Average sea-surface temperature and the amount of CO2 dissolved in the ocean are rising due to increasing concentrations of atmospheric CO2. Many coral reef fishes appear to be living close to their thermal optimum, and for some of them, even relatively moderate increases in temperature (2 – 4°C) lead to significant reductions in aerobic scope (i.e. the difference between resting and maximum rates of oxygen uptake). Reduced aerobic capacity could affect population sustainability since less energy can be devoted to feeding and reproduction. Coral reef fish seem to have limited capacity to acclimate to elevated temperature as adults, but recent research shows that developmental and transgenerational plasticity occur, which might enable some species to adjust to rising ocean temperatures. A mayde even greater worry is that, even at maintained water temperatures, predicted future rises in ocean CO2 levels have been found to alter sensory responses and behaviour of marine fishes. Changes include increased boldness and activity, loss of behavioural lateralization, altered auditory preferences and impaired olfactory function. It has been found that impaired olfactory function makes larval coral reef fish attracted to odours they normally avoid, including ones from predators and unfavourable habitats. These behavioural alterations have significant effects on mortality that may have far-reaching implications for populations. However, the underlying mechanism linking high CO2 to these diverse responses has been unknown until recently. I will present results showing that abnormal olfactory preferences and loss of behavioural lateralization exhibited by two species of larval coral reef fish exposed to high CO2 can be rapidly and effectively reversed by treatment with an antagonist of the GABA-A receptor. GABA-A is a Cl- / bicarbonate channel and a major neurotransmitter receptor in the vertebrate brain. These results indicate that high CO2 interferes with brain ion-balance and thereby neurotransmitter function, a hitherto unrecognized threat to marine populations and ecosystems. Given the ubiquity and conserved function of GABA-A receptors, this predicts that rising CO2 levels could cause sensory and behavioural impairment in a wide range of marine animals. See Munday et al. (2012) and Nilsson et al. (2012) for further reading.