Long-term acclimation to near-future ocean acidification has negligible effects on energetic attributes in a juvenile...

Increased levels of dissolved carbon dioxide (CO₂) drive ocean acidification and have been predicted to increase the energy use of marine fishes via physiological and behavioural mechanisms. This notion is based on a theoretical framework suggesting that detrimental effects on energy use are caused by plasma acid-base disruption in response to hypercapnic acidosis, potentially in combination with a malfunction of the gamma aminobutyric acid type A (GABA_A) receptors in the brain. However, the existing empirical evidence testing these effects primarily stems from studies that exposed fish to elevated CO₂ for a few days and measured a small number of traits. We investigated a range of energetic traits in juvenile spiny chromis damselfish (Acanthochromis polyacanthus) over 3 months of acclimation to projected end-of-century CO₂ levels (~ 1000 µatm). Somatic growth and otolith size and shape were unaffected by the CO₂ treatment across 3 months of development in comparison with control fish (~ 420 µatm). Swimming activity during behavioural assays was initially higher in the elevated CO₂ group, but this effect dissipated within ~ 25 min following handling. The transient higher activity of fish under elevated CO₂ was not associated with a detectable difference in the rate of oxygen uptake nor was it mediated by GABA_A neurotransmitter interference because treatment with a GABA_A antagonist (gabazine) did not abolish the CO₂ treatment effect. These findings contrast with several short-term studies by suggesting that end-of-century levels of CO₂ may have negligible direct effects on the energetics of at least some species of fish.