An ongoing research project exploring the effects of rising levels of oceanic CO2 on squid neurology reveals that exposure to future levels of ocean acidification could shrink their brain volume by around 50%. This severe brain shrinkage appears to be most pronounced in the areas that interpret visual information, correlating with significant reductions in normal feeding behaviours and suggesting serious consequences for the future of squid and other cephalopods.
"Cephalopods are widely regarded as being one of the most intelligent groups of animals living in the ocean," says Dr Garett Allen, an assistant professor at Acadia University, Canada. The Coleoidea subclass of cephalopods that includes squid, cuttlefish and octopuses are believed to be the most intelligent invertebrates on Earth, possessing a similar number of neurons to dogs.
Ocean acidification caused by elevated levels of atmospheric CO2 is known to be pose a serious threat to many marine species, but this project, presented at the Society for Experimental Biology conference in Florence, Italy, reveals a previously unknown impact of ocean acidification on cephalopod neural anatomy.
Preliminary data from this study suggests that bigfin reef squid, Sepioteuthis lessoniana, reared from hatching within elevated levels of dissolved CO2 exhibited significant changes in brain physiology – the most surprising being an average 49% reduction in brain volume compared to a control group.
To investigate the effects of future ocean acidification on the squid neurology, collaborations between Dr Allen and project lead Dr Yung-Che Tseng whose his team is located at Academia Sinica's Marine Research Station reared the squid in one of two parallel water tanks: one representing the modern oceans (pH 8.2) and one representing the oceans in the year 2100 under a predicted climate change scenario (pH 7.8). After 90 days, the squid were removed and their heads were preserved for visual analysis with diffusion magnetic resonance imaging (dMRI).
Once the images were visualized and the brain morphometrics could be assessed, Dr Allen discovered that the brains from the ocean acidification tank squid were substantially smaller than the control tank squid. "I immediately saw that their brains were half the size and had to check the diagnostic output of the software," says Dr Allen. "It was a real surprise - I wasn't expecting that at all."
The study found no effect of CO2 on whole body size, so the brain volume was normalised to mantle length to account for variation in body size. This volume reduction was observed across the whole of the brain, but the greatest reductions were found in regions identified as the optic lobes and optic tracts, which were 52% and 62% less voluminous than squid reared under modern ocean conditions, respectively.
These new findings follow on from an earlier study that linked rising CO2 levels to reductions in hunting behaviours in bigfin reef squid, finding that an acute 7-day exposure to high CO2 levels resulted in a 65% reduction in hunting behaviours, and squid exposed to a full 90 days exposure from hatching showed a 42% reduction in hunting behaviours compared to controls.
The ability to rapidly capture and interpret visual information is vitally important for bigfin reef squid, as they rely on their eyesight for tracking and catching their prey. "We think that the reduced willingness to feed may be linked to a decline in visual acuity," says Dr Allen. "Not because of the retina itself, which looks to stay the same, but perhaps because the optic lobe is shrinking."
The factors causing this brain volume shrinkage are still being investigated, but Dr Allen believes that they may be likely due to energetic constraints within the brain or oxidative damage, which could mean that the brain is unable to relay information correctly and lead to the abnormal feeding behaviour that the team have observed.
Dr Allen and Dr Tseng are currently carrying out further studies on the brains of squid reared in the same future conditions at 30 and 60 days to further validate these findings and examine how this neural shrinkage manifests as the squid grows and matures.
