| Leatherback sea turtle | |
|---|---|
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Sauropsida |
| Order: | Testudines |
| Suborder: | Cryptodira |
| Superfamily: | Chelonioidea |
| Family: | Dermochelyidae |
| Genus: | Dermochelys Blainville, 1816[2] |
| Species: | D. coriacea |
| Binomial name | |
| Dermochelys coriacea (Vandelli, 1761)[1] |
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| Synonyms | |
|
Testudo coriacea Vandelli, 1761 |
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The leatherback sea turtle (Dermochelys coriacea) is the largest of all living sea turtles and the fourth largest modern reptile behind three crocodilians.[3][4] 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.
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Leatherback turtles have the most hydrodynamic body design of any sea turtle, with a large, teardrop-shaped body. A large pair of front flippers power the turtles through the water. 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 metres (8.9 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 the lack of a bony carapace. Instead of scutes, it has thick, leathery skin with embedded minuscule osteoderms. Seven distinct ridges rise from the carapace, crossing from the anterior to posterior 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.[5][6]
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.
Dermochelys coriacea adults average 1–1.75 metres (3.3–5.74 ft) in carapace length, 1.83–2.2 metres (6.0–7.2 ft) in total length and weigh 250 to 700 kilograms (550 to 1,500 lb).[5][7] The largest ever found, however, was over 3 metres (9.8 ft) from head to tail, including a carapace length of over 2.2 metres (7.2 ft), and weighed 916 kilograms (2,020 lb).[8] That specimen was found on a beach on the west coast of Wales.[9]
Dermochelys coriacea exhibits a suite of anatomical characteristics believed to be associated with a life in cold waters, including an extensive covering of brown adipose tissue,[10] temperature independent swimming muscles,[11] counter-current heat exchangers between the large front flippers and the core body, as well as an extensive network of counter-current heat exchangers surrounding the trachea.[12]
Leatherbacks have been viewed as unique among reptiles for their ability to maintain high body temperatures using metabolically generated heat, or endothermy. Initial studies on leatherback metabolic rates found leatherbacks had resting metabolisms around three times higher than expected for a reptile of their size.[13] However, recent studies using reptile representatives encompassing all the size ranges leatherbacks pass through during ontogeny discovered the resting metabolic rate of a large Dermochelys coriacea is not significantly different from predicted results based on allometry.[14]
Rather than use a high resting metabolism, leatherbacks appear to take advantages of a high activity rate. Studies on wild D.coriacea discovered individuals may spend as little as 0.1% of the day resting.[15] This constant swimming creates muscle-derived heat. Coupled with their counter-current heat exchangers, insulative 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 18 °C (32.4 °F) above the water they were swimming in.[16]
Leatherback turtles are one of the deepest diving marine animals. Individuals have been recorded diving to depths as great as 1,280 metres (4,200 ft).[17][18] Typical dive durations are between 3 and 8 minutes, with dives of 30–70 minutes occurring infrequently.[19]
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.[20][21] More typically, they swim at 0.5–2.8 metres per second (1.1–6.3 mph).[22]
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.[5] The leatherback is found in all tropical and subtropical oceans, and its range extends well into the Arctic Circle.[23]
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.[3][24] 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, once had the largest nesting population in the world, hosting 10,000 nests per year. In 2008, though, only two nested at Rantau Abang, and unfortunately the eggs were infertile. The major cause for the decline is 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.[25]
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.[26] These declining numbers have energized efforts to rebuild the species, which currently is critically endangered.[26]
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.[27]
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.[28] 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 in April.[24][29][29] 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.[30] A few hundred nest annually on the eastern coast of Florida.[4] In Costa Rica, the beaches of Gandoca and Parismina provide nesting grounds.[25][31]
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 coasts of California, Oregon, and Washington 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.[24][32]
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.[32] Further north, off the Pacific coast of Canada, leatherbacks visit the beaches of British Columbia.[28]
While little research has been done on Dermochelys populations in the Indian Ocean, nesting populations are known from Sri Lanka and the Nicobar Islands. These turtles are proposed to form a separate, genetically distinct Indian Ocean subpopulation.[25]
The Malaysian nesting population, reduced to less than a hundred individuals as of 2006, has been proposed as a third Pacific subpopulation.[25]
