As oceans become more acidic, creatures could struggle to smell

This article was originally featured on Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.

Dungeness crabs hunt by flicking their chemical-detecting antennae to and fro. Sensing the water—the underwater equivalent of sniffing the air—is a well-trod strategy for homing in on potential prey. But that timeless tactic appears to be at risk, as new research shows that climate change–induced ocean acidification seems to cause Dungeness crabs’ antennae to falter.

Researchers at the University of Toronto Scarborough in Ontario put Dungeness crabs in water just slightly more acidic than normal—conditions that are already present in some coastal ecosystems and could be widespread by the year 2100 if humans continue to emit a high level of greenhouse gases. They found that the animals need to be exposed to cadaverine, a food signaling chemical, at a concentration 10 times higher than normal before they register its presence.

And it’s not just Dungeness crabs that appear to be in trouble. Acidification threatens to deprive a variety of marine species of crucial chemical cues. Research into this phenomenon is still limited, but as the field develops, the scope of the potential consequences is growing clearer.

“Almost every chemical that’s in the sea could be affected,” says Jorg Hardege, a chemical ecologist at the University of Hull in England.

Just like on land, where animals smell and taste chemicals to glean vital information, many marine creatures use chemical cues to spot food, locate potential mates, or avoid nearby predators. Chemoreception works because each of these cues is a molecule with a distinct chemical structure and physical shape. But because all of these chemicals are floating around in water, they’re susceptible to a range of chemical reactions. More acidic water, says Hardege, has more positively charged hydrogen ions floating around. Those hydrogen ions can bind to the cue chemicals, changing their shape—and how they’re detected. Hydrogen ions can also bind to the animals’ chemoreceptors, changing how they sense those chemical cues, Hardege says.

If you think of these chemical cues as a language, Hardege says, it’s as if words start sounding different while, at the same time, your ears are changing how they hear sound.

Unsurprisingly, disrupting an animal’s ability to detect key chemical cues can alter its behavior. Take the European green crab, for example. One study, coauthored by Hardege, shows that a slight increase in water’s acidity can change the shape of chemicals that tell the crabs to fan their eggs with water to provide fresh oxygen and remove waste. Crabs in experimentally acidified water were less sensitive to these cues—they needed at least 10 times as much of these chemicals added to the water before they started fanning their eggs more frequently.

Some fish have also demonstrated having trouble picking up on chemical cues in more acidic water. In one study, juvenile pink salmon seemed less attuned to chemical cues and less able to avoid predators. Gilthead seabream—a commonly eaten European fish—have shown the same trend.

Many of these experiments tested levels of ocean acidification that could be widespread by the end of the century if the world hits extreme climate change projections. But with coastal upwelling, a process that can bring acidic deep-ocean water to the surface, some coastal environments already see this level of acidification occasionally. And even if future carbon emissions are reigned in, the whole ocean will still grow more acidic than it is now. Individual species will likely have different thresholds at which the increasing acidity suddenly derails their ability to detect certain chemicals, Hardege says, and scientists don’t yet know where those thresholds might be.

Christina Roggatz, a marine chemical ecologist at the University of Bremen in Germany, notes that acidification does not always reduce animals’ sensitivity to chemicals. For example, one study found that in more acidic water, hermit crabs seem to be even more attracted to a particular chemical cue.

But with some cues growing stronger and others growing weaker, widespread acidification could upend the balance of chemical communication in the ocean, Roggatz says.

This is on top of the other, more overtly threatening, consequences of changing marine chemistry. In a particularly frightening case, Roggatz discovered that a combination of increasing acidity and rising temperatures actually increases the toxicities of saxitoxin, a potent neurotoxin from contaminated shellfish, and tetrodotoxin, produced by pufferfish, blue-ringed octopuses, and other animals.

Research into acidification’s potential to disrupt underwater chemical communication and sensory perception is really just getting started. Last year, Hardege, Roggatz, and others wrote a paper urging researchers, from chemists to ecologists, to unravel what these changes could mean.

It is possible, Hardege says, that wildlife could adapt to the changing chemical environment. The signal of nearby food, for instance, isn’t often one chemical, but an array of chemicals. Even if a species can no longer detect one of those chemicals, it might still be able to detect the others. Or, it might turn to its other senses, like vision.

Of course, it’s best if we don’t put that to the test. The best way to protect marine ecosystems from ocean acidification is to limit acidification, says Roggatz.

“If we can buy time by reducing the carbon dioxide amounts we emit substantially,” Roggatz says, “I think that is the solution.”

This article first appeared in Hakai Magazine and is republished here with permission.

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