Okay, search your memory here. Remember ocean acidification? It’s been a while, but as you may recall, ocean acidification is the phenomenon in which carbon dioxide from the atmosphere dissolves in the surface waters of the ocean, producing carbonic acid that (in sufficient quantities) shifts the pH balance of the ocean toward acidity. In the past 200 years, the ocean has absorbed nearly a third of carbon dioxide emissions, resulting in a 30% increase in ocean acidity.
The extra acidity impairs the ability of animals like oysters and corals to extract the calcium carbonate they need to build their skeletons or shells. And there’s evidence that ocean acidification is already impacting the health and survival of bay scallops and other shellfish.
But get this: a new study published in the journal Biology Letters suggests that ocean acidification may affect basic life-saving behaviors in fish.
WHAT WE KNOW and HOW WE KNOW IT
The researchers at University of Bristol raised clownfish in modern seawater (390 ppm CO2) and in seawater enriched with carbon dioxide (600-900 ppm CO2). The elevated carbon dioxide situations mimicked levels expected by mid- to late-century if greenhouse gas emissions continue to grow at the current rate.
At the point when juvenile clownfish would normally be choosing a coral reef to settle down on, the researchers played recordings of a busy, predator-filled reef during the day – normally a big turn-off for small fish in the market for a home – and then watched whether the young fish swam away from the speaker. As expected, fish reared in modern-day water showed a strong inclination to get away from the threatening sound. But fish reared in any of the elevated carbon dioxide conditions did not; they spent more than twice as much of their time at the end of the fish tank closest to the speaker.
WHAT WE DON’T KNOW
The results of the experiment were pretty clear-cut, but there’s nothing in this study to explain why the fish failed to respond to the threatening sounds as expected. The researchers didn’t see any major changes in the ear bones of the fish, so speculate that it’s more likely changes in how the brain processes or responds to sounds than an actual change in hearing capabilities. But that’s only speculation at this point.
Also, this is just one study of one species of fish. We have no idea how widespread this type of response might be. Maybe it’s all fish. Maybe it’s just clownfish.
WHAT IT MEANS
As with many laboratory studies of the impacts of ocean acidification, the real-world implications are hard to know. But failing to avoid predators could have drastic consequences (we’ve all seen those nature shows, right?). Lead author Dr Steve Simpson of the School of Biological Sciences at the University of Bristol says the concern is whether fish will be able to adapt to increasingly acidic waters quickly enough to avoid worst-case scenarios.
“What we have done here is to put today’s fish in tomorrow’s environment, and the effects are potentially devastating. What we don’t know is whether, in the next few generations, fish can adapt and tolerate ocean acidification. This is a one-way experiment on a global scale, and predicting the outcomes and interactions is a major challenge for the scientific community.”
UPDATED: A lot of media reports about this study have been saying that ocean acidification leaves clownfish deaf. I just want to be absolutely clear that that is not what the study says. The authors stress that they do not know why the clownfish don’t respond to the threatening sounds, but they suspect it might be due to a change in neural function – how messages about the sound are transmitted to or processed by the brain – brought about by disrupted acid-base balance in the fish. In other words, nerves and brain might be the culprits rather than the ear.