Animals

Researchers have finally discover the mystery of why sea turtles will mainly stay in shallow water to feed and breed instead of going deep into the water. They have often wondered for years why they wouldn’t go out into the deep parts of the water more often when they are built for it.

Deep sea turtles are born with myoglobin-rich blood that helps them to stock up on oxygen. Jonathan Houghton and colleagues from the University of Swansea in Britain conducted experiments to find out why the lumbering sea creatures make these rare forays, and published their findings Friday in the British Journal of Experimental Biology.

The researchers fitted 13 leatherbacks with data loggers which recorded location, temperature, dive depth and duration, and transmitted the information to satellites as the animals surfaced. Of more than 26,000 dives logged all across the North Atlantic Ocean, only 95 — less than half of one percent — went below three hundred meters.

A turtle trying to avoid becoming some fish’s lunch would surely swim a bit more vigorously that usual, but the data collected indicates they were in no hurry as they plunged. Moreover turtles spent several hours at the surface just before deep diving, probably to boost oxygen efficiency.

“Hanging out at the surface would be a daft strategy for avoiding predators, because that is where they can spot your silhouette,” said Houghton. As for keeping cool, temperatures don’t drop much after the 350-meter mark, so there’s little incentive to go any deeper. But the food hypothesis, the study found, may be at least half right: even if the turtles don’t eat the food they find at extreme depth, they probably find the food they will eat - later on.

Leatherbacks like to dine on surface-dwelling jellyfish, but during the months spent traveling from their tropical breeding grounds in the Caribbean to cooler waters, they rely on jellyfish-like animals that form long colonies during the day at depths of about 600 meters.

The turtles, Houghton speculates, dive when the sun is out to find the colonies, and then wait for them to surface at night to begin feasting. This would explain why the leatherbacks often loiter in the same area for days or weeks after such a deep dive, he said.