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Marine animals temporarily lose the ability to see when they enter water that is low in oxygen. The finding suggests the animals may struggle in the coming decades, as climate change is causing low-oxygen zones in the ocean to spread.

Lillian McCormick of the Scripps Institution of Oceanography in La Jolla, California and her colleagues studied the larvae of four marine animals: , , and . All these larvae rely heavily on vision.

The team placed each larva in a small water-filled well and inserted tiny electrodes into the retina of one eye. This allowed them to track the electrical signals the retina made in response to light. Then they exposed the larvae to lights of varying intensity, including some very brief flashes, while lowering the amount of oxygen in the water.

Retinas need a lot of energy, and therefore oxygen, to function, so it is not surprising that the larvae鈥檚 vision was impaired 鈥� but the effect was more severe than expected.

鈥淭he retinal function started declining at oxygen levels we weren鈥檛 expecting to be stressful for these marine animals,鈥� says McCormick. In lower oxygen levels, the retinal response fell by 60 to 100 per cent.

However, there was a lot of variation. The market squid and brachyuran crab larvae were the most sensitive. 鈥淎s soon as we started decreasing oxygen availability, their retinal responses started decreasing,鈥� says McCormick. 鈥淭he octopus held on a little bit longer.鈥� The tuna crab was the most tolerant of low oxygen levels.

The larvae鈥檚 retinas began to recover once they were put back into oxygenated water. However, McCormick says prolonged exposure might cause permanent harm, just as a stroke can permanently damage a person鈥檚 eyesight.

We do not know how animals handle low-oxygen zones, says McCormick. 鈥淚 would assume they鈥檙e going to try to avoid those areas,鈥� she says. 鈥淚t鈥檚 probably not very safe for them to have compromised visual function. They might not see a predator and get eaten, or might starve because they can鈥檛 visually detect their prey.鈥�

This poses a problem. The low-oxygen zones in the seas are spreading as a result of climate change triggered by our greenhouse gas emissions. Even if animals caught in these zones do not asphyxiate, they may find themselves partially blinded.