Ocean Acidification Could Make Tiny Fish Lose Their Hearing

An immobilized fish lay between Craig Radford's fingers. The several-week-old Australasian snapper, no longer than a pinkie nail, rested flat on a slab of modeling clay, held down by small staples—“as someone would strap you down on an ambulance bed to hold you there,” says Radford. He stuck tiny electrodes on the fish’s head, then submerged it in a tank and switched on an underwater speaker. It was time to test its hearing. “If you actually put your head underwater and take the time to listen, it's amazing what you'll hear,” Radford says. “From whales to fish to crustaceans—sound plays an important role in many, many different species’ life strategies.”
But Radford’s experiment wasn’t due to curiosity about what the world sounds like to fish. He was worried about how well they could hear it.Life-forms lurking in Earth's oceans depend a lot on what humans do above the surface. After we burn the carbon-rich fuels that nations mine, chop, and slurp out of the ground, they meander into the atmosphere as pollution, such as carbon dioxide. Increased CO2 in the atmosphere leads to more dissolved CO2 in the oceans, too, where it acidifies plant and animal habitats. In some cases, the consequences make intuitive sense: A more acidic ocean corrodes coral reefs and the symbiotic microorganisms hanging around them. But other effects are less straightforward, and Radford and his team found a weird one: The CO2 levels can morph the inner ears of fish, leading to hearing loss.
In a new study, a team of researchers from the University of Auckland, New Zealand, and James Cook University in Australia used neural electrodes and a micro CT scanner to measure the first evidence of what happens to reef fish hearing when larvae develop in a more acidic ocean. They found that the juvenile Australasian snapper can be roughly 10 times less sensitive to sound—a potentially fatal blow to animals that rely on hearing to find their way home. The team’s result highlights a surprising example of knock-on effects of atmospheric change. The work was published last week in the Proceedings of the Royal Society B.“It's not intuitive that fish would not be able to hear, or not be able to smell, or not be able to behave correctly,” says Sean Bignami, a biologist running Concordia University Irvine’s Marine Lab, who was not involved with the study. Bignami studies how acidification affects ocean life, and he studied fish inner ears in his doctoral work. “I think it's fascinating,” he says of the new results.

Many reef fish are actually hatched in the open ocean, and the juveniles must swim back to the reef where they will make their home, Radford says. “A lot of work has shown that sound is an orientation cue to find their way back,” he says. Messing with that sense of hearing can threaten a species’ survival.

Sound is a major concern for marine ecologists studying how human behavior in the air-breathing world affects our finned kin. Warming oceans make snapping shrimp snap louder , creating noisy interference for their underwater neighbors, and an enormous review published in Science in February, on which Radford was a coauthor, concluded that human noise has made the “soundscape” unbearable for sea creatures. Noise pollution drowns out whale sounds, for instance, which complicates socializing and mating.
Fish, including the Australasian snapper, use sound to communicate, procreate, and orientate. Some use it to attract mates or synchronize egg and sperm release. Some baby fish use it to find suitable reefs to live in.