For hundreds of millions of years, sharks have been roaming Earth’s oceans making meals out of a huge range of critters, from the whale shark gobbling up tiny krill to the 60-foot megalodon that could take down whales. Their ancestral line has survived mass extinctions with ease, most notably the catastrophe that took down the dinosaurs.
But nothing could have prepared them for the scourge that is humanity—we’re polluting their waters and snatching up their prey and hunting them to extinction. And now, thanks to climate change, humans may be transforming the very water sharks swim into an existential threat: In findings published today in the journal Scientific Reports, researchers show that prolonged exposure to acidified water corrodes the scales, known as denticles, that make up a shark’s skin. To be clear, this work was done in the lab and on only one species, but the implications are troubling. As we belch still more CO2 into the atmosphere, which reacts with seawater and makes the oceans more acidic, the seas themselves could become yet another threat that pushes sharks over the brink.
These days, the oceans on average have a pH of 8.1, making them 25 percent more acidic than in pre-industrial times. The lower the number, the more acidic the water, so 1 is a strong acid (think battery acid) and 14 is a strong base (milk of magnesia clocking in around 11).
For these experiments, the researchers kept puffadder shysharks—beautiful little mottled creatures that spend their time on the seafloor—in tanks of 7.3 pH water, which is what ocean water could be by 2300, according to one estimate. A shark control group was housed in regular, non-acidic water.
After nine weeks, the researchers scrutinized the sharks’ denticles with a scanning electron microscope, which creates super-detailed images by bombarding a surface with electrons. They found that on average, a quarter of the denticles on the sharks in acidic water were damaged, compared to 9.2 percent on the controls.
This was a lab experiment, so it’s an imperfect representation of what might happen by the year 2300. Nonetheless, it’s cause for concern, since sharks rely on their skin not just as armor but for streamlining, says study coauthor Lutz Auerswald, a biologist at Stellenbosch University in South Africa. In free-swimming sharks like the great white, denticles account for up to 12 percent of their swimming speed. Damaged denticles “may impact their ability to hunt or escape,” Auerswald adds. “In addition, since sharks’ teeth are from the same material, corrosion may impact hunting and feeding.” All sharks—not to mention the closely related rays, skates, and chimaeras—have teeth and denticles made from that same dentin material, meaning they all could be vulnerable to increasingly acidic water.
But because the puffadder shysharks have super tiny teeth, the team did not test them in these experiments, so they don’t actually know if the acidic water caused corrosion there too. And it’s likely that different species will react differently to acidifying water.
Could this be just another bump in the evolutionary road for sharks, which have adapted to hundreds of millions of years of environmental change—including swings in oceanic acidity? Perhaps. But sharks are particularly vulnerable because they tend to have long generations, so they may not have the time to adapt to such a radical transformation of the seas. “The rate of climate change is very fast compared to previous changes,” says Auerswald. “Most likely, shark species will feel the impact differently and some may not be able to cope, whereas others may have the potential to adapt.”
Really, though, sharks have more immediate concerns, says Luiz Rocha, curator of fishes at the California Academy of Sciences, who wasn’t involved in the study. Not only are they hunted for their fins, but they’re frequently snagged as by-catch in, say, tuna nets. So science needs to better understand the movements of sharks to protect their habitats from fishing, and to develop better fishing technology to keep the creatures from becoming collateral damage.
Rocha adds that the 7.3 pH value the researchers used is quite acidic. “It’s probably not going to be an average for the ocean, even in 2300,” he says. “Unless instead of slowing down, we continue racking up the amount of greenhouse gases that we’re dumping in the atmosphere.”
“If it drops to 7.3, we are doomed,” Rocha adds. “Everything is going to fall apart, not just sharks.” Anything with a shell won’t be able to form that shell—think corals and bivalves like clams and tiny shelled phytoplankton that make up the base of the food chain. Sharks’ ecosystems will have devolved into chaos.
Let us hope it never comes to that.
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