Leatherback sea turtle
The leatherback sea turtle, sometimes called the lute turtle, leathery turtle or simply the luth, is the largest of all living turtles and the heaviest non-crocodilian reptile, reaching lengths of up to and weights of. It is the only living species in the genus Dermochelys and family Dermochelyidae. It can easily be differentiated from other modern sea turtles by its lack of a bony shell; instead, its carapace is covered by oily flesh and flexible, leather-like skin, for which it is named. Leatherback turtles have a global range, although there are multiple distinct subpopulations. The species as a whole is considered vulnerable, and some of its subpopulations are critically endangered.
Taxonomy and evolution
Taxonomy
Dermochelys coriacea is the only species in genus Dermochelys. The genus, in turn, contains the only extant member of the family Dermochelyidae.Domenico Agostino Vandelli named the species first in 1761 as Testudo coriacea after an animal captured at Ostia and donated to the University of Padua by Pope Clement XIII. In 1816, French zoologist Henri Blainville coined the term Dermochelys. The leatherback was then reclassified as Dermochelys coriacea. In 1843, the zoologist Leopold Fitzinger put the genus in its own family, Dermochelyidae. In 1884, the American naturalist Samuel Garman described the species as Sphargis coriacea schlegelii. The two were then united in D. coriacea, with each given subspecies status as D. c. coriacea and D. c. schlegelii. The subspecies were later labeled invalid synonyms of D. coriacea.
Both the turtle's common and scientific names come from the leathery texture and appearance of its carapace. Older names include "leathery turtle" and "trunk turtle". The common names incorporating "lute" and "luth" compare the seven ridges that run the length of the animal's back to the seven strings on the musical instrument of the same name, but are more likely derived from the lute's ribbed back which is in the form of a shell.
Evolution
Relatives of modern leatherback turtles have existed in relatively the same form since the first true sea turtles evolved over 110 million years ago during the Cretaceous period. The dermochelyids are relatives of the family Cheloniidae, which contains the other six extant sea turtle species, and studies using fossil-calibrated genetic analyses have shown these families to have split approximately 49 to 70 million years ago. However, the sister taxon of Dermochelyidae is the extinct family Protostegidae that included other species that did not have a hard carapace.Anatomy and physiology
Leatherback turtles have the most hydrodynamic body of any sea turtle, with a large, teardrop-shaped body. A large pair of front flippers powers the turtles through the water. Like other sea turtles, the leatherback has flattened forelimbs adapted for swimming in the open ocean. Claws are absent from both pairs of flippers. The leatherback's flippers are the largest in proportion to its body among extant sea turtles. Leatherback's front flippers can grow up to in large specimens, the largest flippers of any sea turtle.The leatherback has several characteristics that distinguish it from other sea turtles. Its most notable feature is the lack of a bony carapace. Instead of scutes, it has thick, leathery skin with embedded minuscule osteoderms. This scute-loss is only otherwise seen in the Trionychia, the superfamily containing Trionychidae and Carettochelys insculpta. The absence of scutes in Dermochelyidae does not, however, have any function for cutaneous respiration, as is seen in these other clades. Instead, researchers have hypothesized that the flexible skin on its carapace may allow it grow and shrink the fat deposits in its shell, depending on the availability of resources, in order to meet the insulation requirements of its deep dives and the energetic requirements of its long migrations. Seven distinct ridges rise from the carapace, crossing from the cranial to caudal margin of the turtle's back. Leatherbacks are unique among reptiles in that their scales lack β-keratin. The entire turtle's dorsal surface is colored dark grey to black, with a scattering of white blotches and spots. Demonstrating countershading, the turtle's underside is lightly colored. Instead of teeth, the leatherback turtle has points on the tomium of its upper lip, with backwards spines in its throat to help it swallow food and to stop its prey from escaping once caught.
