Sharks use Earth's magnetic field to navigate the seas - Science Magazine

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Sharks like this great white might use a magnetic “sixth sense” to find their way in the open seas.

Nature Picture Library/Alamy Stock Photo

By Nikk OgasaMay. 6, 2021 , 11:00 AM

Most Uber drivers need a smartphone to get to their destinations. But sharks, it seems, need nothing more than their own bodies—and Earth’s magnetic field. A new study suggests some sharks can read Earth’s field like a map and use it to navigate the open seas. The result adds sharks to the long list of animals—including birds, sea turtles, and lobsters—that navigate with a mysterious magnetic sense.

“It’s great that they’ve finally done this magnetic field study on sharks,” says Michael Winklhofer, a biophysicist at the Carl von Ossietzky University of Oldenburg in Germany, who was not involved in the study.

In 2005, scientists reported that a great white shark swam from South Africa to Australia and back again in nearly a straight line—a feat that led some scientists to propose the animals relied on a magnetic sense to steer themselves. And since at least the 1970s, researchers have suspected that the elasmobranchs—a group of fish containing sharks, rays, skates, and sawfish—can detect magnetic fields. But no one had shown that sharks use the fields to locate themselves or navigate, partly because the animals aren’t so easy to work with, Winklhofer says. “It’s one thing if you have a small lobster, or a baby sea turtle, but when you work with sharks, you have to upscale everything.”

Bryan Keller, an ecologist at Florida State University, and his colleagues decided to do just that. The researchers lined a bedroom-size cage with copper wire and placed a small swimming pool in the center of the cage. By running an electrical current through the wiring, they could generate a custom magnetic field in the center of the pool. The team then collected 20 juvenile bonnethead sharks—a species known to migrate hundreds of kilometers—from a shoal off the Florida coast. They placed the sharks into the pool, one at a time, and let them swim freely under three different magnetic fields, applied in random succession. One field mimicked Earth’s natural field at the spot where the sharks were collected, whereas the others mimicked the fields at locations 600 kilometers north and 600 kilometers south of their homes.

When the applied field was the same as at the collection site, the researchers found that the animals swam in random directions. But when subjected to the southern magnetic field, the sharks persistently changed their headings to swim north into the pool’s wall, toward home, the researchers report today in Current Biology (see video, below). “[This] suggests they’re able to use magnetic fields for long-distance migration,” says Neil Hammerschlag, a shark ecologist at the University of Miami who was not involved in the study.

Surprisingly, the researchers found that the sharks didn’t favor any direction when swimming under the northern field. Keller says this might be because the bonnetheads don’t usually migrate north of their home location, and so they rarely have to find their way back south again. “This could support the theory that their ability to orient toward home is a learned behavior,” he says. They might not know what to do in the northern field “because they’ve never been up there.”

Although the researchers used bonnethead sharks for their experiments, Keller says other sharks probably also use Earth’s magnetic field to navigate. Species like the great white shark, which travel longer distances than the bonnetheads, would find even greater use from this ability, Keller says.

With magnetic navigation now demonstrated in so many animals, Winklhofer wants researchers to figure out the underlying mechanism. Some say the sense relies on cells containing a magnetic iron mineral, magnetite, whereas others invoke a protein in the retina called cryptochrome. “All the other major senses have been described and understood,” Winklhofer says. “The key question is how do they do it?”

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May 06, 2021 at 10:00PM
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Sharks use Earth's magnetic field to navigate the seas - Science Magazine

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