Carbon Dioxide Affects Fish Ear Structure

Levels of carbon dioxide, the dangerous greenhouse gas responsible for global warming, are increasing at an alarming rate. And not just in the air. The ocean’s absorption of CO2 from the atmosphere has resulted in increasingly acidic waters. Scientists have already begun to see the global effects of ocean acidification as carbonic acid, a by-product of CO2, has begun to destroy coral reefs as it hinders shell formation by corals, which in turn has lead to a forced habitat relocation for coral-dependent species. Scientists at the University of California in San Diego (UCSD) Scripps Institution of Oceanography led by biological oceanographer David Checkley, have recently discovered that the high levels of carbon dioxide in the oceans are causing mutations in fish, specifically affecting the bones in a fish’s ear.

As in humans, fish’s ears are important for survival as they perform a major role in the animal’s acceleration and orientation. The ear structure in fish is known as otoliths and is composed of minerals. The Scripps team assumed that since acidic waters weaken and dissolve shells made up of minerals, the otoliths of fish living in waters with high levels of CO2 should grow very slowly and even possibly result to smaller otoliths. Fish ears are inside their bodies, and therefore the change would not be noticeable by simply looking at the fish.

Checkley and his research team began their experiment by incubating the eggs of white sea bass in seawater that contained more than six times the normal amount of carbon dioxide. When the fish were between seven and eight days old, the scientists measured their otoliths and were met with quite a surprise. Contrary to the researchers assumptions, the young fish had otoliths that were 15 to 17 percent larger than normal. They repeated the experiment, only to receive the same results. The Scripps team then performed another experiment by reducing the carbon dioxide in the water to roughly 3.5 times the normal level. In contrast to the first experiments results, the otoliths in these fish were only 7 to 9 percent larger than normal.

The Scripps team published their conclusions in the June 26th issue of Science. The researchers state they found that the otoliths extreme growth mutation caused the animals to become disoriented and incapable of surviving in their normal environment. They also found that the fish’s body sizes did not grow proportionally to their ear bones.

Checkley has stated that this finding raises further questions that he and his research team hope to explore. Specifically in determining how the additional carbon dioxide the water affects the otoliths in addition to researching if similar otoliths deformities are occurring in fish other than the sea bass. Checkley also said that they will look at whether having larger ear bones affects a fish’s survival and behavior.

“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 otoliths helps the fish stay upright, navigate and survive… At this point one doesn’t know what the effects are in terms of anything damaging to the behavior or the survival of the fish with larger otoliths. The assumption is that anything that departs significantly from normality is an abnormality and abnormalities at least have the potential for having deleterious effects.”

To read the study:

Checkley, D., Dickson, A., Takahashi, M., Radich, J., Eisenkolb, N., & Asch, R. (2009). Elevated CO2 Enhances Otolith Growth in Young Fish Science, 324 (5935), 1683-1683 DOI: 10.1126/science.1169806