The open ocean has no road signs. Out past the coast, every direction looks like the same endless blue. Yet some sharks swim thousands of miles across it and then turn around and find their way back. So how do they know where they're going?
The short answer is that sharks seem to carry their own navigation system. Scientists think they can sense Earth's magnetic field, using it a bit like a compass and a rough map at the same time. The key tool is a network of tiny pores on the shark's snout called the ampullae of Lorenzini. Researchers are still working out exactly how it all fits together, but the evidence has been building.
Champion Long-Distance Swimmers
Great white sharks are some of the ocean's great travelers. They migrate in response to food and water temperature, and possibly to mate. One great white tagged off the coast of South Africa swam to the southern coast of Australia and back within a year.
Another South African great white was tracked to Australia's northwestern coast and back, a journey of about 12,000 miles in under nine months. In the Pacific, great whites travel between the coasts of the United States and Mexico and the islands of Hawaii. In the Atlantic, they gather between the Gulf of Maine and Cape Hatteras in spring and summer, then shift south toward Florida and the Gulf of Mexico as the weather cools.
The Pores on a Shark's Nose
If you look closely at a shark's snout, you'll see clusters of little pores. Each one leads to a mucus-filled canal and a tiny sac beneath the skin. These are the ampullae of Lorenzini, named after the Italian doctor Stefano Lorenzini, who described them in detail in 1679. Nobody knew what they were for until 1960, when scientists identified them as organs for sensing electric fields.
Sharks have the greatest electrical sensitivity of any animal. They use it to find prey hiding under the sand by picking up the weak electric fields that living things give off. But there's a bonus. As a shark swims through Earth's magnetic field, and as ocean currents move through it, tiny electric signals are created. Those signals may be just what a shark needs to get its bearings.
Putting the Theory to the Test
In one 2004 experiment, a group of about eight sharks was placed in a tank with a magnetic field that could be switched on or off. During training, the field came on whenever food was offered at a target on the tank floor. Later, researchers switched the field on without any food. The sharks swam over the target far more often when the field was on, showing they could detect it.
Earth's magnetic field changes in strength and angle from place to place across the planet. An animal that can read those differences could get a general sense of where it is. That may help explain how sharks keep to such strict migration routes, though scientists still have open questions about exactly how the sense works.
A Nose for the Job, Too
Sharks have other tools as well. Their sense of smell is famously sharp, and some species can detect blood at about one part per million in seawater. Sharks that live in murky or deep water tend to have bigger smell centers in the brain than those living on clear, bright reefs.
And they've had plenty of time to perfect all this. Shark-like fish first appeared more than 400 million years ago, long before trees, dinosaurs, or anyone drawing maps. It turns out the ocean's oldest navigators never needed one.
