Effect of ocean acidification on otolith development in larvae of a tropical marine fish

Abstract

Calcification in many invertebrate species is predicted to decline due to ocean acidification. The potential effects of elevated CO₂ and reduced carbonate saturation state on other species, such as fish, are less well understood. Fish otoliths (earbones) are composed of aragonite, and thus, might be susceptible to either the reduced availability of carbonate ions in seawater at low pH, or to changes in extracellular concentrations of bicarbonate and carbonate ions caused by acid-base regulation in fish exposed to high <i>p</i>CO₂. We reared larvae of the clownfish <i>Amphiprion percula</i> from hatching to settlement at three pH_NBS and <i>p</i>CO₂ levels (control: ~pH 8.15 and 404 μatm CO₂; intermediate: pH 7.8 and 1050 μatm CO₂; extreme: pH 7.6 and 1721 μatm CO₂) to test the possible effects of ocean acidification on otolith development. There was no effect of the intermediate treatment (pH 7.8 and 1050 μatm CO₂) on otolith size, shape, symmetry between left and right otoliths, or otolith elemental chemistry, compared with controls. However, in the more extreme treatment (pH 7.6 and 1721 μatm CO₂) otolith area and maximum length were larger than controls, although no other traits were significantly affected. Our results support the hypothesis that pH regulation in the otolith endolymph can lead to increased precipitation of CaCO₃ in otoliths of larval fish exposed to elevated CO₂, as proposed by an earlier study, however, our results also show that sensitivity varies considerably among species. Importantly, our results suggest that otolith development in clownfishes is robust to even the more pessimistic changes in ocean chemistry predicted to occur by 2100.