| Green sea turtle | |
|---|---|
| Chelonia mydas swimming above Hawaiian coral reef. | |
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Testudines |
| Family: | Cheloniidae |
| Genus: | Chelonia |
| Species: | C. mydas |
| Binomial name | |
| Chelonia mydas (Linnaeus, 1758) |
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| Subspecies | |
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| Synonyms | |
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The Green sea turtle (Chelonia mydas), also known as green turtle, black (sea) turtle, or Pacific green turtle,[3] is a large sea turtle of the family Cheloniidae. It is the only species in the genus Chelonia.[4] Its range extends throughout tropical and subtropical seas around the world, with two distinct populations in the Atlantic and Pacific Oceans.[5] Their common name derives from the usually green fat found beneath their carapace (upper shell).
The green sea turtle is a sea turtle, possessing a dorsoventrally flattened body covered by a large, teardrop-shaped carapace and a pair of large, paddle-like flippers. It is usually lightly colored, although parts of the carapace can be almost black in the eastern Pacific. Unlike other members of its family, such as the hawksbill sea turtle and loggerhead sea turtle, C. mydas is mostly herbivorous. The adults commonly inhabit shallow lagoons, feeding mostly on various species of seagrasses.[6]
Like other sea turtles, they migrate long distances between feeding grounds and hatching beaches. Many islands worldwide are known as Turtle Island due to green sea turtles nesting on their beaches. Females crawl out on beaches, dig nests and lay eggs during the night. Later, hatchlings emerge and walk into the water. Those that reach maturity may live to age 80 in the wild.[5]
C. mydas is listed as endangered by the IUCN and CITES and is protected from exploitation in most countries. It is illegal to collect, harm or kill them. In addition, many countries have laws and ordinances to protect nesting areas. However, turtles are still in danger because of several human practices. In some countries, turtles and their eggs are hunted for food. Pollution indirectly harms turtles at both population and individual scales. Many turtles die caught in fishing nets. Also, real estate development often causes habitat loss by eliminating nesting beaches.
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The green sea turtle is a member of the tribe Chelonini. A 1993 study clarified the status of genus Chelonia with respect to the other marine turtles. The carnivorous Eretmochelys (hawksbill), Caretta (loggerhead) and Lepidochelys (Ridley) were assigned to the tribe Carettini. Herbivorous Chelonia warranted their status, while Natator (flatback) was further removed than previously believed.[7]
The species was originally described by Linnaeus in 1758 as Testudo mydas.[8] In 1868, Bocourt named a particular species of sea turtle as Chelonia agassizii (Chelonia agassizi is a commonly cited misspelling of this taxon).[9] This "species" was referred to as the "black sea turtle".[10] Later research determined the "black sea turtle" was not genetically distinct from C. mydas, and thus taxonomically not a separate species.[11] These two "species" were then united as Chelonia mydas and were given subspecies status. C. mydas mydas referred to the originally described population, while C. mydas agassizi referred only to the Pacific population.[12][13] This subdivision was later determined to be invalid and all species members were then designated Chelonia mydas.[4] The oft-mentioned name C. agassizi remains an invalid junior synonym of C. mydas.
The species' common name does not derive from any particular green external coloration of the turtle. Its name comes from the greenish color of the turtles' fat, which is only found in a layer between their inner organs and their shell.[14] As a species found worldwide, the green turtle is called differently in some languages and dialects. In the Hawaiian language, honu is used to refer to this species.[15]
Its appearance is that of a typical sea turtle. C. mydas has a dorsoventrally flattened body, a beaked head at the end of a short neck, and paddle-like arms well-adapted for swimming.[16] Adult green turtles grow to 1.5 metres (5 ft) long.[17] While individuals have been caught that reached weights of up to 315 kilograms (690 lb), the average weight of mature individuals is 110–190 kilograms (240–420 lb). The largest known C. mydas weighed 395 kilograms (870 lb).[18]
Anatomically, a few characteristics distinguish the green turtle from the other members of its family. Unlike the closely related hawksbill turtle, the green turtle's snout is very short and its beak is unhooked. The sheath of the turtle's upper jaw possesses a denticulated edge, while its lower jaw has stronger, serrated, more defined denticulation. The dorsal surface of the turtle's head has a single pair of prefrontal scales. Its carapace is composed of five central scutes flanked by four pairs of lateral scutes. Underneath, the green turtle has four pairs of inframarginal scutes covering the area between the turtle's plastron and its shell. Mature C. mydas front appendages have only a single claw (as opposed to the hawksbill's two), although a second claw is sometimes prominent in young specimens.[19]
The carapace of the turtle has various color patterns that change over time. Hatchlings of C. mydas, like those of other marine turtles, have mostly black carapaces and light-colored plastrons. Carapaces of juveniles turn dark brown to olive, while those of mature adults are either entirely brown, spotted or marbled with variegated rays. Underneath, the turtle's plastron is hued yellow. C. mydas limbs are dark-colored and lined with yellow, and are usually marked with a large dark brown spot in the center of each appendage.[5][20]
The range of C. mydas extends throughout tropical and subtropical oceans worldwide. There are two major subpopulations, the Atlantic and the eastern Pacific subpopulations. Each population is genetically distinct, with their own set of nesting and feeding grounds within the population's known range.[5]
Chelonia mydas can generally be found throughout the entire Atlantic Ocean. Individuals have been spotted as far north as Canada in the western Atlantic, and the British Isles in the east. The subpopulation's southern range is known until past the southern tip of Africa in the east and Argentina in the western Atlantic. The major nesting sites can be found on various islands in the Caribbean, along the eastern shores of the continental United States, the eastern coast of the South American continent and most notably, on isolated North Atlantic islands.
