Ocean acidification is shrinking octopuses’ brains

Our carbon dioxide emissions are causing the world’s oceans to become increasingly acidic. Researchers have now discovered unexpected side effects of this in octopuses – their brains are shrinking dramatically.

It is well known that, in the long term, ocean acidification could lead to what are essentially catastrophic conditions for many marine organisms. At present, the pH level – the measure used to indicate acidity – in the oceans is around 8.1. However, there is a clear risk that this will fall to around 7.8 by the year 2100, which would mean a sharp increase in acidification.

The consequences of this are unclear, but for many cephalopods at least, it could prove disastrous, judging by experimental studies carried out by researchers at Acadia University in Canada and Academia Sinica in Taiwan. They investigated the sensitivity to increasing acidification of a common ten-armed species, the large-finned reef cuttlefish (Sepioteuthis lessoniana), which is widely found in coastal waters of the Indian and Pacific Oceans.

The cuttlefish were placed in two different water tanks, one with a pH of 7.8 and one with a pH of 8.2. After 90 days, the animals were removed and the effects examined.

Immediately, something striking was discovered – the brains were significantly smaller in the cuttlefish that had been kept in the more acidic tank. No effect was observed on other organs, but brain volume had decreased dramatically – by a total of around 50 per cent – likely as a result of so-called oxidative stress. The greatest reduction was seen in the regions that control vision: the optic lobes and the optic nerve.

Cuttlefish hunt primarily using their sight, and the animals showed a clear reduction in their inclination to seek out prey following their exposure to the more acidic water. The long-term consequences remain unclear, but if the brain is unable to process information from the outside world, the result could be abnormal behaviour. Cuttlefish are recognised as intelligent creatures with a particularly great need for a functioning cognitive system. At present, many species are abundant, but unless carbon dioxide emissions are reduced, the future may look bleak for them.