The leatherback turtle (Dermochelys coriacea) is the largest of all living sea turtles and the fourth largest modern reptile behind three crocodilians.^[2][3] It is the only living species in the genus Dermochelys. It can easily be differentiated from other modern sea turtles by its lack of a bony shell. Instead, its carapace is covered by skin and oily flesh. Dermochelys coriacea is the only extant member of the family Dermochelyidae.

Anatomy and physiology

Leatherback turtles follow the general sea turtle body plan of having a large, dorsoventrally flattened, round body with two pairs of very large flippers and a short tail. Like other sea turtles, the leatherback's flattened forelimbs are 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 2.7 meters (9 ft) in large specimens, the largest flippers (even in comparison to its body) of any sea turtle.

The leatherback has several characteristics that distinguish it from other sea turtles. Its most notable feature is that it lacks the bony carapace of other sea turtles. Instead of scutes, thick, leathery skin with embedded minuscule bony plates covers its carapace. Seven distinct ridges rise from the carapace, crossing from the anterior to posterior margin of the turtle's back. 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.^[4][5]

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. Leatherback turtles can dive to depths as great as 4,200 feet (1,280 metres).^[6]

Dermochelys coriacea adults average 1–2 metres (3.3–6.6 ft) long and weigh 250 to 700 kilograms (550 to 1,500 lb).^[4] The largest ever found however was over 3 meters (10 ft) meters from head to tail and weighed 916 kilograms (2,019 lb).^{[citation needed]} That specimen was found on a beach on the west coast of Wales.

Physiology

Its metabolic rate is about four times higher than predicted for a reptile of its size; this, coupled with counter-current heat exchangers and the insulation provided by its oily flesh and large body size, allow it to maintain a body temperature as much as 18 °C (64.4 °F) above water temperature. Its large size also gives the leatherback more capacity to maintain its body temperature than smaller, more ectothermic reptiles.^[2]

Leatherbacks are the reptile world's deepest-divers. Individuals have been discovered deeper than 1,200 meters (3,937 ft).^[2]

They are also the fastest-moving reptiles. The 1992 edition of the Guinness Book of World Records lists the leatherback turtle moving at 35.28 kilometres per hour (21.92 mph) in the water.^[7][8]

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 the Cape of Good Hope in Africa and the southernmost tip of New Zealand.^[4] The leatherback is found in all tropical and subtropical oceans, and its range extends well into the Arctic Circle.^[9]

There are three major, genetically distinct populations. The Atlantic Dermochelys population is separate from the ones in the Eastern and Western Pacific, which are also distinct from each other.^[2][10] A third possible Pacific subpopulation has been proposed, those that nest in Malaysia. This subpopulation however, has been almost eradicated. The beach of Rantau Abang in Terengganu, Malaysia, had once had the largest nesting population in the world, hosting 10,000 nests per year. However in 2008 only 2 nested at Rantau Abang and unfortunately the eggs were infertile. The major cause for the decline is that the eggs are collected for food. While nesting beaches have been identified in the region, leatherback populations in the Indian Ocean remain generally unassessed and unevaluated.^[11]

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.^[12] These declining numbers have energized efforts to rebuild the species, which currently is critically endangered.^[12]

Atlantic subpopulation

The leatherback turtle population in the Atlantic Ocean ranges across the entire region. They range as far north as the North Sea and to the Cape of Good Hope in the south. Unlike other sea turtles, leatherback feeding areas are in colder waters where there is an abundance of their jellyfish prey, which broadens their range. However, only a few beaches on both sides of the Atlantic provide nesting sites.^[13]

Off the Atlantic coast of Canada, leatherback turtles feed in the Gulf of Saint Lawrence near Quebec and as far north as Newfoundland and Labrador.^[14] The most significant Atlantic nesting sites are in Suriname, French Guiana and Trinidad and Tobago in the Caribbean and Gabon in Central Africa. The beaches of Mayumba National Park in Mayumba, Gabon host the largest nesting population on the African continent and possibly worldwide, with nearly 30,000 turtles visiting its beaches each year to April.^[10][15][15] Off the northeastern coast of the South American continent, a few select beaches between French Guiana and Suriname are primary nesting sites of several species of sea turtles, the majority being leatherbacks.^[16] A few hundred nest annually on the eastern coast of Florida.^[3] In Costa Rica, the beaches of Gandoca and Parismina provide nesting grounds.^[11][17]

Pacific subpopulation

Pacific leatherbacks divide into two populations. One population nests on beaches in Papua, Indonesia and the Solomon Islands and forage across the Pacific in the Northern Hemisphere, along the coast of Oregon in North America. The Eastern Pacific population forages in the Southern Hemisphere, in waters along the western coast of South America, nesting in Mexico and Costa Rica.^[10][18]

