Projected near-future CO2 levels increase activity and alter defensive behaviours in the tropical squid Idiosepius pygmaeus

Abstract

Carbon dioxide (CO2) 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 CO2 levels (626 and 956 µatm) on the behaviour of male two-toned pygmy squid, Idiosepius pygmaeus. Exposure to elevated CO2 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 CO2. Squid vigilance and defensive behaviours were also altered by elevated CO2 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 CO2. In addition, more escape responses were chosen over threat behaviours in body pattern displays at elevated CO2 and individuals were more than twice as likely to use ink as a defence strategy at 956 µatm CO2, 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.

Keywords: Anti-predator behaviour; Avoidance; Cephalopod; Escape; Ocean acidification; Startle response.

Fig. 1.. Photograph of pygmy squid.

Photograph of squid employing defensive anchor posture and with darkened mottle display body pattern. Note posture and body pattern type can occur independently and result in additional combinations to those seen here. Photograph reproduced with the permission of the copyright holder, Ria Tan.

Fig. 2.. Effect of elevated CO₂ on resting frequency of squid.

Proportion of squid that were at rest (zero line crosses), displayed low activity levels (1–25 line crosses), and high activity levels (more than 25 line crosses) in behavioural trials for individuals exposed to control (447 µatm), moderate (626 µatm), or high (956 µatm) CO₂. Sample sizes are displayed above the bars.

Fig. 3.. Effect of elevated CO₂ on the movement levels of squid.

Mean number of line crosses (± standard error) in 5-minute trials for squid exposed to control (447 µatm), moderate (626 µatm), or high (956 µatm) CO₂. Mean movement includes individuals that were at rest throughout the trial (zero line crosses) in light grey (left y-axis) and excludes individuals at rest in dark grey (right y-axis). Sample sizes are displayed above the bars.

Fig. 4.. Effect of elevated CO₂ on squid response to startle stimulus.

Proportion of squid that (a) produced ink as a defensive mechanism, (b) chose a defensive “anchor” posture or jet response, (c) displayed various body patterns in response to a visual startle stimulus from three CO₂ treatment levels: control (447 µatm), moderate (626 µatm), or high (956 µatm). Sample sizes are displayed above the bars.