Adaptive responses of fishes to climate change: Feedback between physiology and behaviour

The adaptive capacity of individuals, from their cells to their overall performance, allows species to adjust to environmental change. We assess a hierarchy of responses (from cells to organismal growth and behaviour) to understand the flexibility of adaptive responses to future ocean conditions (warming and acidification) in two species of fish with short lifespans by conducting a long-term mesocosm/aquarium experiment. Fishes were exposed to elevated CO₂ and temperature in a factorial design for a five-month period. We found a feedback mechanism between cellular defence and behavioural responses. In circumstances where their antioxidant defence mechanism was activated (i.e. warming or acidification), increased feeding rates prevented oxidative damage (i.e. during warming Sp. 1). However, when feeding rates failed to increase to provide additional energy needed for antioxidant defence, oxidative damage could not be prevented (warming + acidification Sp. 1). In contrast, when the activation of antioxidant defence was not required, energy intake from increased feeding was redirected to increased fish growth (acidification Sp. 2, warming + acidification Sp. 2), whilst no gain in growth rate was observed where feeding remained unchanged (acidification Sp. 1 or warming Sp. 2). This adaptive strategy seems to rely on the inherent behavioural response of fishes to their environment and such adjustability shows the kind of responses that organisms may express to prevail in future ocean climate. Indeed, assessing the link between responses from cellular to organismal levels, using a diversity of fitness indicators and behaviour, provides a fundamental understanding of how organisms as a whole may adjust to prevail in a future world.