Finding Nemo's gone deaf owing to high CO2

Hollywood's "Finding Nemo" charted the quest of Dory, a fish with a memory problem, to track down his missing clownfish friend. But rising CO₂ levels might mean that more fish could go "awol" like Nemo, according to British and Australian scientists.

Writing in the journal Biology Letters, Bristol-based researcher Steve Simpson, working with collaborators at the James Cook University in Queensland, has discovered that exposing newborn clownfish to the sorts of CO₂ levels predicted for the end of the century, based on present trends, causes the fish to ignore the normal auditary instincts that help them to find a home and avoid danger in the wild.

So rather than swimming away from the sounds of reef predators played to them through an underwater speaker like control animals did, the fish reared under enriched CO₂ conditions swam equally in all directions. And if this were their home reef, that would probably mean straight into the open mouth of just one of those predators.

The researchers don't know why the elevated CO₂ levels are having this effect.

"Previously it's been shown that raising carbon dioxide levels causes fish to lose their ability to follow olfactory [smell] cues to find their way around, although the mechanism isn't known" explains Simpson. "Now it seems that it has the same effect on hearing."

The team have examined the hearing organs in the affected fish, which appear to be grossly normal, suggesting that perhaps the high CO₂ levels instead trigger more subtle disruption of the sensory nervous system, or stress the animals causing them to turn their backs on their instincts.

The team are now beginning to look at the progeny of fish reared under elevated carbon dioxide conditions to determine whether the observations are merely a short-term response from which the animals can recover and adapt.