Shark teeth can resist ocean acidification

Over the last century, the rapid increase in carbon dioxide concentrations in our atmosphere, thanks to the combustion of fossil fuels, has led to an observed increase in the acidity and surface temperature of seawater. “The oceans have absorbed about a third of all carbon dioxide (COtwo) emissions related to human activities since the 1700s. Estimates of future levels of carbon dioxide, based on normal emission scenarios, indicate that by the end of this century ocean surface waters could be almost 150% more acidic, resulting in at a pH that the oceans have not experienced for over 20 million years,” explains NOAA.

For good reason, ocean acidification is the “evil twin” of climate change. This overload of carbon dioxide in our oceans is often compared to osteoporosis, eating away at the minerals used by oysters, clams, lobsters, shrimp, coral reefs and others to build their shells and skeletons. This is leaving them weak, fragile… and more at risk than ever.

However, it remains unclear whether the ‘corrosive’ effect of acidified seawater also affects organisms that have body parts made of calcium phosphate minerals (eg shark teeth). Professor Sean Connell of the University of Adelaide set out to answer the question whether or not ocean acidification and warming had an effect on the mechanical properties of shark teeth “because theory suggested that ocean acidification could reduce the force of shark teeth and their ability to feed.”

The team decided to focus on the dental properties of the Port Jackson shark (Heterodontus portusjacksoni), evaluating whether its mineralogical properties can be modified in response to predicted changes in pH (-0.3 units) and temperature (+3°C) of seawater in the near future. Found in the coastal region of southern Australia, this blunt-headed shark is well known for its dark brown markings on its body. Feeding on hard-shelled molluscs, crustaceans, sea urchins and fish, the Port Jackson shark’s teeth are one of its most distinguishing features. While many shark species have the same tooth morphology, the Port Jackson has teeth that look different in the front and back. Its front teeth are small, sharp and pointed, while its back teeth are flat and blunt. The different shapes allow this predator to hold, break and crush and shred the shells and any crunchy bits of other prey this species feeds on. Juveniles have also been shown to have sharper teeth, as they eat more soft-bodied animals than adults.

To see how ocean acidification affects biomineralization in elasmobranchs, scientists collected eggs from Port Jackson at a similar developmental stage and placed them in ponds filled with sand-filtered natural seawater in a temperature-controlled room until hatching. They then chose two nominal pH levels (8.0 and 7.7) and temperature (16°C and 19°C) to expose the juvenile sharks for two months, with the high acidity and temperature representing the predicted levels for the year 2100.

“We found that warming resulted in the production of more fragile teeth (higher modulus of elasticity and lower mechanical resilience) that were more vulnerable to physical damage,” the authors report in their publication in the journal Global Change Biology. “However, when combined with ocean acidification, the durability of the teeth increased (i.e., less prone to physical damage due to the production of more elastic teeth), so they did not differ from those bred under environmental conditions.”

But how? The teeth were found to be made primarily of fluorapatite (the same phosphate mineral that makes human teeth hard), and the crystallinity of their teeth increased with fluoride content under ocean acidification. Increased precipitation of this highly insoluble mineral under ocean acidification suggests that sharks could modulate and enhance biomineralization to produce more corrosion-resistant teeth. “THE [most surprising finding of this study was the] nature’s ability to adapt to environmental changes,” commented Connell. “This extremely ancient lineage of marine life [has] a surprising ability to persist, but we can’t take that for granted when fishing is so intense.”

This adaptive mineralogical adjustment may allow some shark species to maintain the durability and functionality of their teeth, which is vitally important when you depend on them for sustenance. While Connell and his team are amazed that these predators “were able to persist for so long because of their adaptability to planetary changes,” he emphasizes that modern pressures from overfishing are causing these ancient animals to disappear. Today, more than a third of the world’s shark and ray species are now facing the threat of extinction, according to the International Union for Conservation of Nature (IUCN).

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