Defensive behaviours and activity levels of the tropical squid Idiosepius pygmaeus are altered by projected near-future CO2 levels

Chair: Martin Grosell

Blake Spady (1,2)*, Sue-Ann Watson (2), and Philip Munday (2)

1 College of Marine and Environmental Sciences, James Cook University, Townsville, QLD 4811, Australia
2 ARC Centre of Excellence for Coral Reef Studies, James Cook University, Townsville, QLD 4811, Australia

Background
Carbon dioxide (CO2) levels projected to occur in the oceans by the end of this century cause a range of behavioural effects in fish and some invertebrates, but whether other highly active marine organisms, such as cephalopods, are similarly affected is unknown. Here, we tested the effects of projected future CO2 levels on the behaviour of two-toned pygmy squid, Idiosepius pygmaeus.

Methods
Squid were collected from Cleveland Bay in Townsville, Queensland and habituated for five days in ambient CO2 conditions (447 µatm) and in elevated CO2 treatments consistent with lower (626 µatm) and upper (956 µatm) end of century projected CO2 levels. Squid were then video recorded in a tank to determine their activity levels followed by recording their escape response from a visual startle cue.

Findings
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.

Conclusions
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.