Behavioural abnormality in fishes has been proposed as a significant threat of the increasing levels of carbon dioxide occurring in the oceans. Negative effects of elevated CO2 have been reported for behaviours such as predator-prey interactions, foraging, hearing and behavioural lateralisation. Importantly, the effects vary greatly both within and between species, and some recent studies have shown minimal effects of CO2 on behaviour. Whether the effect of CO2 also varies between males and females is...