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This article was first published Hakai Magazine.
When the ruins of a 13th-century coastal fishing ground on Santa Catalina Island in southern Brazil were threatened by real estate development in 1996, archaeologists rushed to excavate it. They quickly collected everything they could from the ruins of Rio do Meio, including pottery, tools, and animal carcasses. This historic site is now located below a popular waterfront property. But fortunately, these artifacts are safely kept in the museum of the Federal University of Santa Catarina (UFSC).
Approximately 750 to 500 years ago, indigenous peoples came to this area for fishing. Sharks were an important part of their diet, and after a successful hunt, fishermen would slaughter bull sharks, great whites, sandbar sharks, and other species in Rio do Meio and carry away the meat. Fortunately for UFSC ecology graduate student Guilherme Burg Meyer, the hunters left behind the shark’s decapitated head. Shark teeth are a treasure trove of information, with the collection of the University Museum of Rio do Meio containing teeth from at least eight different species.
In a recent paper, Berg-Meyer and his colleagues compared these excavated teeth to modern shark teeth collected in the same area between 1975 and 2010. Using a method called stable isotope analysis, which provides insight into an animal’s diet and environment, the research team determined that: The differences between the two sets of teeth are thought to reflect how the region’s marine ecosystem has changed over the centuries, and how the sharks have adapted.
“I think a lot of people think of shark teeth as triangular, pointy things,” says John, a stable isotope biogeochemist at the University of California, Merced, who also studies shark teeth. Sora Kim, who was not involved in Rio do Meio’s analysis, said: This approach is relatively new, and scientists can learn a lot from the shape and chemistry of shark teeth, Kim said.
Like human teeth, shark champers contain collagen, a type of protein that contains various elements such as nitrogen and carbon that animals get from their food. Scientists can’t tell exactly what an animal ate by analyzing teeth, but Berg-Meyer’s team was able to compare different forms of nitrogen, called isotopes, between two sets of teeth. The researchers were able to glean clues about sharks’ diets and their ranking as predators. . High nitrogen isotope values suggest that the animal is feeding higher up the food chain, meaning it is more likely to eat carnivorous or herbivorous prey than to eat plants.
Researchers discovered that centuries ago, different species of sharks fed on animals that occupied the same level of the food web – prey that lived close to each other and ate similar things. Ecologists consider this kind of dietary overlap to be a good thing, Berg-Meyer says. That’s because it shows a more resilient food web, with predators munching on lots of different prey. For example, if one type of delicious fish is gone, sharks have plenty of other fish.
In contrast, there is little overlap in the diets of modern Brazilian sharks. Instead, sharks appear to be munching on prey that receives little attention from other sharks, or eating a more diverse combination of prey from different levels of the food web. A possible explanation for this change is that sharks became less picky as fish species declined, forcing them to snack on new foods, Berg-Meier said. To researchers, this change indicates that ecosystems are becoming less resilient.
The researchers focused on a species called the sandbar shark and found that it has moved up the food chain over time, becoming one of the ecosystem’s top predators. Burg Meyer says modern species have learned to choose prey from higher up the food chain. The number of sand tiger sharks that can compete for the biggest and best dietary options is decreasing. Sandwich sharks have been overhunted in Brazil for decades and were listed as an endangered species in the region in 2018. (This type of fishing was banned several years ago, but the meat is still sold illegally.)
However, as Kim points out, stable isotope analysis can only show that a change has occurred. I can’t explain why. Overfishing may or may not be the cause of changes in sharks’ diets. And because sharks regularly lose and grow teeth, a single tooth is more like a snapshot that covers weeks or months of an animal’s life, rather than the entirety of its life. , she added. Ultimately, Kim says, without additional research, it’s hard to say exactly why sharks eat different things now than they did centuries ago.
Either way, Berg-Meyer says the findings reflect a disruption in the ecosystem and merit further investigation. The study also shows the resilience of sand tiger sharks even under difficult conditions.
“This particular group is adapting to change,” Berg-Meyer says. “It’s trying to survive.”
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