Tuna, which migrate thousands of miles each year through the featureless depths of the Atlantic and Pacific Oceans, appear to find their way by using a kind of biological compass consisting of microscopic magnets embedded in their skulls.

It has been known for some years that certain migratory birds and homing pigeons rely on a similar apparatus.

The new findings, from the University of Hawaii and the National Marine Fisheries Service, show the natural compass is a more widespread evolutionary phenomenon than had been supposed.

Richard Brill, a fisheries service biologist, said the compass in the yellowfin tuna consists of about 10 million crystals of the iron-based mineral magnetite. The crystals, which are tiny magnets, are situated in a microscopically small organ embedded in a bone of the tuna's snout. There appear to be nerves linking the organ to the fish's brain.

Somehow, Brill said, the magnetic organ responds to the Earth's magnetic field, telling the fish which way it is pointing. He said the organ may also help the fish form a mental map of the ocean floor by sensing its varied patterns of magnetism.

These patterns exist because, as new molten rock wells up along mid-ocean ridges and spreads to the sides, magnetic crystals in the rock align themselves with the Earth's magnetic field. When the rock hardens, the magnetic alignment is frozen. But, at irregular intervals over thousands of years, the Earth's magnetic polarity has reversed. As a result, the ocean floors consist of stripes of normal and reversed magnetic polarity, preserving the ancient magnetic fields.

"We think this old magnetism may give the yellowfin a map while the current magnetic field gives them a compass," Brill said.