Going where few air-breathers dare to go

(originally posted 16 April 2007 on Deep-Sea News)

Life in the deep sea is as far removed from a source of atmospheric oxygen as there is on Earth, but a select few animals do not let their need to breathe air limit the depths of their exploration. (No, I’m not referring to intrepid deep-sea human researchers.) These extraordinary critters frequently venture into the hostile conditions of the deep-sea, despite being vitally tied to air the above the ocean’s surface.

When you hear about deep-diving, air-breathing animals, you might first think of colossal sperm whales plunging over 1,000 meters to battle giant squid in the dark abyss. Or perhaps you think of massive elephant seals spending over an hour at depths over a half a mile down chasing prey. Maybe you’ve even heard of the deepest known diver of them all, the beaked whales, with recorded dives to over 2,000 meters. But what about turtles? Could a shelled reptile be suited for making dives where only a few whales and seals dare to go?

Leatherback turtles, like some whales and seals, possess remarkable adaptations for long and deep dives, with large onboard stores of oxygen in their blood and muscle, and special features like collapsible lungs (to avoid ‘the bends’), flexible shell (to respond to increased pressure at depth), and slowed heart rate (to conserve energy and oxygen stores). In fact, researchers have recorded leatherbacks diving to over 1,000 meters in different ocean basins (maximum reported dive was 1,230 m in the North Atlantic). Apparently, anything a whale can do, a turtle can, too.

To be clear, leatherbacks spend most of their time within the top 300 m of the ocean. These diving tendencies are likely due to leatherbacks’ air-dependence and because they feed primarily on jellyfish, like lion’s mane jellies and sea nettles, which tend to be concentrated at or near the ocean’s surface. However, leatherbacks are clearly capable of – and undertake – much more ambitious diving. The obvious question is: what are leatherbacks doing down there?

At this point, we aren’t sure what motivates a leatherback to plunge thousands of meters away from their most critical resource, but there are a few possibilities. Like other deep-divers, they might be pursuing prey. (Let’s face it; food is a strong incentive for any animal.) Perhaps they shuttle between different water temperatures in order to keep their body temperature stable, so an occasional deep dive into frigid waters could offset the heating effects of strenuous diving. Or maybe, like many other marine animals, they use the deep-sea as refuge to evade hungry predators, such as sharks and orcas.

Currently, we are using satellite telemetry to track several leatherback turtles migrating away from their Costa Rican nesting beaches to their high-latitude foraging grounds in the Eastern Tropical Pacific. Check out www.greatturtlerace.com to follow leatherbacks on their migration and to learn more about leatherbacks, the threats they face, and what is being done to protect them. Led by Stanford researcher George Shillinger, our group is analyzing leatherback dive profiles in relation to several oceanographic features to figure out just what drives leatherback diving. So far, all leatherbacks in our study have dived well beyond 300 meters on several occasions, and many have reached depths in excess of 1,000 meters. While leatherbacks have yet to reveal all of their secrets to us, hopefully our current efforts and those of our colleagues in other parts of the world will shed light on the dark side of leatherbacks’ deep-sea lives.

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