In the Caribbean, major nesting sites have been identified on Aves Island, the U.S. Virgin Islands, Puerto Rico and Costa Rica. One of the region's most important nesting grounds is in Tortuguero in Costa Rica.[21] In fact, the majority of the Caribbean region's C. mydas population hails from a few beaches in Tortuguero.[22] Within United States waters, minor nesting sites have been noted in the states of Georgia, North Carolina, South Carolina and all along the east coast of Florida. Hutchinson Island in particular is a major nesting area in Florida waters. Notable locations in South America include secluded beaches in Suriname and French Guiana.[23] In the Southern Atlantic Ocean, the most notable nesting grounds for Chelonia mydas are found on the island of Ascension,[16] hosts 6-13,000 turtle nests.[24][25][26]
In contrast with the sporadic distribution of nesting sites, feeding grounds are much more widely distributed throughout the region. Important feeding grounds in Florida include Indian River Lagoon, the Florida Keys, Florida Bay, Homosassa, Crystal River and Cedar Key.[16][27]
In the Pacific, its range reaches as far north as the southern coast of Alaska and as far south as Chile in the east. The turtle's distribution in the western Pacific reaches north to Japan and southern parts of Russia's Pacific coast, and as far south as the northern tip of New Zealand and a few islands south of Tasmania. The turtles can be found throughout the Indian Ocean.[28]
Significant nesting grounds are scattered throughout the entire Pacific region, including Mexico, the Hawaiian Islands including O'ahu's Turtle Bay,[29] the South Pacific, the northern coast of Australia and Southeast Asia. Major Indian Ocean nesting colonies include India, Pakistan and other coastal countries. The east coast of the African continent hosts a few nesting grounds, including islands in the waters around Madagascar.[28]
Nesting grounds are found all along the Mexican coast. These turtles feed in seagrass pastures in the Gulf of California.[30] Green turtles belonging to the distinct Hawaiian subpopulation nest at the protected French Frigate Shoals some 800 kilometers (500 mi) west of the Hawaiian Islands.[15] In the Philippines, green turtles nest in the Turtle Islands along with closely related hawksbill turtles.[31] Indonesia has a few nesting beaches, one in the Meru Betiri National Reserve in East Java.[32] The Coral Sea has nesting areas of world significance.[33] The Great Barrier Reef has two genetically distinct populations; one north and one south. Within the reef, twenty separate locations consisting of small islands and cays were identified as nesting sites for either population of C. mydas. Of these, the most important is on Raine Island.[34][35] Major nesting sites are common on either side of the Arabian Sea, both in Ash Sharqiyah, Oman, and along the coast of Karachi, Pakistan. Some specific beaches there, such as Hawke's Bay and Sandspit, are common to both C. mydas and L. olivacea subpopulation. Sandy beaches along Sindh and Balochistan are nesting sites. Some 25 kilometers (16 mi) off the Pakistani coast, Astola island is another nesting beach.[6][36][37]
On 30 December 2007, fishermen, using a hulbot-hulbot (a type of fishnet) accidentally caught an 80 kg (180 lb), 93 cm (37 in) and 82 cm (32 in) wide, turtle off Barangay Bolong, Zamboanga City, Philippines. December is breeding season near the Bolong beach.[38]
As one of the first sea turtle species studied, much of what is known of sea turtle ecology comes from studies of green turtles. The ecology of C. mydas changes drastically with each stage of its life history. Newly emerged hatchlings are carnivorous, pelagic organisms, part of the open ocean mininekton. In contrast, immature juveniles and adults are commonly found in seagrass meadows closer inshore as herbivorous grazers.
