High levels of carbon dioxide cause fish to grow abnormally large ear bones — known as otoliths — according to new research from the Scripps Institution of Oceanography at the University of California – San Diego.

Otoliths serve a vital function in fish by helping them sense orientation and acceleration.

Before conducting their experiments, Scripps researchers had hypothesised that otoliths in young white seabass raised in waters with elevated carbon dioxide would grow more slowly than a comparable group growing in seawater with normal CO2 levels. Instead, they discovered the reverse, finding “significantly larger” otoliths in fish developing in high-CO2 water.

The high-CO2 fish themselves weren’t larger than normal — only their otoliths.

“At this point one doesn’t know what the effects are in terms of anything damaging to the behaviour or the survival of the fish with larger otoliths,” said David Checkley, lead author of the study. “The assumption is that anything that departs significantly from normality is an abnormality and abnormalities at least have the potential for having deleterious effects.”

With carbon dioxide levels rising due to human activities, particularly fossil-fuel burning, resulting in both increased ocean CO2 and ocean acidification, the researchers intend to broaden their studies to examine specific areas, such as determining whether the otolith growth abnormality exists in fish other than white seabass; locating the physical mechanism that causes the enhanced otolith growth; and assessing whether the larger otoliths have a functional effect on the survival and the behaviour of the fish.

“Number three is the big one,” said Checkley. “If fish can do just fine or better with larger otoliths then there’s no great concern. 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.”

In addition to serving in orientation and acceleration, otoliths help reveal physical characteristics of fish. Because otoliths grow in onion-like layers, scientists use otoliths to determine the age of fish, counting the increments similar to tree-ring dating.

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