pH of 5 and 9 versus the control of pH 7. The results demonstrate the ability of these micro-invertebrates to actively avoid environmental stressors and complement toxicological studies that quantify the toxicity in a homogeneous environment. They also allow for a better understanding of the spatial distribution of zooplankton in the field.