Bonnethead sharks swam in the direction of their home waters when placed in a tank charged with an electromagnetic field.
By Alex Fox SMITHSONIANMAG.COM
MAY 6, 2021
Every December, great white sharks swimming off the coast of California make a beeline for a mysterious spot in the middle of the Pacific roughly halfway to the Hawaiian islands. The sharks travel roughly 1,000 miles to the so-called white shark cafe. Tracking data has revealed that their routes are remarkably direct considering their paths traverse apparently featureless open ocean. Tiger sharks, salmon sharks and multiple species of hammerheads also make lengthy journeys to and from precise locations year after year.
Pete Klimley, a retired shark researcher who worked at the University of California, Davis calls the ability of some animals to find their way to pinpoint locations across the globe “one of the great mysteries of the animal kingdom.”
Now, new research published today in the journal Current Biology provides new support for a longstanding hypothesis that sharks use the Earth’s magnetic field to navigate during their long-distance migrations. Scientists caught bonnethead sharks off the coast of Florida and put them in a tank surrounded by copper wires that simulated the magnetic fields sharks would experience in locations hundreds of miles from their home waters. In one key test, the bonnetheads were tricked into thinking they were south of their usual haunts and in response the sharks swam north.
Iron and other metals in Earth’s molten core produce electrical currents, which create a magnetic field that encircles the planet. The north and south poles have opposing magnetic signatures and invisible lines of magnetism arc between them. The idea that sharks can navigate by sensing these fields rests on the fact that Earth’s geomagnetism isn’t evenly distributed. For example, the planet’s magnetism is strongest near the poles. If sharks can somehow detect the subtle perturbations of Earth’s magnetic field, then they might be able to figure out which way they’re heading and even their position.
Sharks are known to have special receptors—tiny jelly-filled pits called ampullae of Lorenzini that are clustered around their noses—which can sense changes in voltage in the surrounding environment. In theory, these electroreceptors, which are usually used to detect the electrical nerve impulses of prey, could pick up Earth’s magnetic field. Prior experiments have shown that, one way or another, sharks can indeed perceive and react to magnetic fields, but figuring out whether sharks can use them to navigate long distances or as a kind of map is another matter.
Scientists’ expanding sense of how sharks perceive their environment may even one day help researchers understand if humans are blocking or confusing the animals’ navigation as offshore infrastructure continues to grow in scope and complexity.
“One of the things that makes this work important is that they’re putting in wave farms and offshore wind farms and all of these projects have big high-voltage cables leading to shore,” says Klimley. “Those cables put off their own electric fields and if that’s how sharks navigate, we need to find out how that undersea infrastructure might impact migratory sharks.”