One Planet News: Losing its bite - how a top predator may lose its place in the food chain
By Annette J Beveridge
When you think about sharks generally, what is your immediate thought? Its sleek perfectly designed body, unblinking eyes or that mouthful of sharp teeth?
Sharks are the most feared predators in the ocean. There are hundreds of species, all playing an important role in the ocean ecosystems. Their predatory skills have been finely tuned over millions of years but sharks are under pressure. The overfishing of sharks is a huge issue with an estimated 100 million sharks killed each year but ocean acidification is yet another threat - one that scientists are studying closely.
The science
Fossil fuels have been linked to the increased acidification of the ocean which is mainly caused by carbon dioxide gas within the atmosphere, and which is dissolved in the ocean. This leads to the ocean’s pH being lowered.
Carbon dioxide is being produced faster than nature can remove it, and the additional ocean acidity is making sharks’ teeth weaker. If they are more prone to breakage, this could push sharks off the list as a top ocean predator.
Toothless sharks?
One study into ocean acidification and shark teeth has been published.
The study's lead author, Maximilian Baum, a marine biologist at Heinrich Hein University Dusseldorf, said: “The ocean will not become populated with toothless sharks overnight. But the possibility of weaker teeth is a new hazard to sharks that already face pollution, overfishing, climate change and other threats.”
Over time, sharks’ could no longer be at the top of the food web.
Baum said: “We found there is a corrosion effect on sharks' teeth. Their whole ecological success in the ocean as the rulers of other populations could be in danger.”
Previous studies have found that shark teeth are relatively resistant to ocean acidification but are susceptible to visible corrosion when experiments utilise acidified conditions.
Tooth roots are not protected by soft tissue so are exposed in water making them susceptible to pH-induced degradation.
The research study was published in the journal Frontiers in Marine Science.
The research included the study of more than 600 discarded teeth from an aquarium that houses blacktip reef sharks. These sharks live in the Pacific and Indian oceans and typically grow to about 1.7 metres long. The teeth were exposed to water with the acidity levels of today and the projected acidity of 2300.
Sharks can replace their teeth continuously but the rate of this varies depending on the species. For the blacktip reef shark, this is unknown.
Baum said: “Shark teeth are highly developed weapons built for cutting flesh, not resisting ocean acid.
“Sharks will go through thousands of teeth in a lifetime, and the teeth are critical for allowing sharks to regulate populations of fish and marine mammals in the oceans.”
Blacktip reef sharks swim with its mouth open to allow oxygen-rich water to flow through the gills. In young sharks, it is possible to see that several rows of teeth are exposed and that the teeth are not embedded deeply in the tissue of the jaw.
Scans were taken which observed the corrosive effects of acidification on different tooth structures. This included the root, primary and secondary serrations, and the crown. Through this, scientists concluded how ocean acidification could impact sharks’ foraging efficiency, and energy uptake plus ascertain potential fitness in future oceans.
There is a bigger picture
Ocean acidification also threatens coral reefs and shell-building marine creatures. Already, coral reefs - tropical and sub tropical have lost 43% of habitat. Highly sensitive, acidification leads to a reduction of growth, weaker skeletons, and a greater vulnerability to bleaching or erosion.
Ocean acidity can also impact the sensory systems of some fish which could make them less resistant to predators. In addition, plankton and pteropods which form the foundations of the food chain could be seriously affected subsequently impacting whales and fish food sources.