Climate Change Ocean Acidification

Does increasing ocean acidification mean sharks need a trip to the dentist?

Leung, J.Y.S., Nagelkerken, I., Pistevos, J.C.A., Xie, Z., Zhang, S., and S.D. Connell. 2021. Shark teeth can resist ocean acidification. Global Change Biology 28: 2286-2295 https://onlinelibrary.wiley.com/doi/epdf/10.1111/gcb.16052

 

Increasing levels of CO2 in the atmosphere are causing problems for animals on land and in the ocean. The planet is warming, and oceans are becoming more acidic as carbon dioxide enters the water. This acidified seawater can have a “corrosive” effect on many marine organisms that use calcium carbonate to make shells, and may also have an effect on shark teeth. This corrosive effect may make teeth more brittle and easier to break, compromising how well the teeth can grab prey. Sharks are important predators and often help control ecosystem dynamics. Researchers from China, Australia, and French Polynesia set out to investigate whether sharks could modify the properties of their teeth in order to adapt to warming and ocean acidification.

The Port Jackson shark is a benthic (bottom-dwelling), temperate shark. By Mark Norman / Museum Victoria – http://www.fishesofaustralia.net.au/home/species/1982, CC BY 3.0

The researchers collected eggs of the Port Jackson shark, a bottom-dwelling shark, and raised the eggs under different pH and temperature conditions. Sharks were raised under 4 different types of conditions, at either pH 8.0 or 7.7, and temperatures of either 16°C or 19°C. pH 8.0 and 16°C represent current ocean conditions, while pH 7.7 and 19°C represent a warmer, more acidic ocean, similar to what may be experienced in the future. Once the eggs hatched juvenile sharks were transferred to larger tanks and raised for two months under the same treatment condition.

After the two months of exposure, sharks from each treatment were randomly selected and humanely euthanized. The scientists then dissected out the jaws to measure a number of tooth properties, including hardness (resistance), elasticity (stiffness), durability (number of teeth that were damaged), elemental composition, and crystallinity (the degree of structural order).

Dentition and oronasal grooves of a Port Jackson shark (Heterodontus portusjacksoni). Fairy Bower, Manly, NSW

Sharks raised in acidic seawater for two months showed reduced hardness, increased elasticity, increased crystallinity, and more fluoride content in their teeth. The scientists hypothesized that these sharks were increasing the fluoride content in their teeth in response to the more acidic seawater, because fluoride is a more stable component and promotes more structured, crystalline teeth. Increased crystallinity makes teeth stronger and more durable, so increasing the fluoride content in teeth may make shark teeth more resistant to the “corrosive” effect of acidified seawater.

Even as the oceans continue to warm and become more acidic, these results seem to be good news for sharks. These researchers provide some evidence that sharks may be able to adjust the mineral properties of their teeth to maintain durability. If this is the case, sharks like the Port Jackson could adapt to a more acidic ocean in the future. This top predator would still have the same impact on the food web, maintaining ecosystem stability.

 

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