Sea turtles are known to rely on magnetic signatures to make their way thousands of miles to the very beaches where they hatched. Now the researchers report in the journal Current biology On May 6, some of the first strong evidence shows that sharks also depend on magnetic fields for their long-range forays across the sea.
“How the sharks managed to navigate during migration to targeted locations had not been resolved,” said Bryan Keller, project manager for the Save Our Seas Foundation, also from the Florida State Coastal and Marine Laboratory. University. “This research supports the theory that they are using the Earth’s magnetic field to help them find their way; it is nature’s GPS.”
Researchers knew that some species of sharks travel long distances to reach very specific places year after year. They also knew that sharks are sensitive to electromagnetic fields. As a result, scientists had long speculated that sharks used magnetic fields to navigate. But the challenge was to find a way to test this in sharks.
“To be honest, I’m surprised it worked,” Keller said. “The reason this question has held up for 50 years is that sharks are difficult to study.”
Keller realized that the necessary studies would be easier to do on small sharks. They also needed a known species to return to specific places every year. He and his colleagues opted for bonnetheads (Sphyrna tiburo).
“The Bonnethead returns to the same estuaries every year,” Keller said. “It shows that sharks know where ‘home’ is and can return there from a remote location.”
The question then was whether the bonnetheads managed these return journeys by relying on a magnetic card. To find out, the researchers used magnetic displacement experiments to test 20 juvenile bonnetheads caught in the wild. In their studies, they exposed the sharks to magnetic conditions representing locations hundreds of kilometers from where the sharks were actually caught. Such studies allow simple predictions of how sharks should orient themselves later if they actually relied on magnetic signals.
If the sharks derive positional information from the geomagnetic field, the researchers predicted a north orientation in the south magnetic field and a south orientation in the north magnetic field, as the sharks attempted to compensate for their perceived displacement. They did not predict any orientation preference when sharks were exposed to the magnetic field corresponding to their capture site. And, it turned out that the sharks acted as they predicted when exposed to fields in their natural range.
The researchers suggest that this ability to navigate based on magnetic fields may also contribute to the structure of the shark population. Findings in bonnetheads probably also help explain the impressive feats of other shark species. For example, a great white shark has been documented to migrate between South Africa and Australia, returning to the exact same location the following year.
“Is it cool that a shark can swim 20,000 kilometers round trip in a three-dimensional ocean and come back to the same site?” Keller asked. “It’s really mind-blowing. In a world where people use GPS to navigate almost anywhere, this ability is truly remarkable.”
In future studies, Keller says he would like to explore the effects of magnetic fields from anthropogenic sources such as underwater cables on sharks. They would also like to study if and how sharks rely on magnetic signals not only during long-distance migration, but also during their daily behavior.
This work was supported by the Save Our Seas Foundation and the Florida State University Coastal and Marine Laboratory.
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