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.