The continental United States offers two major leatherback feeding areas. One well-studied area is just off the northwestern coast near the mouth of the Columbia River. The other American area is located in the state of California.^[18] Further north, off the Pacific coast of Canada, leatherbacks visit the beaches of British Columbia.^[14]

Indian Ocean subpopulation

While little research has been done on Dermochelys populations in the Indian Ocean, nesting populations are known from Sri Lanka and the Nicobar Islands. It is proposed that these turtles form a separate, genetically distinct Indian Ocean subpopulation.^[11]

The Malaysian nesting population, reduced to less than a hundred individuals as of 2006, has been proposed as a third Pacific subpopulation.^[11]

Ecology and life history

Habitat

Leatherback turtles can be found primarily in the open ocean. Scientists tracked a leatherback turtle that swam from Indonesia to the U.S. in an epic 20,000 kilometers (12,427 mi) foraging journey over a period of 647 days.^[4][19] The turtles prefer deep water but are most often seen within sight of land. Feeding grounds are in nearshore waters. Unusually for a reptile, leatherbacks can survive and actively swim in colder waters; individual turtles have been found in waters as cold as 4.5 °C (40.1 °F).^[4][20]

Its favored breeding beaches are mainland sites facing deep water and they seem to avoid those sites protected by coral reefs.^[21]

Feeding

Adult Dermochelys coriacea subsist almost entirely on jellyfish.^[4] Due to its obligate feeding nature, it has been hypothesized that leatherback turtles help control jellyfish populations.^[2] Leatherbacks also feed on other soft-bodied organisms such as tunicates and cephalopods.^[20]

Death and decomposition

Dead leatherbacks that wash ashore are micro-ecosystems on their own while decomposing. In 1996, a drowned carcass held sarcophagid and calliphorid flies after being picked open by a pair of Coragyps atratus vultures. Infestation by carrion-eating beetles of the Scarabaeidae, Carabidae, and Tenebrionidae families soon followed suit. After days of decomposition, beetles from the families Histeridae and Staphylinidae and anthomyiid flies invaded the corpse as well. Organisms from more than a dozen families took part in consuming the carcass.^[22]

Life history

Like all sea turtles, leatherbacks start as hatchlings climbing out of the sands of their nesting beaches. Birds, crustaceans, other reptiles, and people prey on hatchlings before the new turtles reach the water. Once in the ocean they are rarely seen before maturity. Few turtles survive this period. Juvenile Dermochelys spend a majority of their time in more tropical waters than do adults.^[20]

Adults are prone to long-distance migration. Migration occurs between the cold waters in which mature leatherbacks feed, to the tropical and subtropical beaches in the regions where they hatch. In the Atlantic, females tagged in French Guiana have been recaptured on the other side of the ocean in Morocco and Spain.^[16]

Mating takes place at sea. Males never leave the water once they enter it, unlike females which nest on land. After encountering a female (who possibly exudes a pheromone to signal her reproductive status) the male uses head movements, nuzzling, biting, or flipper movements to determine her receptiveness. Females mate every two to three years. However, leatherbacks can breed annually. Fertilization is internal, and multiple males usually mate with a single female. This polyandry does not provide the offspring with any special advantages.^[23]

While other sea turtle species almost-always return to their hatching beach, leatherbacks may choose another beach within the region. Leatherbacks choose beaches with soft sand since their softer shells and plastrons are easily damaged by hard rocks. Nesting beaches also have shallower approach angles from the sea. This is a vulnerability for the turtles because such beaches easily erode.

Females excavate a nest above the high-tide line with their flippers. One female may lay as many as nine clutches in one breeding season. About nine days pass between nesting events. Average clutch size is around 110 eggs, 85% of which are viable.^[4] After laying, the female carefully back-fills the nest. disguising it from predators with a scattering of sand.^[20][24]

Cleavage of the cell begins within hours of fertilization, but development is suspended during the gastrulation period of movements and infoldings of embryonic cells, while the eggs are being laid. Development then resumes, but embryos remain extremely susceptible to movement-induced mortality until the membranes fully develop after incubating for 20 to 25 days. The structural differentiation of body and organs (organogenesis) soon follows. The eggs hatch in about sixty to seventy days. As with other reptiles, the nest's ambient temperature determines the sex of the hatchlings. After nightfall, the hatchlings dig to the surface and walk to the sea.^[25][26]

Leatherback nesting seasons vary from place-to-place. Nesting occurs in February to July in Parismina, Costa Rica.^[17] Farther east in French Guiana, Dermochelyss nest from March to August.^[16] Atlantic leatherbacks nest between February and July from South Carolina in the United States to the United States Virgin Islands in the Caribbean and to Suriname and Guyana.^{[citation needed]}