As climate change causes oxygen levels to plunge in the world’s oceans, a variety of sea creatures could lose much of their eyesight, harming their ability to survive, according to a new study published in the Journal of Experimental Biology.
Lillian McCormick, the lead researcher and author of the study, wanted to understand the relationship between sight and oxygen availability in various cephalopods and crustaceans that absorb oxygen through the water, as opposed to animals that surface to breathe in oxygen.
“It’s important to research sub-lethal effects of low oxygen on animals,” she told Global Citizen, referring to effects that are serious but not necessarily deadly.
“A lot of research has been done on critical limits that cause mass die-offs, but I think it’s also really interesting to look at some of these sub-lethal effects that affect behavior and survival before it gets to the point of actually killing the animal," she said.
Oxygen levels vary throughout the oceans, depending on depth, temperature, and other factors, and often dictate where marine animals go to reproduce and find food.
As the world’s oceans absorb the majority of greenhouse gas emissions from human activity, they’re undergoing chemical reactions that make waters more acidic, a process known as ocean acidification. This phenomenon, in combination with warming waters driven by climate change, decreases overall oxygen levels.
To conduct her experiment, McCormick first gathered larvae of two-spot octopus, tuna crab, and brachyuran crab with the help of a specialized net. She also dove to get market squid eggs and then hatched them in the lab.
Once the larvae were prepped, she put them one at a time in a contained well on a microscope stand in a pitch black room. She could control the oxygen levels in the well through a pump that allowed her to change out the seawater. She then shined a light on the creatures.
“Then I would stick an electrode just under its eye until I had a stable visual recording,” she said, explaining a method called “in vivo electroretinogram” that recorded the larvae’s retinal sensitivity to light.
“We knew that terrestrial animals, when exposed to lower oxygen levels, suffer visually,” McCormick said. “Humans might lose color vision or lower sensitivity to light in higher altitudes.
“We didn’t know what would happen in marine animals, but we learned that whenever I decreased the oxygen, the visual response also decreased,” she added.
Different species reacted differently to changes in oxygen levels.
“There were very large differences between species,” she said. “We expected that there would be a uniform response, but we didn’t know it would be so different between different species.”
The squid and the rock crab were particularly sensitive, McCormick explained.
“As soon as I decreased the oxygen, their retinal responses started to decrease as well, even at levels that we didn’t think they would be sensitive at,” she said.
Certain larvae placed for 30 minutes in reduced oxygen environments nearly went blind, and McCormick found that some larvae lost 60% of their vision while in oxygen conditions reminiscent of the parts of the ocean where they live. The larvae were able to largely recover their vision when returned to normal oxygen levels, but the findings have profound implications for a future in which oxygen levels are expected to decline by as much as 6% in the world’s oceans.
McCormick also tested how well animals responded to flashes of light in different oxygen conditions and found that this ability, known as temporal resolution, was also negatively affected.
“[These animals] feed on very fast-moving prey items with very fast darting movements,” she said. “If they can’t detect them enough to have successful capture, then they might have issues with starvation.”
McCormick said it’s too soon to use these findings to gain insight into adult animal behavior in the wild, but she said that it would make sense that species would be negatively affected.
“All these larvae are highly visual,” she said. “They rely on vision for predator avoidance and prey detection, as well as light cues for vertical migration.”
“These larvae are swimming up and down the water column on a daily cycle,” she said. “They swim up to the surface to feed there, and then they sink back down to avoid predators.”
She plans to study the effects of oxygen on fish and adult animals in the future, and hopes that her research one day informs marine management plans.
Marine animals are being affected in other ways by ocean warming and acidification.
In eastern Australia, green sea turtles are giving birth almost exclusively to females because of warmer water, a bizarre biological shift that could undermine the species’ survival.
The planet’s polar regions are melting so fast that entire marine ecosystems could be destroyed.
Ocean acidification, meanwhile, is making it harder for crustaceans to form shells and reproduce.
Oxygen-deprived marine environments aren’t inevitable. If countries work together to reduce carbon emissions into the atmosphere, then the oceans could begin to stabilize, allowing marine creatures to flourish.
“Ocean deoxygenation is very much a concern,” McCormick said. “But it depends on the time scale it happens over. If it happens on a faster time scale, these organisms might not be able to develop mechanisms — learning to avoid lower oxygen areas, or evolving to develop higher tolerance to oxygen — to cope with the changing conditions.”