Over the last century, the rapid increase of carbon dioxide concentrations in our atmosphere thanks to the combustion of fossil fuels has led to an observed increase in acidity and surface temperature of seawater. “The oceans have absorbed roughly one-third of all carbon dioxide (CO2) emissions related to human activities since the 1700s. Estimates of future carbon dioxide levels, based on business-as-usual emission scenarios, indicate that by the end of this century the surface waters of the ocean could be nearly 150 percent more acidic, resulting in a pH that the oceans haven’t experienced for more than 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 likened 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 then ever before.
However, it is still unclear whether the ‘corrosive’ effect of acidified seawater also impacts organisms that have body parts made of calcium phosphate minerals (e.g. shark teeth). Professor Sean Connell from the University of Adelaide set out to answer the question of whether or not ocean acidification and warming had an effect on the mechanical properties of shark teeth “because theory suggested that ocean acidification might reduce the strength of shark teeth and their ability to feed themselves.”
The team decided to focus on the toothy properties of the Port Jackson shark (Heterodontus portusjacksoni), assessing whether their mineralogical properties can be modified in response to predicted near-future seawater pH (–0.3 units) and temperature (+3°C) changes. Found in the coastal region of southern Australia, this blunt-headed shark is well-known for its dark brown harness-like markings on its body. Feeding on hard-shelled mollusks, crustaceans, sea urchins, and fish, the teeth of the Port Jackson shark are one of its most distinguishable features. While many shark species have the same tooth morphology throughout, the Port Jackson has teeth that looks different in the front and back. Their front teeth are small, sharp and pointed, while their back teeth are flat and blunt. The different shapes allow this predator to hold, break then crush and grind the shells and any crunchy outer bits of other prey upon which this species feeds. Juveniles have also been shown to have sharper teeth since they eat more soft-bodied animals than their adult selves.
In order to see how ocean acidification affects biomineralization in elasmobranchs, the scientists collected Port Jackson eggs at a similar developmental stage and then placed them into tanks filled with sand-filtered natural seawater in a temperature-controlled room until hatching. They then chose two nominal pH (8.0 and 7.7) and temperature levels (16°C and 19°C) to expose the young sharks to for two months, with the elevated acidity and temperature representing the predicted year 2100 levels.
“We found that warming resulted in the production of more brittle teeth (higher elastic modulus and lower mechanical resilience) that were more vulnerable to physical damage,” report the authors in their publication via the Global Change Biology journal. “Yet, when combined with ocean acidification, the durability of teeth increased (i.e. less prone to physical damage due to the production of more elastic teeth) so that they did not differ from those raised under ambient conditions.”
But, how? The teeth were found to be chiefly made of fluorapatite (the same phosphate mineral that makes human teeth hard), and crystallinity of their teeth increased with fluoride content under ocean acidification. The increased precipitation of this highly insoluble mineral under ocean acidification suggests that the sharks could modulate and enhance biomineralization to produce teeth which are more resistant to corrosion. “The [most surprising finding of this study was the] ability for nature to adapt to environmental change,” commented Connell. “This extremely old linage of marine life [has] a surprising ability to persist, but we cannot take that for granted when fishing is so intense.”
This adaptive mineralogical adjustment could allow some shark species to maintain durability and functionality of their teeth, which is vitally important when you rely on them for sustenance. While Connell and his team are in awe that these predators “have been able to persist for so long because of their adaptability to planetary change,” he does stress that the modern pressures of over fishing is causing these ancient animals to disappear. Today, more than one-third of the world’s shark and ray species are now facing the threat of extinction according the International Union for the Conservation of Nature (IUCN).