Thursday, March 7, 2013

What’s big, aquatic, warm-blooded, and has rubbery skin? Why, the leatherback sea turtle, of course.

If there was ever a turtle that wanted to be a whale, it would have to be the leatherback turtle. They are gigantic, the largest turtles in the world and the fourth-largest reptile; they are deep-diving and can be found worldwide in virtually every climate; they go on tremendous migrations both to breed and feed; they even have oily, smooth skin in place of a hard shell.

That doesn’t sound like any sea turtles we’re so familiar with, does it? Where did such an offshoot from the “typical” sea turtles come from? It’s not so much that the leatherback is an offshoot from modern turtles; rather, leatherbacks represent a lineage of turtles which were once abundant in prehistoric shallow seas. Close relations to the leatherback include the “ruling turtle” Archelon, which grew to lengths of 4m (13ft) and sported a flipper-span of 5m (16ft). Sadly, the leatherback’s closest relatives are all extinct; other extant sea turtles belong to an entirely different family.

The turtle that wanted to be a whale. Photograph from Projeto TAMAR, IBAMA's sea turtle conservation program.
The family Dermochelyidae, of which the leatherback is the sole living member, has likely been around since the Cretaceous. As such, the family as a whole has had an incredibly long time to adapt to an oceanic lifestyle. The leatherback’s adaptations for living in the open ocean are remarkable, especially for a reptile. Most notably, the leatherback, as its name suggests, completely lacks a shell. Rather, its thick, oily, rubbery skin is embedded with tiny osteoderms. These little bumps in its back form distinct ridges, which give the appearance of a shell; however, skeletally, they appear much like other tetrapods: their ribs and vertebrae are still able to move freely, unlike other turtles in which these bones are fused to the shell.

The leatherback can be found worldwide, from the Arctic Circle to south of New Zealand. They are impressive divers, sometimes submerging over 900m and staying underwater for up to 80 minutes in pursuit of jellyfish, the main staple of their diet. (Sale et. al, 2006) Clearly, they have very different feeding strategies than other sea turtles, which mostly feed on foliage in tropical and subtropical waters. The leatherback’s feeding strategy and range expose it to waters just shy of freezing; one individual was recorded as diving to 61m and experiencing 0.4oC (~33oF) water. (James et. al, 2006) Such temperatures pose a huge threat to the physical health of any normal reptile. But, as we have seen, the leatherback is anything but a normal reptile.

No, the leatherback is something truly special. While most reptiles depend on the temperature of their environment to regulate their body temperature, the leatherback has much more control over its body. They have two distinct layers of fat which insulate its body, a trait found in many cold-dwelling mammals, as well as some open-water fish. (Goff & Stenson, 1988) Under their fat, they have an even more remarkable adaptation for surviving in cold waters. The muscles used in swimming operate completely dependent of temperature. Generally, reptiles require heat in order to keep their metabolism up and, in turn, move around. However, the pectoral muscles of the leatherback were found to operate totally independent of temperatures ranging from 5-38oC (~41-100oF). Interestingly, while the leatherback’s muscles showed higher rates of metabolism at lower temperatures when compared to other sea turtles, they also displayed lower rates of metabolism at higher temperatures. (Penick et. al, 1998) This could mean that they are not only well-adapted to cold water, but in fact, they survive much better in such conditions.

Photograph from Projeto TAMAR.
One surprising part of the leatherback’s body contributes a great deal to thermoregulation: its throat. While leatherback hatchlings have typical reptilian tracheae, composed of rings connected by connective tissue, adults have, in essence, one long, elliptical tube containing plates of cartilage. These plates can close when the turtle dives, eliminating problems faced by all deep-diving animals when venturing far below the surface. When making such dives, the lungs and main airways face the danger of collapsing under the immense water pressure to which the animal is exposed.  The adult leatherback’s trachea is also lined with a network of blood vessels; any air caught in the lungs or trachea is warmed by these vessels, allowing the turtle to breath comfortable, humid air even in frigid environments. (Davenport et. al, 2009)

I hope you can tell why I’m suddenly so fascinated by this turtle. They’re more than just your average sea turtle: their adaptations go above and beyond those of other sea turtles. They aren’t just well-adapted for a life at sea, they’re pretty much perfect for it. Sadly, these giants are critically endangered and face a number of threats, including pollution. We’ll discuss more about the feeding habits and endangerment of the great leatherback in the next post.

Davenport, John, John Fraher, Ed Fitzgerald, Patrick McLaughlin, Tom Doyle, Luke Harman, Tracy Cuffe, and Peter Dockery. 2009. “Ontogenetic Changes in Tracheal Structure Facilitate Deep Dives and Cold Water Foraging in Adult Leatherback Sea Turtles.” Journal of Experimental Biology 212 (21) (November 1): 3440–3447. doi:10.1242/jeb.034991.

Goff, Gregory P., and Garry B. Stenson. 1988. “Brown Adipose Tissue in Leatherback Sea Turtles: A Thermogenic Organ in an Endothermic Reptile?” Copeia 1988 (4) (December 28): 1071–1075. doi:10.2307/1445737.

James, Michael C., John Davenport, and Graeme C. Hays. 2006. “Expanded Thermal Niche for a Diving Vertebrate: A Leatherback Turtle Diving into Near-freezing Water.” Journal of Experimental Marine Biology and Ecology 335 (2) (August 8): 221–226. doi:10.1016/j.jembe.2006.03.013.

Penick, David N., James R. Spotila, Michael P. O’Connor, Anthony C. Steyermark, Robert H. George, Christopher J. Salice, and Frank V. Paladino. 1998. “Thermal Independence of Muscle Tissue Metabolism in the Leatherback Turtle, Dermochelys coriacea.” Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 120 (3) (July 1): 399–403. doi:10.1016/S1095-6433(98)00024-5.

Sale, Alessandro, Paolo Luschi, Resi Mencacci, Paolo Lambardi, George R. Hughes, Graeme C. Hays, Silvano Benvenuti, and Floriano Papi. 2006. “Long-term Monitoring of Leatherback Turtle Diving Behaviour During Oceanic Movements.” Journal of Experimental Marine Biology and Ecology 328 (2) (January 24): 197–210. doi:10.1016/j.jembe.2005.07.006.

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