Sharks Have a Magnetic Sense of Where They Are
Vladimir Wrangel via Shutterstock
(Inside Science) -- The world's oceans contain few natural corridors to shepherd marine animals on their migratory journeys. Birds and mammals can use sunlight, moonlight and landmarks to orient themselves, but visibility is minimal below the ocean surface, so marine animals must use cues like sound waves or a powerful sense of smell. A new study shows for the first time that some sharks traverse the oceans with the aid of Earth itself, by sensing its magnetic field.
Bonnethead sharks are found along the North American Atlantic coast, often in shallow estuaries where the water is murky and dark, said Bryan Keller, a researcher at Florida State University's Coastal and Marine Laboratory. Stretching 3 to 4 feet long and weighing around 25 pounds, the sharks migrate back and forth more than 600 miles, swimming between favorite locations on the coast of South Carolina to the Gulf Coast of Florida year after year. Their journeys suggest they know where they are going and how to get there, but researchers couldn't tell how they did it, Keller said.
Scientists suspected the sharks might somehow glean maplike information from the Earth's magnetic field, by sensing its direction and intensity.
"So many species travel so far in the ocean, and the magnetic field is such a good cue to use. It just seemed like they must," said Catherine Lohmann, a biologist at the University of North Carolina at Chapel Hill who studies sea turtle migration and was not involved in the new work. But uncovering a magnetic sense in marine animals, especially large creatures like sharks, is extraordinarily hard.
To do it, Keller wrapped pieces of lumber in copper wire and placed them around a tank of water. This surrounded the water with a magnetic field. Using an electric current, the researchers were able to manipulate the magnetic field's intensity and direction, simulating the Earth's internal magnetic field for any given location. In the ocean, the field varies with latitude and can be affected by seamounts, shallow areas and other ocean features.
Then Keller and his crew captured 20 bonnetheads off Florida's west coast and took them to a holding pool to acclimate them for two weeks. Then, during the experiment, Keller exposed the sharks to a magnetic field that matched locations about 375 miles south of their summer grounds. He filmed the sharks as they swam around the tank and then persistently swam northward.
"They are thinking, 'We are not home, because the magnetic field is different, and in order to go home I need to migrate north,'" he said. "Their orientation was toward their preferred habitat, which tells us they are using this information to navigate to home."
The sharks are the first solely marine creature shown to have this ability, he said. Previous studies have shown a similar capability in salmon, sea turtles and eels, but all those animals spend part of their time in freshwater or on land. The new results were published today in the journal Current Biology.
Kyle Newton, a marine biologist at Washington University in St. Louis, had previously trained rays to respond to changes in a magnetic field in order to reach a food reward. His research with yellow stingrays, found in the same family as sharks and skates, showed that they learned to follow the field direction to reach a snack. If they couldn't sense the field, they would not have responded, he said. But Keller's experiment demonstrates that sharks can not only detect the magnetic field but also actually use it with purpose, Newton said. It's akin to having both a compass and a map.
Newton said he's wanted to perform a similar study for years. "I'm just jealous -- I wish I could have done it," he said.
"It demonstrates finally, and I think unequivocally, that they can actually use the magnetic field to gain an idea of where they are relative to where they want to go, and that they can use that to reorient themselves."
Researchers still don't know how the sharks do this, however. Other sensory receptors were relatively easy to find because they are external -- your sense of smell, vision, taste, touch and hearing are driven by receptors on your body's exterior.
"But magnetic fields go right through an animal. You could have it buried in the brain. You could have it in the bones. You could have it anywhere," Lohmann said. "That means there is no obvious place to look for a magnetic field receptor."
There's also no good analogy for what a magnetic sense might feel like. Though many animals detect sound waves we can't hear and light we cannot see, we can still relate to those capabilities, understanding they are just outside the spectrum we can sense. But so far as anyone knows, no human has a magnetic field detector, certainly not one we can notice. The cells that perform this function might be found in electroreceptors in skin or magnetoreceptors we have yet to find, or maybe through cells in the retina. If that's the case, maybe detection of magnetic fields would be like looking through a polarized lens, Newton said.
"It could be like a vignetting effect, and the angle of the vignette tells you the angle of the magnetic field, and the amount of vignetting tells you the intensity. That's the closest thing we can think of," he said.
Keller speculates that it might feel akin to standing in a freezing cold room and sensing a heater on one wall: You would naturally be drawn in the heater's direction, without even thinking about it.
"It's so hard to imagine what this feels like. And I have thought about it so much," he said.
Now that animals like sharks are known to also use a magnetic sense, humans should be aware of changes we make to those senses, too, Keller and Newton said. Seafloor power lines carrying electricity from offshore wind farms will produce magnetic fields that cannot be shielded, so biologists need to know more about how the animals detect those fields and how they are used.
"We need to do all of these baseline studies to determine if there is an effect, and if there is, how do we mitigate it," Newton said.