As expected, the otoliths of fish exposed to higher levels of carbon dioxide were slightly larger – Radford was not surprised. “More importantly,” he adds, “the symmetry between the left side and the right side was different.”
Symmetry is vital for bilateral animals such as fish and humans. “If you look at someone’s face, most people are symmetrical, the left side is the right side. It’s the same with all human sensory systems, ”says Radford. The brains of fish depend on symmetrical anatomy to calculate their perception of audience raw sound. If you let go of this symmetry, the mental arithmetic changes and the hearing of the fish becomes less sensitive. “If you have different shaped otoliths,” says Radford, “then one is going to feel something different from the other, which is going to make any form of sound localization more difficult.” If you’ve ever lost your balance because water is clogging an ear, you’ve experienced something similar.
Radford’s team measured how ocean acidification can impair hearing. Radford placed tiny sensors on each fish immobilized in plasticine, right next to their brainstem. Then, after the fish returned to the aquarium, the researchers played sounds and measured “auditory evoked potentials” – the electrical signals the brain receives.
“We found that the low frequency part of the hearing had dropped,” says Radford. At frequencies between 80 and 200 hertz, hearing sensitivity collapsed by about 10 decibels. Most vocal fish communicate at frequencies between 100 and 300 Hz, a deep buzz has a soft ooo. And the decibels run on a logarithmic scale: a decrease of 10 decibels means a tenfold decrease. “It’s bad news, especially for fish, if they can’t hear at these low frequencies,” says Radford.
Bignami takes the model-defying results of the new study in stride. A larger otolith should make hearing more sensitive, as his model found, but the surprising contribution of the asymmetry between the left and right otoliths ends up being much more important. “The measurement of actual neurological signals in juveniles is really hard to do, ”he says of the new study. “It’s pretty convincing. They’re seeing a pretty clear change here.
The full effect on fish behavior will result from a combination of overgrown otoliths, asymmetric anatomy and neurochemical effects. Ocean acidification makes some fish brains less receptive to a neurotransmitter controlling impulsive behavior. (In a study, the larvae reared in the acidified waters swam towards the smell of predators.)
“You have to remember that this is about looking at the relationship at a really critical point in life,” says Sara Shen, a marine scientist who now works for an environmental consulting company that has not been involved in the study. Shen’s previous research has shown the connection between otolith size and a type of balance called a vestibulo-ocular reflex in a sensitive transition point for fish larvae. The period chosen by Radford, when the young fish settle on the reefs, is very important for the maintenance of their populations. “It’s really a great job,” she says.
So what does this experience tell us about how climate change affects reef fish? The tenfold decrease in hearing was observed in fish exposed to a 120% increase in dissolved carbon dioxide from 450 to 1000 micro-atmospheres of pressure in the water. From CO2 is a dissolved gas, this value represents the pressure it would have in an otherwise empty container, where 1 million micro-atmospheres equals normal atmospheric pressure. Such an average increase in concentration will not occur in surface waters over the next few years, although it does correspond to longer-term trends if CO2 broadcasts continue unabated.
But even a smaller drop in hearing acuity, caused by milder acidification, would still be significant. Juvenile deaf fish may have difficulty finding reefs when they migrate after hatching on the high seas. If they cannot settle, they cannot survive and reproduce. And these fish play an important role in maintaining reefs. Predatory reef fish, for example, eat herbivores, which in turn control the growth of algae. The invaded algae suffocate the coral. The coral dies and erodes. Fish shelters and spawning surfaces disappear with it. “This ecosystem is disappearing,” says Yvonne Sadovy, a marine biologist at the University of Hong Kong who was not involved in the study.