Just when you thought you’d heard it all about the effects of greenhouse gases, researchers have found that higher atmospheric carbon dioxide levels could lead to changes in fish anatomy involved in navigation.

A study of white sea bass eggs and larvae showed that the developing fish grew larger ear bones (otoliths, which don't field sound, but rather help fish sense speed and direction) under higher CO2 concentrations. The results appear in the latest issue of Science.

The researchers raised the fish at different CO2 saturations and then examined their otoliths under scanning electron microscopes. The find may spell trouble for fish down the road. It also highlights the unpredictable nature of biological changes in CO2-rich waters, as they had expected the ear bones to shrink rather than grow.

Ocean acidification, driven by rising CO2 in the atmosphere, is occurring faster than had previously been predicted. The change in seawater is retarding coral growth and corroding shells. But as the study authors note, "The specific effects of elevated CO2, not simply acidification, should be considered."

Although the researchers still don't know what effect the overgrown otoliths will have on the fish, "fish have evolved to have their bodies the way they are," David Checkley, lead study author and professor at the Scripps Institution of Oceanography, said in a statement. "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."

Image of 3- to 4- month-old juvenile white sea bass courtesy of Hubbs-SeaWorld Research Institute