Author: Natasha Hynes
The ocean is sometimes referred to as a “carbon sink”, meaning it can dissolve carbon from the atmosphere. When this happens, carbonic acid is formed, which increases the acidity of the ocean. As carbon dioxide levels have increased in the atmosphere, so has the acidity of the ocean. This has caused problems for lots of marine life, especially those who has calcified body parts (i.e. animals with shells!). The acidic water can eat away at animals’ shells or make it difficult for them to grow one. Sharks don’t have shells, but they do have teeth made of calcium. This leads to an important question – how will ocean acidification affect shark teeth? An international group of scientists were wondering this exact thing, so they developed an experiment to study what would happen to shark teeth under higher levels of ocean acidity.
The researchers in this study used Port Jackson sharks for their experiment. They collected Port Jackson shark eggs in South Australia and brought them back to the lab, where they were placed in tanks to hatch. The tanks were carefully monitored for temperature, pH and nitrogenous compounds. They chose four treatments for their experiment:
(1) a tank at a temperature of 16°C and a pH of 8.0 (control group)
(2) a tank at a temperature of 16°C and a pH of 7.7 (acidification effects)
(3) a tank at a temperature of 19°C and a pH of 8.0 (temperature effects)
(4) a tank at a temperature of 19°C and a pH of 7.7 (acidification and temperature effects)
Between 1-4 sharks were reared in each treatment until they were around 2 months old. Two sharks from each tank were humanely euthanized and their jaws were extracted. The scientists then prepared the jaws to be tested for hardness and elasticity, using tools to make a series of indentations on the teeth. The area of a shark’s tooth that would be most affected is the tip, so this is where the scientists examined for any damage. They also determined the elements that the teeth were made of and the crystallinity of the teeth.
The scientists found that acidification softened teeth and increased the elasticity of teeth. Meanwhile, warming had no effect on the hardness of the teeth, but it did decrease the elasticity of the teeth. The number of damaged tooth tips increased in warmer water, but did not increase under more acidified water. The combined effects of acidification and warming were the most interesting. While the warm water alone caused an increase in tooth damage, when the water was both warm and acidified, the change in the level of tooth damage was not significant. The composition of the teeth didn’t vary significantly under the four treatments, except for fluoride, which was higher in the teeth exposed to acidified water. Teeth were also more crystalized in acidic water.
These sharks’ teeth are made mainly of a material called fluorapatite, which is a compound of calcium and fluoride. The scientists in this study showed that when faced with increased ocean acidification, these benthic sharks increased the crystallization of their teeth by increasing their fluoride content. As our oceans continue to warm and acidify, this increased crystallization could help Port Jackson sharks keep being able to crack open their hard-shelled prey!
Leung, J.Y.S., Nagelkerken, I., Pistevos, J.C.A., Xie, Z., Zhang, S., Connell, S.D., 2022. Shark teeth can resist ocean acidification. Glob. Change Biol. 28, 2286–2295. https://doi.org/10.1111/gcb.16052