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.
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.
References
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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