Marine biologists have confirmed sharks can detect changes in magnetic fields.
Hammerheads move over great distances
This ability has long been suspected by researchers who have observed the fish migrating huge distances in the ocean along straight lines.
A Hawaii University team has trained captive sharks to swim over targets in their tank whenever an artificial magnetic field is activated.
The new study, by Dr Carl Meyer and colleagues, is reported in Interface, a journal of the UK's Royal Society.
"This significant advance in demonstrating the existence of a 'compass' sense should now make it possible to investigate exactly how this sense works and how sensitive sharks are to the Earth's magnetic field," the team tells Interface.
The Hawaii group used six sandbar sharks and one scalloped hammerhead in their research. They kept the animals in a 7m-diameter tank.
The fish were trained to associate the presence of food in a 1.5m by 1.5m target area on the enclosure floor with the switching on of a magnetic field, derived from a copper coil surrounding the tank
In a series of trials, the field was then activated at random times and the fish were seen to move on the feeding zone even when there was no food present, proving the existence of their "compass".
"Activating the artificial field produced an immediate response in the conditioned sharks," the team says.
"They changed from swimming steadily around the perimeter of the tank to swimming faster, turning rapidly and converging on the target in anticipation of a food reward."
Tiger sharks, blue sharks and scalloped hammerhead sharks are all known to swim in straight lines for long periods across hundreds of kilometres of open ocean, and then later orient themselves to underwater mountains, or seamounts, where geomagnetic anomalies exist.
Scientists want to understand how sharks are able to detect magnetic fields. Other animals that do it, such as trout and pigeons, possess the iron mineral magnetite in their bodies.
Sharks, however, do not possess magnetite. It is possible electro-receptors in their heads are employed instead.