The ocean is getting more acidic, and it could affect sharks’ teeth

How will the climate crisis affect one of the ocean’s fiercest predators? New research published Wednesday has examined what might happen to sharks’ highly specialized, flesh-cutting teeth.

As carbon emissions increase, the ocean is absorbing more carbon dioxide from the atmosphere, lowering pH levels and making seawater more acidic, a process that can affect many ocean species and ecosystems.

“Since ocean acidification is known to damage calcified structures like corals and shells, we wanted to investigate whether shark teeth, especially in species … that swim with their mouths open to ventilate their gills and have constant seawater exposure, might also be vulnerable,” said Maximilian Baum, a biologist at Heinrich Heine University in Düsseldorf, Germany, and lead author of the study, which was published in the journal Frontiers in Marine Science.

“Our main takeaway is that not only small organisms like corals or mollusks are at risk: even the teeth of apex predators show visible damage under acidified conditions, suggesting that ocean acidification could impact sharks more directly than previously assumed,” Baum said via email.

Baum and his colleagues collected 600 naturally shed teeth from 10 blacktip reef sharks (Carcharhinus melanopterus) housed at the Sea Life Oberhausen aquarium in Germany. Most shark species lose and regrow their teeth, but the rate of replacement varies from a few days to several weeks depending on the species.

The researchers selected 16 undamaged teeth and 36 teeth with limited damage and placed them in two separate 20-liter water tanks for eight weeks with different pH levels.

The saltwater in the control tank had a pH level of 8.2, close to the current ocean average, while the other tank contained more acidic water, with a pH level of 7.3, the projected seawater pH in the year 2300 per a 2003 study published in the journal Nature.

According to the National Oceanic and Atmospheric Administration, since the industrial revolution began more than two centuries ago, the pH of surface ocean waters (the upper layer of water in the ocean) has fallen by 0.1 pH units, representing an approximately 30% increase in acidity.

Compared with the teeth incubated at 8.2 pH, the teeth exposed to more acidic water had “visible surface damage such as cracks and holes, increased root corrosion, and structural degradation,” said Sebastian Fraune, a professor at Heinrich Heine University and leader of its Institute of Zoology and Organismic Interactions,” in a statement. He was the senior author of the study.

This damage, the researchers’ reasoned, could lead to changes in how sharks find and digest food.

“Many shark species use several rows of teeth at once, and individual teeth can remain in use for weeks or even month, so cumulative damage can reduce feeding efficiency and increase energy demands, especially in species with slower replacement cycles and many rows of teeth that are used at the same time,” Baum explained.

As the study focused on teeth shed from sharks in an aquarium, it had limitations, said Ivan Nagelkerken, a professor of marine ecology at the University of Adelaide in Australia.

First, it was unclear whether the experiment represented the same conditions as those in the mouth of a living shark. Second, the study had used an “extreme” ocean acidification scenario, which assumed that greenhouse gases would be emitted at the current rate until 2300.

Nagelkerken was not involved in the latest research but was part of a 2022 study, based on a different shark species hatched in tanks with various pH levels, that found that sharks’ teeth were relatively resistant to ocean acidification.

“The (new) study tested extreme corrosive effects on already shed shark teeth — this may not necessarily represent what sharks will experience in a future ocean, or indicate whether it will affect their food consumption,” Nagelkerken told CNN in an email.

Baum and his coauthors agreed their study had limitations, noting that “our findings represent the purely chemical effects of ocean acidification on non-living, mineralized tissue.” He said, however, that his team approached the question from a different perspective and their results aligned with other work that showed visible effects.

“Our study focused on naturally shed teeth because there’s currently very little data on this topic. By isolating the chemical effects of acidified seawater on the mineralized structure itself, we want to provide a baseline for understanding vulnerability of shark teeth,” Baum said.

“This approach complements previous studies on live animals and helps highlight the potential damage to exposed hard tissues like teeth,” he added.

Sharks’ teeth are central to their ecological success, and as apex predators, sharks are important for the overall health of marine ecosystems, Baum said.

“If sharks and their resilience were to be weakened by ocean acidification in combination with other stressors such as overfishing and plastic pollution, this could trigger cascading and domino effects across many marine ecosystems.”

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