Our planet’s oceans are in a crisis. When we think about global warming, we usually think about the continents and the air, but the oceans are heating up much more than the atmosphere. The ocean is storing an estimated 91% of the excess heat energy trapped in the Earth’s climate system and the consequences are already affecting wildlife.

We see this already. Numerous creatures are migrating to the deeper, cooler parts of the ocean. But octopuses are particularly vulnerable.

If temperatures continue to rise as expected, heat stress in the oceans could be devastating for octopuses. In addition to all the obvious problems, octopuses could lose their eyesight.

Heated vision

Octopuses are some of the smartest creatures in the ocean, and they may seem like unlikely victims of climate change. However, octopuses are ectothermic animals, meaning their body temperature and metabolic processes are directly influenced by the temperature of their surrounding environment. This characteristic makes them particularly sensitive to the warming oceans, which can disrupt their metabolic balance, growth, and reproduction.

In the new study, researchers from Australia and the US wanted to see how warming temperatures will affect octopuses. They focused on their eyes because octopuses (like humans) are highly visual creatures. In fact, octopuses dedicate about 70% of their brains to vision alone.

The team, led by Dr. Qiaz Hua, a recent Ph.D. graduate from the University of Adelaide’s School of Biological Sciences, exposed unborn octopuses and their mothers to three different temperatures:

• 19°C — the control temperature;
• 22°C — a common summer temperature for oceans;
• 25°C — the forecast summer ocean temperatures by 2100.

“We found several proteins important for vision that were affected by thermal stress,” says Hua.

Octopuses exposed to 25°C water were found to produce significantly fewer vision proteins than the other two groups. The researchers expect this to significantly affect their vision.

“One of them is a structural protein found in high abundance in animal eye lenses to preserve lens transparency and optical clarity, and another is responsible for the regeneration of visual pigments in the photoreceptors of the eyes.

“The levels of both of these proteins were significantly reduced under projected ocean warming conditions, which suggests that octopus vision is likely to be impaired under thermal stress.”

More than a vision problem

The study also showed that temperature affects unborn octopuses in other ways. The higher the temperatures got, the higher the risk of unborn octopuses and premature deaths of pregnant mothers. At the highest temperature, less than half of the octopus eggs were viable and mothers in this group were showing “visible signs” of stress. Even the hatchlings that did survive exhibited an “immense amount of thermal stress”, the researchers say. Most of them are unlikely to survive into adulthood.

“We found a high mortality rate under future warming conditions. Out of three replicate octopus broods, none of the eggs hatched for two of them and less than half of the eggs hatched for the remaining brood,” Dr. Hua says. “In the broods where none of the eggs hatched, the mothers died naturally while the eggs were still in early development stages.

“Because maternal care of embryos occurs in octopuses, global warming could have a simultaneous impact on multiple generations, with the low survival rate of the embryos caused by the direct effect of thermal stress as well as the indirect effect of thermal stress on the mothers. Our study shows that even for a highly adaptable taxon like octopuses, they may not be able to survive future ocean changes.”

This study also fits with previous research that found that ocean heat is a major stressor of octopuses, affecting their metabolic rate, size, and survival rate.

Qiaz Q. H. Hua et al, Projected ocean temperatures impair key proteins used in vision of octopus hatchlings, Global Change Biology (2024). DOI: 10.1111/gcb.17255