A recent study, published in the Journal of Experimental Biology , discovered that the amount of oxygen available to marine invertebrates – squids, crabs and octopuses – may be a key element that enables their vision. In the study, published online April 24, researchers saw a significant drop in retinal activity in four species of marine larvae (two crabs, an octopus and a squid) when the animals were exposed to reduced-oxygen environments for as little as 30 minutes.

For the study, McCormick and her team investigated the following specimens: the market squid (Doryteuthis opalescens), two-spot octopus (Octopus bimaculatus), the tuna crab (Pleuroncodes planipes) and the graceful rock crab (Metacarcinus gracilis). These species are all local to the Pacific Ocean near Southern California, and they all engage in a daily diving routine known as vertical migration. By night, they swim near the surface to feed; by day, they descend to greater depths to hide from the sun. As these creatures migrate up and down the water column, the oxygen availability changes dramatically.


For some species, even a minuscule drop in oxygen levels resulted in almost immediate vision loss, eventually causing near-total blindness before the oxygen was cranked back up again. Turning light particles into visual information is hard work, and your body relies on oxygen to get the job done. This is true whether you walk the land on two limbs or swim through the sea with eight.

According to lead study author Lillian McCormick (doctoral candidate at the Scripps Institution of Oceanography in La Jolla, California, some form of vision impairment may be a daily reality for these species, which migrate between the ocean’s highly oxygen-saturated surface and its hypoxic (low-oxygen) depths during their daily feeding routines. And as ocean oxygen levels continue to drop around the globe, in part due to climate change, the risks to these creatures could intensify.

It’s likely that because the Pacific naturally experiences a lot of low-oxygen conditions near Southern California, these highly sensitive species grapple with some form of vision impairment every day, McCormick said. (More research is needed to know for sure, though). These species are naturally developing avoidance behaviors so that they swim to higher-oxygen parts of the ocean when severe vision impairment sets in.

Turning light particles into visual information is hard work, and your body relies on oxygen to get the job done. This is true whether you walk the land on two limbs or swim through the sea with eight.


To find out whether these daily swings in oxygen affect the animals’ vision, McCormick attached small electrodes to the eyes of each one of her test larvae, none of which measured longer than 0.15 inches (4 millimeters). The electrodes recorded the electrical activity in each larva’s eyes as its retinas reacted to light. It’s “kind of like an EKG (electrocardiogram), but for your eyes instead of your heart,” McCormick said.

Each larva was then placed in a tank of water and made to look at a bright light while the water’s oxygen level was steadily decreased. Levels fell from 100% air saturation, oxygen levels you’d expect to find at the surface of the ocean, down to about 20% saturation, which is lower than what they currently experience. After 30 minutes of this low-oxygen condition, the oxygen levels were increased back to 100%.

While each of the four species showed a slightly different tolerance, all four took a marked blow to vision when exposed to the low-oxygen environment. Overall, each larva’s retinal activity dropped between 60% and 100% in low-oxygen conditions. Some species, particularly the market squid and the rock crab, proved so sensitive that they started losing their vision as soon as the researchers started decreasing the oxygen in the tank.

“By the time I reached the lowest oxygen levels, these animals were almost blinded,” McCormick said.

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