Baby turtles journey across the Pacific.

Off the coast of Baja California, 10,000 juvenile loggerhead turtles munch on the area's plentiful crabs and grow into adults. These young reptiles rest at the heart of a long-standing puzzle in marine biology: There are no known loggerhead nesting sites on the eastern side of the Pacific.

The nearest sites lie in Japan and Australia. Young turtles "head out into the water as 3-inch-long hatchlings,'' says Brian Bowen of the University of Florida in Gainesville, and researchers found it hard to accept that they could survive the 10,000-kilometer swim to Baja, a distance spanning more than one-third of the globe. The trek to Baja would rank among the longest documented marine migrations.

But fishermen often catch juvenile loggerheads in the North Pacific, causing scientists to rethink their disbelief in the voyage. And now a team led by Bowen appears to have proved that this incredible journey does take place.

Bowen's group obtained mitochondrial DNA samples from the loggerhead turtles at the Japanese and Australian nesting grounds and compared them to samples taken from 26 Baja juveniles and 34 ones caught inadvertently by North Pacific fishermen. Ninety-five percent of the juveniles carried the same distinctive genetic sequences as the baby turtles in Japan; the rest matched the DNA markers of the Australian turtles, they report in the April 25 Proceedings of the National Academy of Sciences.

The case for a trans-Pacific migration is not completely nailed down, the authors acknowledge, since a remote possibility still exists of a hidden East Pacific nesting ground whose the turtles have the same DNA sequences.

Also unclear is whether the Baja turtles, after maturation, recross the Pacific and return to their nesting grounds. Knowledge of the migratory routes, the authors stress, will help in assessing the impact of commercial fishing on an already dwindling species.