Projected near-future CO(2) levels increase activity and alter defensive behaviours in the tropical squid Idiosepius pygmaeus

Carbon dioxide (CO(2)) levels projected to occur in the oceans by the end of this century cause a range of behavioural effects in fish, but whether other highly active marine organisms, such as cephalopods, are similarly affected is unknown. We tested the effects of projected future CO(2) levels (626 and 956 µatm) on the behaviour of male two-toned pygmy squid, Idiosepius pygmaeus. Exposure to elevated CO(2) increased the number of active individuals by 19–25% and increased movement (number of line-crosses) by nearly 3 times compared to squid at present-day CO(2). Squid vigilance and defensive behaviours were also altered by elevated CO(2) with >80% of individuals choosing jet escape responses over defensive arm postures in response to a visual startle stimulus, compared with 50% choosing jet escape responses at control CO(2). In addition, more escape responses were chosen over threat behaviours in body pattern displays at elevated CO(2) and individuals were more than twice as likely to use ink as a defence strategy at 956 µatm CO(2), compared with controls. Increased activity could lead to adverse effects on energy budgets as well as increasing visibility to predators. A tendency to respond to a stimulus with escape behaviours could increase survival, but may also be energetically costly and could potentially lead to more chases by predators compared with individuals that use defensive postures. These results demonstrate that projected future ocean acidification affects the behaviours of a tropical squid species.