Sharks may be losing their bite

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Sharks are known for their toothy and terrifying smiles. Unfortunately for Jaws, the future may pose a dental challenge for the fearsome fish. Acidic marine conditions brought on by climate change could corrode the teeth of blacktip reef sharks (Carcharhinus melanopterus). Based on tests conducted in a study published August 27 in the journal Frontiers in Marine Science, the seawater pH forecast for the year 2300 damages shark teeth, leaving them full of tiny holes and cracks. 

“It’s not only shells, corals, and mussels that are impacted by ocean acidification,” Maximilian Baum, lead study author and a biologist who completed the work as part of his bachelor’s thesis at the Heinrich Heine University Düsseldorf in Germany, tells Popular Science. “The best, highest developed, and highly mineralized weapons of top predators are also affected. We don’t know what the consequences will be, but we can see and we can measure damage.” 

Why the ocean is getting more acidic

It’s no secret: human-cause climate change is heating up the oceans. As atmospheric carbon dioxide (CO₂) levels rise and the salty seas warm, the water absorbs more dissolved CO₂. The dissolved gas reacts with seawater, releases hydrogen ions, and triggers a drop in pH. 

Throughout most of human history, the ocean has been slightly alkaline, with an average pH around 8.1-8.2, similar to that of baking soda. Now, depending on the region, it’s hovering closer to 8.05. It may not sound like much, but pH is a logarithmic scale, meaning every single 1.0 jump represents a 10-fold shift in acidity or alkalinity. 

Since the 1880s, human activities have increased the ocean’s concentration of acidic hydrogen ions by more than 26 percent. In response, the ocean’s surface pH has dropped every decade since the beginning of the Industrial Age, and continues to, according to the most recent IPCC report. The ocean is currently acidifying 10 times faster than at any prior time in the last 300 million years. 

Already, the change has started eating away at the oceans’ physical infrastructure– from shellfish aggregations and coral reefs to the seafloor. So, Baum wondered how even larger animals, usually in the business of eating others, might struggle with the direct difficulties of acidification. 

A steady supply of teeth

In addition to his studies, Baum works as a diver in a local aquarium, helping to maintain the animal habitats. While diving in the shark tank (don’t worry, the sharks are “very shy”), he’d routinely notice fallen teeth scattered around the enclosure. This dental confetti is the product of the natural tooth loss that happens throughout a shark’s lifetime. The predatory fish are constantly regrowing and replacing their teeth. This steady supply of teeth presented the perfect opportunity for Baum to follow his curiosity. 

He collected intact, undamaged samples from blacktip reef sharks and set out to assess how the teeth would fare under future ocean conditions. First, Baum and his colleagues took a close look at hundreds of teeth, and picked the most pristine, cataloging their starting state with high resolution photographs taken under a microscope. 

They narrowed their tooth collection down to 16 representative teeth, matched in pairs. One half of each set went into a control tank, mimicking an ocean pH of 8.1. The other half went into an experimental tank kept at a 7.3 pH for eight weeks. The IPCC predicts marine pH could fall to 7.3 by 2300 if humans fail to rein in emissions, under a worst case scenario. Both tanks’ temperature and salinity were kept as close to identical as possible. 

After two months, they compared the teeth once again– this time looking under a scanning electron microscope. Those kept in the more acidic water were significantly more corroded, and had far more cracks and holes, especially at the root and tooth base. Every one of the eight teeth held at a pH of 7.3 was visibly damaged, compared with less than half of the control teeth. 

While sharks do replace their teeth continually, the findings suggest that they would need to do so more frequently in the more acidic oceans caused by climate change. Unfortunately, swimming through lower pH waters may also make it more difficult for those teeth to grow and mineralize. 

Teeth “are key for shark’s ecological success,” Baum says. If maintaining that critical adaptation becomes more energy intensive, sharks are likely to suffer for it. Already, sharks face many threats in our rapidly shifting world, including habitat loss, fishery declines, poaching, and even impaired senses. Weakened teeth would only add to the pile-on. 

[ Related: When the ocean got hot, the sharks bulked up. ]

An uncertain future

The study provides initial evidence that acidification may stress sharks in a previously undocumented way, though the findings come with a big caveat. “We don’t know what fitness impact it would have, really,” Baum says. He and his colleagues didn’t assess the mechanical strength of the teeth, nor conduct tests in living sharks, and not all research is in agreement with the new results. 

One 2019 study assessed the effects of pH on shark skin denticles, specialized, scale-like structures embedded in shark skin made up of the same material as their teeth that aid sharks’ hydrodynamic and efficient movement through the water. In that study, scientists did find that acidic conditions also corrode denticles. However, a separate 2022 study of Port Jackson sharks (Heterodontus portusjacksoni) determined that the species’ teeth can resist ocean acidification, in part because the effects of rising temperature and falling pH may cancel out in some contexts. For example, heat makes the teeth more brittle, while acid makes the teeth softer.

Baum notes that the 2022 research relied on very different methods from his team’s study, and exposed sharks to a less acidic environment. Additionally, the species of shark examined likely makes a big difference. Port Jackson sharks’ teeth are less exposed to the ambient water than those of blacktip reef sharks, and have a different anatomy. Which sharks would best and least tolerate souring seas remains an open question. And more research is needed to understand how a single experiment with 16 loose teeth might translate to the real world. 

Sharks have been around for more than 400 million years and they’ve lived through it all– periods warmer and cooler than the present, in lower pH and higher pH environments, higher and lower sea levels, and more. But all those past shifts took their toll. Megalodon is gone for a reason.

“There are many extinct shark species,” says Baum, alongside the hundreds of currently living ones. Looking forward to the not-so-distant future, “maybe one species will manage it,” he says. “Another, maybe not.”

 

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