The Skin of Sharks
You might think sharks are smooth like dolphins or whales, but you will be surprised. Unlike softer fish scales, shark skin consists of hard scales that mimic microscopic teeth, known as denticles. These denticles protect the shark from injuries as well as reduce drag when swimming, ensuring the shark is streamlined and has minimum resistance in the water. The denticles are curved in shape, with grooved teeth, which is what gives the sharks a sandpaper-like texture when touched (and often lead to “shark burn” – when researchers get an abrasion after handling sharks).
Ocean Acidification Impacting Sharks
Our oceans are continuing to heat up and absorb more carbon dioxide due to climate change and an increased amount of atmospheric carbon dioxide emissions. This results in ocean acidification due to absorbed carbon dioxide being converted into carbonic acid, which is having an observed and well-studied impact on coral reefs and crustaceans. Since the beginning of the industrial revolution, acidity in the oceans has increased by more than 30% due to a sharp increase in atmospheric carbon dioxide levels. This impacts calcium carbonate production in marine animals, and their abilities to form shells. Ocean acidification may literally cause coral reefs to dissolve, and may also disrupt the behavior of bony fish.
Sadly, the impact of ocean acidification does not stop there, and researchers have discovered that shark skin is susceptible to corrosion when exposed to acidic conditions for a prolonged period of time. This can have some significant negative impacts on the sharks.
Denticles are made up of dentine and other enamel-like substances, similar to those found in human (and shark) teeth. Acid conditions result in corrosion of dentine – similar to when you expose your teeth to acidic cola drinks which may result in cavities. Until recently, the impact of acidification in our oceans on sharks was only hypothetical, however new research has revealed some astonishing results.
Studying Shark Skin In The Lab
A group of scientists decided to test the effects of ocean acidification on catsharks under laboratory conditions, and published their findings in the journal Nature. They caught 80 wild puffadder shysharks (Haploblepharus edwardsii) and transferred them to the laboratory. The reason this species was selected was due to their high tolerance for acidic conditions, and also their alkaline blood which prevents carbon dioxide from dissolving.
The sharks were placed in tanks with a water pH level of 8, which is the current acidity level of the ocean, as well as tanks with a water pH level of 7.3, which is the projected future acidity level. For 36 hours, 66 of the sharks were housed in the tank filled with the water with a pH of 7.3. Three of the sharks remained in these tanks with a pH of 7.3 for nine weeks, to allow researchers to compare physiological changes brought on by the increased acidity and to mimic chronic exposure to more acidic ocean water. Although the sharks were able to prevent dissolved carbon dioxide from building up in their blood due to their alkaline blood, the sharks in the tank with a pH level of 7.3 suffered from serious corrosion of their denticles. Of all the denticles observed under an electron microscope, 25% of the denticles showed permanent damage.
Impacts of Corroded Denticles
The impacts resulting from corroded denticles goes beyond that of a mild aesthetic inconvenience. As mentioned above, denticles assist in streamlining the shark and reducing resistance from the water when swimming, and therefore corroded denticles can reduce the swimming speed of sharks. Although smaller shark species, like the puffadder shysharks in the study, are not dependent on fast swimming speeds as they are bottom-feeders, larger species that are apex predators in their ecosystem – such as Great White sharks (Carcharodon carcharias) – require speed when hunting. Corroded skin can therefore mean the difference between a meal or starving to death. Shark teeth are also made of similar material as denticles and although not studied yet, it is hypothesized that should the teeth be similarly corroded, it will influence the shark’s ability to feed.
Puffadder shysharks have specially adapted alkaline blood that prevents carbon dioxide from acidic seawater dissolving into their blood, but sadly other sharks are not as fortunate. As more carbon dioxide enters the ocean, it will end up inside the blood and cells of many marine animals, including other shark species. High levels of carbon dioxide in the blood means their ability to absorb oxygen will be limited. Experiments of this nature have already been done on fish, and showed that high levels of carbon dioxide in the water will result in a form of intoxication known as hypercapnia. Sharks exposed to high carbon dioxide levels also showed altered blood chemistry, as they accumulate bicarbonate to keep their blood acid levels normal.
Not being able to feed will impact individual sharks, however acidification in the blood has the potential to destroy an entire species if they are unable to quickly adapt to new ocean conditions.
The Future for Sharks
According to the latest estimates, the pH of the ocean will only drop below the study conditions of 7.3 pH level by the year 2300, so hopefully sharks will still be mostly unaffected by these impacts until then. Unfortunately, should we reach this point, the impacts on sharks (and other marine life) will be irreversible. Sharks are already facing many other threats, including habitat loss and degradation, reduction in food resources due to commercial fishing, accidental bycatch and illegal fishing for sharks, and the results of the corrosion study could be the final nail in the coffin for many shark species.
It is important that further studies be conducted on the corrosion of denticles, as the sample size of 80 sharks was very small. It will be especially important to study shark species that swim in open waters, as their survival is more dependent on speed.