Ocean acidification could eat away at sharks’ teeth and scales

The fishes’ ability to swim and feed could be compromised

Sharks are some of the world’s most formidable predators, but their place at the top of the marine food chain may be threatened by ocean warming and acidification. As carbon dioxide levels in the oceans increase, upping the acidity of the water, shark teeth and scales may begin to corrode, compromising their ability to swim, hunt and feed, according to research published today in Scientific Reports.

Ultimately, sharks could be displaced as apex predators, disrupting entire ocean food webs, says the study’s senior author Lutz Auerswald, a fisheries biologist at Stellenbosch University in South Africa and the nation’s Department of Agriculture, Forestry and Fisheries. “Some of the bigger species, like great white sharks, are also already highly endangered, so this might wipe them out.”

Auerswald and Sarika Singh, an ocean researcher at South Africa’s Department of Environmental Affairs, stumbled upon the idea for their study over beers. Realizing that the high acidity of beer and many other carbonated beverages causes human teeth to erode, Singh wondered what effect more acidic ocean water might have on shark teeth.

Most studies on ocean acidification examine species that build shells or other calcium-based structures, including corals and shellfish. Possibly because sharks are large and difficult to work with—and because many of them are endangered—only a few studies to date have looked at how acidification might impact the animals. Just one paper has examined the effect of pH (the scale that measures how basic or acidic a substance is) on sharks’ skin denticles, or scales. That study, conducted on small-spotted cat sharks, a species in the North Atlantic, did not find a significant impact. But its results were possibly constrained by the relatively low carbon dioxide concentration the researchers used, compared with the high levels of acidity already present in many places in the world’s oceans.

Auerswald, Singh and their colleagues focused on puff adder shy sharks, a small, bottom-dwelling South African species of cat shark that is easily handled and not endangered. The sharks’ teeth are very small, so the team decided to investigate acidification’s effects on the animals’ comparatively bigger scales. (Because shark teeth and scales are both made of a calcium phosphate material called dentin, the researchers would expect the effects on teeth to be similar to any impact on the scales.)

The researchers captured puff adder shy sharks in a harbor in Cape Town, South Africa, and transported them to a government research aquarium, where the fish acclimated for four months. They divided 13 of the sharks into control and experimental groups. Control animals stayed in an aquarium with a mildly basic pH of 8.1, matching that of the ocean, while the researchers gradually dropped the pH of the experimental animals’ water to 7.3, the level that ocean water is predicted to hit by 2300 if carbon dioxide emissions continue. (In some areas, including the waters off South Africa and California, the pH can already drop to 7.3 or lower, depending on prevailing currents and winds.)

Rachel Nuwer, Scientific American, 19 December 2019. Full article.

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