Pilots flying at high altitudes need extra oxygen, or they'll start to lose vision — and eventually pass out.

Similarly, creatures dwelling in the oceans also require oxygen to see. Unfortunately for them, the seas are now gradually losing oxygen, a problematic marine event known as deoxygenation. Recognizing that this loss of oxygen could also cause blindness in sea organisms, scientists at the Scripps Institution for Oceanography tested how reduced oxygen levels impacted the vision of squid, octopus, and crab species. Their results, published in the Journal of Experimental Biology, showed that these organisms did indeed experience varying degrees of blindness, including near total blindness.

"It's a pretty significant issue in the oceans because organisms rely on oxygen to survive," said Lillian McCormick, a marine scientist at Scripps and lead author of the study, emphasizing that oxygen is needed for more than breathing. "Vision is a very demanding sensory process. It requires oxygen to function."

It's not as if sea creatures are now on the brink of blindness, suffocation, or death as they wander the dark seas without vision. But as oxygen levels gradually fall — due to the planet's accelerating warming — these creatures' vision may very well deteriorate.

"They might not be dead, but if they can’t see well, they potentially won't see predators, light changes, or things on the sea floor," said Karen Wishner, a biological oceanographer at the University of Rhode Island who had no role in the research. "It's potentially of critical importance."

A young marine organism tested in the vision experiment. Image: Scripps institution of oceanography

Out in the vast open oceans, there are two big drivers of oxygen loss. One is simple physics. "As the ocean absorbs heat — excess heat from the greenhouse effect — the water generally holds less gas," said Takamitsu Ito, an oceanographer at Georgia Tech who also had no role in the study.

The other is a decline in the mixing of oxygen into deeper ocean waters. The planet's oceans absorb over 90 percent of the heat trapped in the atmosphere by human-created carbon emissions, and most of this heat is absorbed near the surface. Oxygen is soaked up near the surface, too. This creates a robust, warm layer of water atop the sea. But now this layer of excessively warmed, buoyant water is more resistant to mixing in with the ocean depths. This deprives deeper dwelling animals of oxygen, explained Ito.

But unlike pilots, who can use supplemental oxygen up in the sky, sea creatures can't escape a water world with declining oxygen. And as this new research suggests, it won't just impact their breathing. It can blind them. "Here’s a whole different type of physiological response that can affect what these animals can do and how they live," said Wishner.

"Clearly, this study suggests there are other physiological issues besides breathing," added Jeremy Owens, a marine biogeochemist at Florida State University who had no involvement in the study.

To see how a species of octopus and squid, and two species of crab, reacted to incrementally lowered levels of oxygen, Scripps' McCormick collected dozens of these creatures from the ocean and brought them back to the institute's lab. In their tiny, young larvae stages, she placed them in seawater — which oxygen levels she could control — and then stuck a small electrode into their eyes. This allowed her, in real time, to measure the retinal function of their eyes (retinas contain visual cells), their sensitivity to light, and the ability of their retinas to respond to fast-moving objects.

Projected changes in sea surface temperatures (Celsius) by century's end if carbon emissions continue unchecked. Image: noaa

This technique, called an electroretinogram, is like the common medical practice of using an EKG to detect the health of a human heart, said McCormick.

The squid and brachyuran crab lost nearly all their sight, when exposed to the lowest oxygen levels tested. The octopus held out for a while, but eventually started going blind. Meanwhile, the tuna crab experienced the smallest loss, at 60 percent of its vision. "That's still a decent amount of loss," noted McCormick.

For a wild creature, losing eyesight is always a terrible option. But especially so for young creatures, who are battling competitors for food and seeking to avoid larger, voracious predators. In many marine creatures relatively few young larvae survive to adulthood as it is, so a decline or loss of vision can be detrimental, explained McCormick.

Fortunately, when she returned oxygen levels to normal, the critters studied regained their vision. But in a deoxygenated ocean, marine organisms won't have such luck.

Taken alone, gradual blindness is a big problem. But ocean dwellers also contend with both rising temperatures and acidifying waters, both of which are direct consequences of skyrocketing carbon emissions in the atmosphere.

"[Marine life] has multiple stressors — how do organisms respond to all these stressors?" asked Owens.

Another marine organism tested during the study. Image: scripps Institution of Oceanography

Humanity will find out. Though modern civilization has the potential and ability to fend off the grimmest of environmental scenarios, it's highly unrealistic — if not bordering on impossible — that nations around the world will curb warming at levels that would limit major impacts to the oceans.

What's more, there's no evidence that the ocean's oxygen loss will taper off. The rates of loss over the last half-century are already pretty extraordinary. "The current rate we’re losing oxygen is likely faster than we’ve seen in the geologic record," said Owens.

"It's not catastrophic at the moment," he added. "But will it continue to go down this path?"