Diel CO cycles reduce severity of behavioural abnormalities in coral reef fish under ocean acidification.

Elevated CO levels associated with ocean acidification (OA) have been shown to alter behavioural responses in coral reef fishes. However, all studies to date have used stable pCO treatments, not considering the substantial diel pCO variation that occurs in shallow reef habitats. Here, we reared juvenile damselfish, Acanthochromis polyacanthus, and clownfish, Amphiprion percula, at stable and diel cycling pCO treatments in two experiments. As expected, absolute lateralization of A. polyacanthus and response to predator cue of Am. percula were negatively affected in fish reared at stable, elevated pCO in both experiments. However, diel pCO fluctuations reduced the negative effects of OA on behaviour. Importantly, in experiment two, behavioural abnormalities that were present in fish reared at stable 750 µatm CO were largely absent in fish reared at 750 ± 300 µatm CO. Overall, we show that diel pCO cycles can substantially reduce the severity of behavioural abnormalities caused by elevated CO. Thus, past studies may have over-estimated the impacts of OA on the behavioural performance of coral reef fishes. Furthermore, our results suggest that diel pCO cycles will delay the onset of behavioural abnormalities in natural populations.