Exposure to high pCO2 or low pH alters sensation and behaviour in many marine animals. We show that crab larvae lose their ability to detect and/or process predator kairomones after exposure to low pH over a time scale relevant to diel pH cycles in coastal environments. Previous work suggests that acidification affects sensation and behaviour through altered neural function, specifically the action of γ-aminobutyric acid (GABA), because a GABA antagonist, gabazine, restores the original behaviour. Here, however, gabazine resulted in a loss of kairomone detection/processing, regardless of pH. Our results also suggest that GABAergic signalling is necessary for kairomone identification in these larvae. Hence, the mechanism for the observed pH effect varies from the original GABA hypothesis. Furthermore, we suggest that this pH effect is adaptive under diel-cycling pH.