Rising carbon dioxide affects ear structure of fish

Listen up! Carbon dioxide being absorbed by the oceans is having a puzzling effect on fish -- their ears get bigger.

The ear structure in fish, known as an otolith, is made up of minerals. Scientists knew that increasing carbon dioxide in the oceans -- absorbed from the atmosphere -- is making the sea more acidic, which can dissolve and weaken shells. They wondered if it also would reduce the size of the otoliths.

It turned out to be just the opposite, according to a study published Friday in the journal Science.

Biological oceanographer David M. Checkley of the Scripps Institution of Oceanography at UC San Diego and colleagues incubated the eggs of white sea bass in seawater and measured the otoliths when the fish were 7 to 8 days old.

In the first test, the water had more than six times the usual amount of carbon dioxide, and the little fish grew otoliths 15% to 17% larger than normal. The surprised researchers repeated the experiment and got the same results.

So they reduced the carbon dioxide to about 3.5 times the current level -- a concentration that could occur by the year 2100 at the rate CO2 is being added to the atmosphere and then the sea. Those fish had ear bones 7% to 9% larger than fish raised in seawater with current gas concentrations.

As in humans, fish ears perform a major role in sensing movement and whether the animal is upright -- abilities that are important for survival.

“If fish can do just fine or better with larger otoliths, then there’s no great concern,” Checkley said. “But fish have evolved to have their bodies the way they are. The assumption is that if you tweak them in a certain way it’s going to change the dynamics of how the otolith helps the fish stay upright, navigate and survive.”

Researchers now will try and figure out how the added carbon dioxide in the water causes the ear bones to enlarge, whether this is happening to other types of fish, and whether the long-term effect will be good or bad, he said.