Behavioural impairment following exposure to ocean acidification-relevant CO2 levels has been noted in a broad array of taxa. The underlying cause of these disruptions is thought to stem from alterations of ion gradients (HCO3Cl) across neuronal cell membranes that occur as a consequence of maintaining pH homeostasis via the accumulation of HCO3Cl. While behavioural impacts are widely documented, few studies have measured acid-base parameters in species showing behavioural disruptions. In addition, current studies examining mechanisms lack resolution in targeting specific neural pathways corresponding to a given behaviour. With these considerations in mind, acid-base parameters and behaviour were measured in a model organism utilized for decades as a research model to study learning, the California sea hare (Aplysia californica). Aplysia exposed to CO2 elevated hemolymph HCO3Cl, achieving full and partial pH compensation at 1200 and 3000 μatm CO2, respectively. Increased CO2 did not affect self-righting behaviour. In contrast, both levels of elevated CO2 reduced the time of the tail-withdrawal reflex, suggesting a reduction in antipredator response. Overall, these results confirm that Aplysia are promising models to examine mechanisms underlying CO2-induced behavioural disruptions since they regulate HCO3-Cl- and have behaviours linked to neural networks amenable to electrophysiological testing.