Diel CO2 cycles reduce severity of behavioural abnormalities in coral reef fish under ocean acidification

Abstract

Elevated CO₂ 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 pCO₂ treatments, not considering the substantial diel pCO₂ variation that occurs in shallow reef habitats. Here, we reared juvenile damselfish, Acanthochromis polyacanthus, and clownfish, Amphiprion percula, at stable and diel cycling pCO₂ 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 pCO₂ in both experiments. However, diel pCO₂ 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 CO₂ were largely absent in fish reared at 750 ± 300 µatm CO₂. Overall, we show that diel pCO₂ cycles can substantially reduce the severity of behavioural abnormalities caused by elevated CO₂. 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 pCO₂ cycles will delay the onset of behavioural abnormalities in natural populations.

Figure 1

Effects of stable vs diel cycling elevated pCO₂ on behavioural responses in experiment one. (a) Absolute lateralization and (b) relative lateralization in juvenile Acanthochromis polyacanthus (n = 60 per treatment) were determined using a two-way T-maze. (c) Response to predator cue of juvenile Amphiprion percula (n = 16 per treatment) was determined using a two-choice flume. Different letters represent significant differences between treatments (Tukey, P < 0.05). Boxplots are sized according to the 25th and 75th quartiles, where the line identifies the median and the whiskers indicate the minimum and maximum values. +signs represent means.

Figure 2

Effects of stable vs diel cycling elevated pCO₂ on behavioural responses in experiment two. (a) Absolute lateralization and (b) Relative lateralization in juvenile Acanthochromis polyacanthus (n = 60 per treatment) were determined using a two-way T-maze. (c) Response to predator cue of juvenile Amphiprion percula (n = 24 per treatment) was determined using a two-choice flume. Different letters represent significant differences between treatments (Tukey, P < 0.05). Boxplots are sized according to the 25th and 75th quartiles, where the line identifies the median and the whiskers indicate the minimum and maximum values. +signs represent means.