Scientists have found that high carbon dioxide levels cause squid to bungle attacks on their prey.

The oceans absorb more than one-quarter of all the excess carbon dioxide (CO₂) released into the atmosphere by humans and this uptake of additional CO₂ causes seawater to become more acidic, said Blake Spady, PhD candidate from the ARC Centre of Excellence for Coral Reef Studies (Coral CoE) at James Cook University (JCU) who led the investigation.

He adds: "Climate models project that unless there is a serious commitment to reducing emissions, CO₂ levels will continue increasing this century to reach levels that will have far-reaching effects on sea life.”

Close up of adult bigfin reef squid, Sepioteuthis lessoniana. Credit: Blake Spady.

The team chose to study cephalopods (a group that includes squid, cuttlefish and octopuses) because while most previous behavioral studies have focused on fishes, the effects of elevated CO₂ on highly active invertebrates is largely unknown.

"Cephalopods also prey on just about anything they can wrap their arms around and are themselves preyed upon by a wide range of predator species, so they occupy an important place within marine food webs."

The scientists tested the effects of elevated CO₂ on the hunting behaviors of pygmy squid and bigfin reef squid.

Spady explains: "For pygmy squid, there was a 20% decrease in the proportion of squid that attacked their prey after exposure to elevated CO₂ levels. They were also slower to attack, attacked from further away, and often chose more conspicuous body pattern displays at elevated CO₂ conditions.

Bigfin reef squid showed no difference in the proportion of individuals that attacked prey, but, like the pygmy squid, they were slower to attack and used different body patterns more often."

The researchers found that both species showed increased activity at elevated CO₂ conditions when they weren't hunting, which suggests that they could also be adversely altering their 'energy budgets'.

"Overall, we found similar behavioral effects of elevated v on two separate cephalopod orders that occupy largely distinct niches. This means a variety of cephalopods may be adversely affected by rising CO₂ in the oceans, and that could have significant consequences in marine ecosystems," said co-author Dr Sue-Ann Watson.

"However, because squid have short lifespans, large populations, and a high rate of population increase, they may have the potential to adapt to rapid changes in the physical environment," added Spady.

"The fast lifestyle of squid could mean they are more likely to adapt to future ocean conditions than some other marine species, and this is the next question we intend to investigate."