Climate change is rewiring fish brains — and probably ours, too – Grist Magazine

Imagine you are a clown fish. A juvenile clown fish, specifically, in the year 2100. You live near a coral reef. You are orange and white, which doesn’t really matter. What matters is that you have these little ear stones called otoliths in your inner ear, and when sound waves pass through the water and then through your body, these otoliths move and displace tiny hair cells, which trigger electrochemical signals in your auditory nerve. Nemo, you are hearing.

But you are not hearing well. In this version of century’s end, humankind has managed to pump the climate brakes a smidge, but it has not reversed the trends that were apparent a hundred years earlier. In this 2100, atmospheric carbon dioxide levels have risen from 400 parts per million at the turn of the millennium to 600 parts per million — a middle‑of‑the-road forecast. For you and your otoliths, this increase in carbon dioxide is significant, because your ear stones are made of calcium carbonate, a carbon-based salt, and ocean acidification makes them grow larger. Your ear stones are big and clunky, and the clicks and chirps of resident crustaceans and all the larger reef fish have gone all screwy. Normally, you would avoid these noises, because they suggest predatory danger. Instead, you swim toward them, as a person wearing headphones might walk into an intersection, oblivious to the honking truck with the faulty brakes. Nobody will make a movie about your life, Nemo, because nobody will find you.

It’s not a toy example. In 2011, an international team of researchers led by Hong Young Yan at the Academia Sinica, in Taiwan, simulated these kinds of future acidic conditions in seawater tanks. A previous study had found that ocean acidification could compromise young fishes’ abilities to distinguish between odors of friends and foes, leaving them attracted to smells they’d usually avoid. At the highest levels of acidification, the fish failed to respond to olfactory signals at all. Hong and his colleagues suspected the same phenomenon might apply to fish ears. Rearing dozens of clown fish in tanks of varying carbon dioxide concentrations, the researchers tested their hypothesis by placing waterproof speakers in the water, playing recordings from predator-rich reefs, and assessing whether the fish avoided the source of the sounds. In all but the present-day control conditions, the fish failed to swim away. It was like they couldn’t hear the danger.

In Hong’s study, though, it’s not exactly clear if the whole story is a story of otolith inflation. Other experiments had indeed found that high ocean acidity could spur growth in fish ear stones, but Hong and his colleagues hadn’t actually noticed any in theirs. Besides, marine biologists who later mathematically modeled the effects of oversize otoliths concluded that bigger stones would likely increase the sensitivity of fish ears — which, who knows, “could prove to be beneficial or detrimental, depending on how a fish perceives this increased sensitivity.” The ability to attune to distant sounds could be useful for navigation. On the other hand, maybe ear stones would just pick up more background noise from the sea, and the din of this marine cocktail party would drown out useful vibrations. The researchers didn’t know…