Ocean acidification could degrade sharks’ tough skin

The tough, toothy skin of sharks may be no match for the acidified oceans of the future.

After nine weeks of exposure to seawater doctored to mimic projected acidic levels in 2300, corrosion had frayed the edges of many denticles — the toothlike protrusions that make up sharkskin — on three puffadder shysharks, researchers report December 19 in Scientific Reports. Damaged denticles could make sharks more vulnerable to infection or injury and increase the drag on shark’s sleek skin.

Oceans gradually acidify as the seawater absorbs increasing amounts of carbon dioxide from the atmosphere and converts it into carbonic acid (SN: 6/2/19). Climate change scientists estimate that, if humans continue to burn fossil fuels and emit CO₂ at current levels, the average pH of oceans will dip from 8.1 today to 7.3 by 2300. Ocean acidification can cause a host of problems for marine life: It can weaken the calcium carbonate shells of clams and other bivalves (SN: 8/26/19), make corals more brittle (SN: 2/23/16) and even cause some creatures to behave erratically (SN: 2/2/17). But little had been known about how sharks might be affected, until now.

“Shark denticles are made from dentin, which we know from human dentistry is susceptible to degradation from carbonic acid,” says Lutz Auerswald, a fisheries biologist at Stellenbosch University in South Africa. “That could make [sharks] especially vulnerable.”

He and his colleagues caught puffader shysharks (Haploblepharus edwardsii) just off the coast of South Africa. In those waters, sharks periodically experience drastic dips in pH, as low as 6.6, because of strong upwelling of colder, more acidic water.

For 36 hours, the team housed 66 sharks in a tank filled with ocean water acidified to a pH of 7.3. That exposure was meant to mimic acute exposure to lower pH. Three other sharks were kept in larger, 1,000-liter tanks for nine weeks to simulate chronic exposure to pH 7.3 ocean water.

Both groups of sharks adapted physiologically to the lower pH by pumping bicarbonate, a base, into their bloodstream to keep internal pH steady. In the sharks exposed for nine weeks, scanning electron microscope images of the sharks’ denticles revealed that, on average, 25 percent of denticles looked corroded, with duller edges and rougher surfaces. By comparison, 9.2 percent of denticles on three sharks kept nine weeks in normal ocean water were corroded. It takes longer than nine weeks for new denticles to form, Auerswald says, so the team can’t say whether chronic exposure impacts denticle development. The researchers did not look for denticle damage after acute exposure.

Valentina Di Santo, a marine biologist at Stockholm University who wasn’t involved in the study, was surprised to see this level of corrosion after just nine weeks. But “we should be careful not to generalize, because the sample size is very small,” she says. She’d like to see similar studies in other shark species, especially those that swim in open waters and whose survival is more dependent on speed.