This story is excerpted from THE WEIGHT OF NATURE: How a changing climate is changing our brainsavailable April 9, 2024, from Dutton, an imprint of Penguin Publishing Group.
Imagine you are a clown fish. A young clownfish, specifically, in the year 2100. You live near a coral reef. You’re 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 travel through the water and then through your body, these otoliths move and displace tiny hair cells, which activate electrochemical signals in your auditory nerve. Nemo, you hear.
But you don’t hear well. In this version of the end of the century, humanity has managed to pump the climate brakes a little, but it has not reversed the trends that were evident 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 prediction. For you and your otoliths, this increase in carbon dioxide is significant because your otoliths are made of calcium carbonate, a carbon-based salt, and ocean acidification is making them bigger. Your ear stones are large and clumsy, and the clicks and chirps of resident crustaceans and all the larger reef fish have gone completely nuts. Normally you would avoid these sounds because they indicate 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. No one will make a movie about your life, Nemo, because no one will find you.
This is 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 found that ocean acidification could compromise young fish’s ability to distinguish between friend and foe odors, leaving them attracted to odors they normally avoid. At the highest levels of acidification, the fish did not respond to odor signals at all. Hong and his colleagues suspected the same phenomenon might apply to fish ears. The researchers raised dozens of clownfish in tanks with different carbon dioxide concentrations and tested their hypothesis by placing waterproof speakers in the water, playing recordings of predator-rich reefs and determining whether the fish avoided the source of the sounds. In all but the contemporary control conditions, the fish failed to swim away. It was as if they could not hear the danger.
However, in Hong’s study, it is not exactly clear whether the whole story is a tale of otolith inflation. Indeed, other experiments have found that high ocean acidity can spur growth in fish ear stones, but Hong and his colleagues actually didn’t notice anything in theirs. Moreover, marine biologists who later mathematically modeled the effects of oversized otoliths concluded that larger stones probably Increase the sensitivity of fish ears – which, who knows, “could be beneficial or detrimental, depending on how a fish experiences this heightened sensitivity.” The ability to tune in to distant sounds can be useful for navigation. On the other hand, earstones might just pick up more background noise from the ocean, and the noise of this marine cocktail party would drown out useful vibrations. The researchers didn’t know.
The uncertainty with the otoliths led Hong and his colleagues to conclude that the carbon dioxide might be doing something else—something more sinister in its subtlety. Perhaps, instead, the gas directly interfered with the fish’s nervous systems: Perhaps the problem with their hearing was not solely a problem of sensory organs, but rather a manifestation of something more fundamental. Maybe the fish brains couldn’t process the auditory signals they received from their inner ears.
The following year, a colleague of Hong’s, one Philip Munday at James Cook University in Queensland, Australia, appeared to confirm this suspicion. His theory had the appearance of a hijacking.
A neuron is like a house: insulated, sometimes permeable, maybe a little leaky. Just as one might open a window during a stuffy party to let in some cool air, brain cells take advantage of physical differences across their walls to keep the neural conversation going. In the case of nervous systems, however, the differences do not come with respect to temperature; they are electric. Inside living bodies float various ions – potassium, sodium, chloride, and the like – and because they have gained or lost an electron here or there, they are all electrically charged. The relative balance of these atoms inside and outside a given neuron causes a voltage difference across the cell’s membrane: Compared to the outside, the inside of most neurons is more negatively charged. But a brain cell’s walls also have windows, and when you open them, ions can flow through, prompting electrical changes.
In practice, a neuron’s windows are proteins that span their membranes. Like a house’s, they come in a plethora of shapes and sizes, and while you can’t fit a couch through a porthole, a window is still a window when it comes to those physical differences . If it’s hot inside and cold outside, opening one will always cool you down.
Until it doesn’t.
Here is the clownfish neural hijacking proposed by Philip Munday. What he and his colleagues hypothesized was that excess carbon dioxide in seawater leads to an irregular accumulation of bicarbonate molecules within fish oceans. The problem for neuronal signaling is that this bicarbonate also carries an electrical charge, and too much of it in the cells eventually causes a reversal of the normal electrical conditions. At the neural house party, it’s now colder inside than outside. When you open the windows – the ion channels – atoms flow in the opposite direction.
Munday’s theory applied to a specific type of ion channel: one responsible for inhibiting neural activity. One of the things that all nervous systems do is balance excitation and inhibition. Too much of the former and you get something like a seizure; too much of the latter and you get something like a coma — it’s in the balance that we find the richness of experience. But with a reversal of electrical conditions, Munday’s inhibitory channels become excitable. And then? All bets are off. To a brain it would be like pushing a bunch of random buttons in a cockpit and hoping the plane stays in the air. In clownfish, if Munday is right, the acidic seawater seems to short-circuit the fish’s sense of smell and hearing, and they swim toward danger. It’s hard to ignore the question of where the rest of us can swim.
Of THE WEIGHT OF NATURE: How a changing climate is changing our brains by Clayton Page Aldern, to be published on April 9, 2024 by Dutton, an imprint of Penguin Publishing Group, a division of Penguin Random House, LLC. Copyright © 2024 by Clayton Page Aldern.
