The Secret of Turtle Island

In the Mediterranean Sea off the coast of Libya, there's an area local fishermen call "Turtle Island." It's real enough, but you'd be foolish to try to sail there. The island is never in precisely the same place, and it changes size from one minute to the next. In fact, you never know when its gleaming shore might disappear altogether, because it's made up entirely of the half-exposed shells of basking loggerhead sea turtles.

The funny thing is, sea turtles, unlike the Mediterranean's human denizens, aren't supposed to like to float, says Sandra Hochscheid, a marine biologist at Stazione Zoologica Anton Dohrn in Naples, Italy. Scientists believe that loggerhead sea turtles (Caretta caretta), the dominant sea turtle in the Mediterranean, spend most of their lives underwater or on the sea floor, coming to the surface only for brief gasps of air. "Everything we'd found out about turtles: feeding, mating, migrating, searching for good places, resting … has all taken place underwater," she says.

Hochscheid knew about Turtle Island and had heard about basking turtles from tourists in Naples, but she'd never seen the behavior for herself. So she decided to figure out just what the loggerheads were doing spending so much time among the waves. "I've been looking 10 years into what they do under the water surface," Hochscheid says. "But I've never looked at the time they spend at the surface."

Hochscheid and colleagues collected 10 loggerheads from various locations in the Mediterranean and glued tracking devices—small black boxes with flexible antennas—to their shells. Over a year of observations, it became clear why the turtles had developed a reputation as bottom dwellers. The tagged loggerheads spent about 98% of their time underwater. And when they did come up, it was irregularly, sometimes twice in 2 days and sometimes not for a week.

There was a hint to the reason for the behavior, however. The turtles almost always surfaced during the day, 82% of the time, in fact, and mostly at about midday when the sunlight is most direct. Another clue: Daylight basking sessions almost always occurred after the turtles had returned from a dive below the thermocline, the transitional zone between the sun-warmed shallows and frigid depths. Hochscheid believes that daylight surfacing helps the loggerheads warm up quickly; she speculates it could also aid digestion, though she's not sure how.

Night surfacing, on the other hand, almost always followed dives that were long enough to cause the turtles to run out of air, a duration that the team calculated based on the turtles' weight. Night sessions, then, would help the turtles clear large amounts of lactic acid, produced when the turtles keep swimming when they've run out of oxygen, the team reports today in The Journal of Experimental Biology.

Basking may have a downside for the turtles, says Brendan Godley, a conservation biologist at the University of Exeter, Cornwall, in the United Kingdom, because it makes the turtles more vulnerable to boat strikes. He says that if more research uncovers a preference for certain spots or times of year, the findings could feed into conservation efforts. The work also exemplifies how technology is transforming marine biologists' understanding of the turtles, he says. "Especially with satellite tracking, the more we look, the more we find that highlights how complicated these animals are."