Low-oxygen levels in the world’s oceans could cause some marine invertebrates, including crabs and octopuses, to lose their eyesight – at least temporarily.

We’ve known for some time that land-based animals are impacted by oxygen levels. For example, humans can lose visual function in low-oxygen environments. Think of fighter jet pilots – if they’re not given supplemental oxygen while at high altitudes, they experience vision impairment and other issues like high blood pressure and strokes. Now, researchers at the Scripps Institute of Oceanography demonstrate that marine animals are also highly sensitive to the amount of available oxygen in the water.

"With all of this knowledge about oxygen affecting vision in land animals, I wondered if marine animals would react in a similar manner," said lead author Lillian McCormick in a statement.

Publishing her work in the Journal of Experimental Biology, McCormick found that four local California marine invertebrates – market squid, two-spot octopus, tuna crab, and brachyuran crab – experienced vision reduction by between 60 and 100 percent under low-oxygen conditions.

"I was surprised to see that even within a few minutes of being exposed to low oxygen, some of these species became practically blind," said McCormick.

To test these animals’ responses to short-term reduced oxygen, McCormick placed larval specimens on a microscope stage with seawater that was gradually reduced in oxygen levels. She then exposed them to light conditions and recorded their visual responses using an “EKG machine for the eye” in which electrodes were connected to their retinas. In every case, she observed immediate responses when oxygen availability decreased. Brachyuran crab and squid lost almost all of their vision when the oxygen level was decreased to just 20 percent of surface levels. Octopuses were better able to tolerate oxygen depletion, and tuna crabs were found to be the most resilient (albeit still affected). When oxygen was restored, most specimens recovered at least some visual function.

This likely has to do with a concept called phototransduction, a highly complex visual system that translates energy from light into neural signals that give animals their vision. Altogether, it is one of the most “energetically expensive processes” and without it, many species could face life-threatening conditions. For example, many larval species migrate vertically depending on the time of day, heading to the depths during the lightest hours and ascending to the surface at night. Without vision, they might become lost, confused, and disrupt natural cycles.

Oxygen levels in the world’s oceans change due to natural processes, but these are being accelerated by human-influenced climate change and pollution. Atmospheric warming caused by greenhouse gas emissions decreases a process called upwelling, whereby well-oxygenated surface water is mixed with nutrient-poor water from the depths. Additionally, pollution has been linked to eutrophication in nearshore environments that cause plankton species to bloom and deplete oxygen levels.