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 km (12,000 mi) foraging journey over a period of 647 days.[5][33] Leatherbacks follow their jellyfish prey throughout the day, resulting in turtles "preferring" deeper water in the daytime, and shallower water at night (when the jellyfish rise up the water column).[15] This hunting strategy often places turtles in very frigid waters. One individual was found actively hunting in waters that had a surface temperature of 0.4 °C (32.7 °F).[34]
Its favored breeding beaches are mainland sites facing deep water and they seem to avoid those sites protected by coral reefs.[35]
Adult D. coriacea turtles subsist almost entirely on jellyfish.[5] Due to their obligate feeding nature, leatherback turtles help control jellyfish populations.[3] Leatherbacks also feed on other soft-bodied organisms, such as tunicates and cephalopods.[36]
Pacific leatherbacks migrate about 6,000 miles (9,700 km) across the Pacific from their nesting sites in Indonesia to eat California jellyfish. One cause for their endangered state is plastic bags floating in the ocean. Pacific leatherback sea turtles mistake these plastic bags for jellyfish; an estimated one third of adult leatherbacks have ingested plastic.[37] Plastic enters the oceans along the west coast of urban areas, where leatherbacks forage; with Californians using upwards of 19 billion plastic bags every year.[38] Several species of sea turtles commonly ingest plastic marine debris, and even small quantities of debris can kill sea turtles by obstructing their digestive tracts.[39] Nutrient dilution, which occurs when plastics displace food in the gut, affects the nutrient gain and consequently the growth of sea turtles.[40] Ingestion of marine debris and slowed nutrient gain leads to increased time for sexual maturation that may affect future reproductive behaviors.[41] These turtles have the highest risk of encountering and ingesting plastic bags offshore of San Francisco Bay, the Columbia River mouth, and Puget Sound.
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 families Scarabaeidae, Carabidae, and Tenebrionidae 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.[42]
Like all sea turtles, leatherbacks start as hatchlings, climbing out of the sands of their nesting beaches. Birds, crustaceans, various carnivorous mammals, other reptiles, and people prey on hatchlings before they reach the water. Once in the water, several varieties of fish will also readily predate the small hatchlings. About 90% of the hatchlings die from predation. Once in the ocean, they are rarely seen before maturity. Dermochelys juveniles spend more of their time in tropical waters than do adults.[36] Once they attain maturity few predators threaten the leatherback, although tiger sharks and killer whales are potential predators.[43]
Adults are prone to long-distance migration. Migration occurs between the cold waters where 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.[30]
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.[44]
While other sea turtle species almost always return to their hatching beach, leatherbacks may choose another beach within the region. They choose beaches with soft sand because 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.[5] After laying, the female carefully back-fills the nest, disguising it from predators with a scattering of sand.[36][45]
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 hatchings. After nightfall, the hatchings dig to the surface and walk to the sea.[46][47]
Leatherback nesting seasons vary by location; it occurs from February to July in Parismina, Costa Rica.[31] Farther east in French Guiana, nesting is from March to August.[30] 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.
Dermochelys coriacea is the only species in genus Dermochelys. The genus, in turn, contains the only extant member of the family Dermochelyidae.[48]
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.[49] In 1816, French zoologist Henri Blainville coined the term Dermochelys. The leatherback was then reclassified as Dermochelys coriacea.[50] In 1843, the zoologist Leopold Fitzinger put the genus in its own family, Dermochelyidae.[51] In 1884, the American naturalist Samuel Garman described the species as Sphargis coriacea schlegelii.[52] The two were then united in D. coriacea, with each given subspecies status as D. coriacea coriacea and D. coriacea schlegelii. The subspecies were later labeled invalid synonyms of D. coriacea.[53][54]
The turtle's common name comes from the leathery texture and appearance of its carapace. Older names include "leathery turtle"[4] and "trunk turtle".[55]
Leatherback turtles have existed in some form since the first true sea turtles evolved over 110 million years ago during the Cretaceous period. The dermochelyids are close relatives of the family Cheloniidae, which contains the other six extant sea turtle species. However, their sister taxon is the extinct family Protostegidae which included other species not having a hard carapace.[56][57]
People around the world still harvest sea turtle eggs. Asian exploitation of turtle nests has been cited as the most significant factor for the species' global population decline. In Southeast Asia, egg harvesting in countries such as Thailand and Malaysia has led to a near-total collapse of local nesting populations.[58] In Malaysia, where the turtle is practically locally extinct, the eggs are considered a delicacy.[59] In the Caribbean, some cultures consider the eggs to be aphrodisiacs.[58] They are also a major jellyfish predator[60][61] which helps keep jellyfish populations in check.
