Deep-sea divers: Leatherback turtle hatchlings are gathered in a bucket prior to their release into the sea on a beach at a Malaysian turtle sanctuary.

Credit: AFP/JIMIN LAI

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PARIS: Biologists have figured out why sea turtles that normally feed and breed in shallow water or on land will, very rarely, go deep sea diving.

People have long puzzled over why leatherbacks (Dermochelys coriacea) are built to plumb the icy depths. Imagine someone donning a complete set of scuba gear – tanks, buoyancy compensator, regulator – only to paddle about the surface of a shallow lagoon. What's the point?

The mystery deepens. Not only are the turtles equipped with myoglobin-rich blood ideal for stocking oxygen, they sometimes plunge more than a kilometre below the surface.

Rare forays

Jonathan Houghton and colleagues from the University of Swansea in Wales conducted experiments to find out why the lumbering sea creatures make these rare forays, and now publish their findings in the 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 0.5 per cent – went below three hundred meters.

Several theories have competed to explain these out-of-character deep dives. Some researchers argue that the egg-laying reptiles go below to escape predators, while others speculate they simply want to cool off. A third hypothesis is that the turtles are on the hunt for deep-sea delicacies.

But Houghton's findings suggest all these theories are off the mark.

Off the mark

A turtle trying to avoid becoming lunch would surely swim more vigorously than 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-metre 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 travelling 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 metres.

Houghton speculates, that the turtles dive when the Sun is out to find the colonies, and then wait form 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