Depleting oxygen in warm oceans can blind marine critters

On high mountains, scarce oxygen can impair eyesight and enfeeble depth perception in climbers. But in oceans, depleting oxygen can almost blind marine invertebrates, according to a recent study.

Those affected were early life stages of California’s tidal species — the market squid, two-spot octopus, tuna crab, and graceful rock crab, whose sophisticated eyes demand more oxygen.

Every spring and summer, these shallow-water creatures naturally witness low-oxygen environments. Seasonal churning of the Pacific Ocean brings oxygen-poor waters from the deep ocean to the surface. But as oceans warm and hold less oxygen, this condition may become an everyday impediment for marine life.

Even if the critters survive (the ordeal), their ability to find food, mates, and avoid predators could be compromised. It’s like removing contact lenses, said Lillian McCormick, a marine biologist at Scripps Institution for Oceanography and lead author of the study. “You can walk around your apartment, find you coat … but you can’t be driving.”

Seeing needs breathing

Animal life in the ocean needs oxygen to breathe. Oceans supply this oxygen by absorbing it from air or circulating the oxygen released by photosynthesizing plant-like organisms.

Critters with complex eyes require high blood-oxygen to power them. In marine systems, these include arthropods like crabs and krill, cephalopods like squid and octopus, and all kinds of fish.

Without this oxygen supply, these creatures run the biggest risk of losing eyesight, said  McCormick.

But as the waters warm, less oxygen dissolves in it. Layers of toasty water sit atop cold oceans and prevent oxygen from reaching the depths. And the hotter it gets, the more oxygen animals need to perform bodily functions.

McCormick and her team set out to test vision in marine animals under changing oxygen levels. And it was the young ones, or larvae, that piqued her interest.

“The larval stage is a huge bottleneck for survival,” said McCormick. “If you don’t survive, you’re obviously not going to recruit to the adult stage.”

The hypoxia experiment

Using fine nets, McCormick collected two-spot octopus, tuna crab, and graceful rock crab larvae off southern California’s shores. And on full or new moon days she dove 100 feet to hoard freshly-hatched market squid eggs. “They looked like mophead piles on the seafloor,” she recalled.

At the University of San Diego’s electrophysiology laboratory, these larvae rested and eggs hatched. As soon as young squids emerged, McCormick began the experiments. And the mysteries unraveled in a dark room, under a microscope lens.

Suspended in a tiny well of seawater, pearl-sized larvae experienced oxygen levels changing between 100 percent to hypoxic, or lacking in oxygen. A miniature electrode inserted into the eye of each live critter recorded their response to one-second flashes of light.

“You can think of it as an EKG,” McCormick said. “Instead of measuring the activity from your heart, it’s measuring the activity in your eye.”

In the ocean, larvae must track fast-moving food and dodge predators. McCormick mimicked these conditions by changing the speed and brightness of the flashing light. She wanted to know if low oxygen environments alter these nuanced seeing abilities.

Blinding larvae

To the researchers’ surprise, all larvae either lost partial vision or turned completely blind in poor-oxygen settings. Graceful rock crabs and market squid responded almost immediately to dialed down oxygen levels.

“We were surprised at how little oxygen drop it took to elicit visual impairment,” said oceanographer Lisa Levin of Scripps Institution for Oceanography, who supervised the research.

By the time oxygen levels dwindled to 3%, the squid, octopus and rock crab larvae had completely lost their eyesight. Although in tuna crabs, a species known for tolerating low oxygen, vision performance never dropped below 60 percent.

“It’s surprising,” said Philip Bickler, principal investigator at the Hypoxia Lab in UC San Francisco who wasn’t involved in the study. Unlike humans, he considered marine invertebrates to be more tolerant of low and fluctuating oxygen levels. “It points out that there might be something about visual systems and their sensitivity to oxygen that we didn’t really appreciate before.”

While the blinding was temporary and resumed as oxygen levels bounced back, oceanographer Tessa Hill of UC Davis, who wasn’t involved in the study, said the results are “obviously important for understanding future impacts.”

“We’re already seeing changes in oxygenation off California’s coast,” she said, referring to ocean depths used by the larvae. “But how fast those changes now occur will be determined by our decisions around global climate change.”

The Scripps research group hopes to understand what this vision loss means for animals’ survival. Could it push them to well-oxygenated areas in search of food? Or like the crucian carp fish temporarily shutting down their eyesight and hibernating in the ice-covered, oxygen-lacking ponds of Europe, marine creatures off California’s coast may adopt new behaviors.

“We’re just at the beginning at this kind of research,” Levin said.