D. coriacea adults average in curved carapace length, in total length, and in weight. In the Caribbean, the mean size of adults was reported at in weight and in CCL. Similarly, those nesting in French Guiana, weighed an average of and measured in CCL. The largest verified specimen ever found was discovered on the Pakistani beach of Sandspit and measured in CCL and in weight. A previous contender, the "Harlech turtle", was purportedly in CCL and in weight, however recent inspection of its remains housed at the National Museum Cardiff have found that its true CCL is closer to, casting doubt on the accuracy of the claimed weight, as well. On the other hand, one scientific paper has claimed that the species can weigh up to without providing more verifiable detail. The leatherback turtle is scarcely larger than any other sea turtle upon hatching, as they average in carapace length and weigh around when freshly hatched.
D. coriacea exhibits several anatomical characteristics believed to be associated with a life in cold waters, including an extensive covering of brown adipose tissue, temperature-independent swimming muscles, countercurrent heat exchangers between the large front flippers and the core body, and an extensive network of countercurrent heat exchangers surrounding the trachea.
Mechanical properties
The carapace of the leatherback sea turtle has a unique design which enables the sea turtles to withstand high hydrostatic pressures as they dive to depths of 1200 m. Unlike other sea turtles, the leatherback sea turtle has a soft, leathery skin which covers the osteoderms rather than a hard keratinous shell. The osteoderms are made up of bone-like hydroxyapatite/collagen tissue and have jagged edges, referred to as teeth. These osteoderms are connected by a configuration of interpenetrating extremities called sutures that provide flexibility to the carapace, enabling in plane and out of plane movement between osteoderms. This is important since the lungs, and thus the carapace, expand when taking in air and contract when deep diving.The sutures connect rigid elements and flexible joints in a zig-zag configuration, so there is no region where teeth can easily penetrate the carapace. There are two main failure mechanisms for the tires in tension: tooth failure corresponding to mineral-brittle failure; and interfacial failure between teeth corresponding to collagen-ductile failure. The triangular tooth geometry is able to evenly distribute load and absorb energy. This leads to a high strength in tension since this geometry takes advantage of the tensile strength of bone and the interface. Additionally, the carapace is tough because sutures prevent crack propagation. Under load, cracks interact with the sutures which can resist crack growth via crack bridging. This phenomenon was observed in sequential compression of osteoderm samples.
Physiology
Leatherbacks have been viewed as unique among extant reptiles for their ability to maintain high body temperatures using metabolically generated heat, or endothermy. Initial studies on their metabolic rates found leatherbacks had resting metabolisms around three times higher than expected for reptiles of their size. However, recent studies using reptile representatives encompassing all the size ranges leatherbacks pass through during ontogeny discovered the resting metabolic rate of a large D. coriacea is not significantly different from predicted results based on allometry.Rather than using a high resting metabolism, leatherbacks appear to take advantage of a high activity rate. Studies on wild D. coriacea discovered individuals may spend as little as 0.1% of the day resting. This constant swimming creates muscle-derived heat. Coupled with their countercurrent heat exchangers, insulating fat covering, and large size, leatherbacks are able to maintain high temperature differentials compared to the surrounding water. Adult leatherbacks have been found with core body temperatures that were above the water in which they were swimming.
Leatherback turtles are one of the deepest-diving marine animals. Individuals have been recorded diving to depths as great as.
Typical dive durations are between 3 and 8 minutes, with dives of 30–70 minutes occurring infrequently.
They are also the fastest-moving reptiles. The 1992 edition of the Guinness Book of World Records lists the leatherback turtle moving at in the water. More typically, they swim at.
Distribution
The leatherback turtle is a species with a cosmopolitan global range. Of all the extant sea turtle species, D. coriacea has the widest distribution, reaching as far north as Alaska and Norway and as far south as Cape Agulhas in Africa and the southernmost tip of New Zealand. The leatherback is found in all tropical and subtropical oceans, and its range extends well into the Arctic Circle.The three major, genetically distinct populations occur in the Atlantic, eastern Pacific, and western Pacific Oceans. While nesting beaches have been identified in the region, leatherback populations in the Indian Ocean remain generally unassessed and unevaluated.
Recent estimates of global nesting populations are that 26,000 to 43,000 females nest annually, which is a dramatic decline from the 115,000 estimated in 1980.