Acidifying Oceans Could Corrode the Tooth-Like Scales on Shark Skin

Shark skin is seriously tough. Blanketing the surface of these fearsome fish are structures called denticles, which resemble scales but are actually modified teeth that comprise one of the zaniest suits of armor in the sea.

But scientists are now concerned that even durable denticles have met their match: ocean acidification. Under the influence of climate change, the pH of the world’s waters is dropping—and potentially corroding these spiky shark scales, according to a study published this week in Scientific Reports.

The wear on shark skin spells far more trouble than a dermatological annoyance. Linked together in tight formation, denticles act as a combination of chain mail and a wetsuit, protecting the sharks while streamlining their swim, study author Lutz Auerswald, a biologist at Stellenbosch University in South Africa, tells Matt Simon at Wired. Like a mouthful of carbonated, sugary soda eating away at teeth, acidified waters may cause wear and tear on denticles in a way that impacts daily life.

Since pre-industrial times, ocean acidity has climbed by about 30 percent, writes University of Sheffield biologist Rory Cooper in The Conversation. A large part of this drop in pH (lower pH is more acidic) is attributable to the gobs of carbon dioxide pumped into the atmosphere each year, where the gas can be absorbed into seawater and converted into carbonic acid, reports Jonathan Lambert for Science News. If emissions continue at their current rate, ocean pH will rise to 7.3 by the year 2300.

Researchers have already shown that ocean acidification, which particularly affects compounds based in calcium, can take a big toll on the shells of bivalves like clams and the skeletons of coral, Lambert reports. But the effects of these changes on sharks, which lack both shells and bony skeletons (shark skeletons are made of cartilage), have been less clear.

So Auerswald and his colleagues decided to simulate the effects of acidification on the denticles of puffadder shysharks (Haploblepharus edwardsii) in the lab. These petite sharks, native to shallow seafloor environments off the coast of South Africa, are already adapted to occasional spells of intense acidity, common where cold, acidic water often wells up to the surface. But these bouts of harshness are usually briefand the researchers wanted to know how the sharks might cope with more chronic exposure.

To test how acidity affects shark skin, they housed 80 puffadder shysharks in tanks full of pH 7.3 water, simulating projected future conditions, or pH 8 water.

Exposed to acidified water, sharks quickly began to pump bicarbonate, a base, into their blood to counteract the acid. But the longer the sharks spent bathing in the harsh liquid, the worse off their denticles were. When the researchers examined three specimens that spent nine weeks in the acidic tank, they found damage on 25 percent of the sharks’ denticles under a microscope, compared to just 9.2 percent in a group that had remained in more neutral water.

That rate is fairly fast for damage to occur, Valentina Di Santo, a marine biologist at Stockholm University who wasn’t involved in the study, explains to Lambert. Real ocean conditions could produce different results, and the findings won’t necessarily translate over to other species.

But the findings may already be cause for concern, Auerswald tells Gege Li at New Scientist. A pH of 7.3 isn’t all that acidic (pH 7 is neutral, and enamel-damaging sodas range around a pH of 3). The seas are changing fast, and even under current conditions, sharks and their denticles may be in peril.

Overfishing, pollution, habitat destruction and more have all played a role in the decline in sharks’ wellbeing. Scientists and conservationists hope to ensure sharks and their neighbors get by—and not just by the skin of their teeth.

Katherine J. Wu | | READ MORE

Katherine J. Wu is a Boston-based science journalist and Story Collider senior producer whose work has appeared in National Geographic, Undark magazine, Popular Science and more. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University, and was Smithsonian magazine's 2018 AAAS Mass Media Fellow.