Adult leatherback turtles have few natural predators once they mature; they are most vulnerable to predation in their early life stages. Birds, small mammals, and other opportunists dig up the nests of turtles and consume eggs. Shorebirds and crustaceans prey on the hatchings scrambling for the sea. Once they enter the water, they become prey to predatory fish and cephalopods. Very few survive to adulthood.
Leatherbacks have slightly fewer human-related threats than other sea turtle species. Their flesh contains too much oil and fat, reducing the demand. However, human activity still endangers leatherback turtles in direct and indirect ways. Directly, a few are caught for their meat by subsistence fisheries. Nests are raided by humans in places such as Southeast Asia.[58]
Many human activities indirectly harm Dermochelys populations. As a pelagic species, D. coriacea is occasionally caught as bycatch. As the largest living sea turtles, turtle excluder devices can be ineffective with mature adults. A reported average of 1,500 mature females were accidentally caught annually in the 1990s.[58] Pollution, both chemical and physical, can also be fatal. Many turtles die from malabsorption and intestinal blockage following the ingestion of balloons and plastic bags which resemble their jellyfish prey.[5] Chemical pollution also has an adverse effect on Dermochelys. A high level of phthalates has been measured in their eggs' yolks.[58]
D. coriacea is listed on Appendix I of CITES, which makes export/import of this species (including parts) illegal.
Conserving Pacific and Eastern Atlantic populations was included among the top ten issues in turtle conservation in the first State of the World's Sea Turtles report published in 2006. The report noted significant declines in the Mexican, Costa Rican and Malaysian populations. The eastern Atlantic nesting population was threatened by increased fishing pressures from eastern South American countries.[62]
The Leatherback Trust was founded specifically to conserve sea turtles, specifically its namesake. The foundation established a sanctuary in Costa Rica, the Parque Marino Las Baulas.[63]
The leatherback sea turtle is subject to differing conservation laws in various countries.
The United States listed it as an endangered species on 2 June 1970. The passing of the Endangered Species Act three years later ratified its status.[64] In Canada, the Species At Risk Act made it illegal to exploit the species in Canadian waters. The Committee on the Status of Endangered Wildlife in Canada classified it as endangered.[28] Ireland and Wales initiated a joint leatherback conservation effort between Swansea University and University College Cork. Funded by the European Regional Development Fund, the Irish Sea Leatherback Turtle Project focuses on research such as tagging and satellite tracking of individuals.[65]
Several Caribbean countries started conservation programs, such as The St. Kitts Sea Turtle Monitoring Network, focused on using ecotourism to highlight the leatherback's plight. On the Atlantic coast of Costa Rica, the village of Parismina has one such initiative. Parismina is an isolated sandbar where a large number of leatherbacks lay eggs, but poachers abound. Since 1998, the village has been assisting turtles with a hatchery program. The Parismina Social Club is a charitable organization backed by American tourists and expatriates, which collects donations to fund beach patrols.[66] [67] Mayumba National Park in Gabon, Central Africa was created to protect Africa's most important nesting beach. More than 30,000 turtles nest on Mayumba's beaches between September and April each year.[29]
In mid-2007, the Malaysian Fisheries Department revealed a plan to clone leatherback turtles to replenish the country's rapidly declining population. Some conservation biologists, however, are skeptical of the proposed plan because cloning has only succeeded on mammals such as dogs, sheep, cats, and cattle, and uncertainties persist about cloned animals' health and life spans.[68] Leatherbacks used to nest in the thousands on Malaysian beaches, including those at Terengganu, where more than 3,000 females nested in the late 1960s.[69] The last official count of nesting leatherback females on that beach was recorded to be a mere two females in 1993.[24]
In Brazil, reproduction of the leatherback turtle is being assisted by the IBAMA's "projeto TAMAR" (TAMAR project), which works to protect their nests and prevent accidental kills by fishing boats. The last official count of nesting leatherback females in Brazil yielded only seven females.[70] In January 2010, one female at Pontal do Paraná laid hundreds of eggs. Leatherback sea turtles had been reported to nest only at Espirito Santo's shore, but never in the state of Paraná. This unusual act brought much attention to the area, so biologists have been protecting the nests and checking their eggs' temperature, although it might be that none of the eggs are fertile.[71]
Australia's Environment Protection and Biodiversity Conservation Act 1999, lists D. coriacea as Vulnerable, while Queensland's Nature Conservation Act 1992 lists it as Endangered.
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