The scales of sharks swimming Earth’s seas may slowly corrode as warmer, more acidic waters increase due to greenhouse gas emissions. Such damage could impair their protection, swimming speed, and perhaps even their ability to feed, say a team in Scientific Reports.
To begin their investigations, scuba divers collected puffadder shysharks off the coast of Simon's Town, South Africa, baiting them with bottles poked with holes and stuffed with sardines. Once captured, the sharks were housed in tanks for nine weeks, with or without acidified seawater. When the team inspected their leaf-shaped scales (called denticles), they found that 25 percent of those swimming in acidified seawater had damage compared to 9.2 percent of those in the control group.
"We did not expect this,” study author Lutz Auerswald of Stellenbosch University told IFLScience. “We know from human dentistry that teeth (also dentin) get damaged by carbonated drinks. However, they are much more acidic than the pH of 7.3 we used in our study. The latter pH is more acidic than the normal global pH of 8.1 but it is still alkaline.”
Global ocean acidification is expected to lower the pH to 7.3 by the year 2300. As the oceans absorb more carbon dioxide, they also become more acidic. Due to upwelling and low-oxygen events, high carbon dioxide concentrations already occur with increasing frequency in the seawater along the South African west and south coasts. This brings nutrient-rich, deep seawater to the surface but also more acidity with it.
In summer, the prevailing winds in South Africa are from a southeasterly direction and increase the region’s upwelling. In the Autumn, these winds stop and can cause algal blooms to inch closer to shore and decay, resulting in oxygen depletion and a further drop in pH. Corrosion to the sharks' denticles from acidification could impair their ability to thrive, although whether it does or not for certain is beyond the scope of this study.
“In the present species, denticles may only be important for protection. In free-swimming, ie pelagic sharks such as the Great White, denticles have been shown to account for up to 12 percent of the swimming speed,” said Auerswald. “Damage may impact their ability to hunt or escape. In addition, since sharks’ teeth are from the same material, corrosion may impact hunting and feeding.”
Blood samples, however, provided encouraging signs. Increased concentrations of carbonate prevented their blood from becoming more acidic for extended periods of time. At least in terms of blood, the sharks seem to be adaptable to high CO2 conditions.
Auerswald hopes the “results will make the general public and decision-makers more aware of the problem of ocean acidification and its impact on marine organisms. Unfortunately, the focus is mostly on ocean warming only. When structures of a resilient material like calcium phosphate (dentin) corrode in near-future acidification levels, people should think twice when, for example, considering sequestration of anthropogenic CO2 into the oceans or, just in [general], their CO2 footprint.”