Green sea turtles move across three habitat types, depending on their life stage. They lay eggs on beaches. Mature turtles spend most of their time in shallow, coastal waters with lush seagrass beds. Adults frequent inshore bays, lagoons and shoals with lush seagrass meadows. Entire generations often migrate between one pair of feeding and nesting areas.[16]
Turtles spend most of their first five years in convergence zones within the open ocean.[6][39] These young turtles are rarely seen as they swim in deep, pelagic waters.[40][41]
Green sea turtles typically swim at 2.5-3kph (0.7-0.8 m/s).[42]
Only human beings and the larger sharks feed on C. mydas adults. Specifically, tiger sharks (Galeocerdo cuvier) hunt adults in Hawaiian waters.[43] Juveniles and new hatchlings have significantly more predators, including crabs, small marine mammals and shorebirds.[5] In Turkey, their eggs are vulnerable to predation by red foxes and golden jackals.[44]
Green sea turtles migrate long distances between feeding sites and nesting sites; some swim more than 2,600 kilometres (1,600 mi) to reach their spawning grounds. Mature turtles often return to the exact beach from which they hatched. Females usually mate every two to four years. Males, on the other hand, visit the breeding areas every year, attempting to mate.[45] Mating seasons vary between populations. For most C. mydas in the Caribbean, mating season is from June to September.[16] The French Guiana nesting subpopulation nests from March to June.[23] In the tropics, green turtles nest throughout the year, although some subpopulations prefer particular times of the year. In Pakistan, Indian Ocean turtles nest year-round, but prefer the months of July to December.[36]
Green sea turtles mating is similar to other marine turtles. Female turtles control the process. A few populations practice polyandry, although this does not seem to benefit hatchlings.[46] After mating in the water, the female moves above the beach's high tide line, where she digs a hole with her hind flippers and deposits her eggs. Clutch size depends on the age of the female and species, but can range between 100 and 200. She then covers the nest with sand and returns to the sea.[5]
At around 45 to 75 days, the eggs hatch during the night, and the hatchlings instinctively head directly into the water. This is the most dangerous time in a turtle's life. As they walk, predators, such as gulls and crabs, feed on them. A significant percentage never make it to the ocean. Little is known of the initial life history of newly hatched sea turtles.[16] Juveniles spend three to five years in the open ocean before they settle as still-immature juveniles into their permanent shallow-water lifestyle.[40][41] It is speculated that they take twenty to fifty years to reach sexual maturity. Individuals live up to eighty years in the wild.[5]
Each year on Ascension Island in the South Atlantic, C. mydas females create 6,000 to 15,000 nests. They are among the largest green turtles in the world; many are more than a metre in length and weigh up to 300 kilograms (660 lb).[47]
Sea turtles spend almost all their lives submerged, but must breathe air for the oxygen needed to meet the demands of vigorous activity. With a single explosive exhalation and rapid inhalation, sea turtles can quickly replace the air in their lungs. The lungs permit a rapid exchange of oxygen and prevent gases from being trapped during deep dives. Sea turtle blood can deliver oxygen efficiently to body tissues even at the pressures encountered during diving. During routine activity, green and loggerhead turtles dive for about four to five minutes, and surface to breathe for one to three seconds.
Turtles can rest or sleep underwater for several hours at a time, but submergence time is much shorter while diving for food or to escape predators. Breath-holding ability is affected by activity and stress, which is why turtles quickly drown in shrimp trawlers and other fishing gear.[19]
Historically, the turtles' skin was tanned and used to make handbags, especially in Hawaii.[15] Ancient Chinese considered the flesh of sea turtles a culinary delicacy, including and especially C. mydas.[48] Particularly for this species, the turtle's calipee, fat and cartilage are sought as ingredients for making turtle soup.[14]
In Java, Indonesia, sea turtle eggs were a popular delicacy. However, the turtle's flesh is regarded as ḥarām or "unclean" under Islamic law (Islam is Java's primary religion). In Bali, turtle meat was a prominent feature at ceremonial and religious feasts. Turtles were harvested in the remotest parts of the Indonesian archipelago.[49] Bali has been importing sea turtles since the 1950s, as its own turtle supplies became depleted.[50] The mostly Hindu Balinese do not eat the eggs, but sell them instead to local Muslims.
Commercial farms, such as the Cayman Turtle Farm in the West Indies, once bred them for commercial sale from eggs removed from nests below the waterline. The farms held as many as 100,000 turtles at any one time. When the markets were closed by regulations that did not allow even farm-bred turtles to be sold, the surviving farms became tourist attractions, supporting 11,000 turtles.[51]
In recent decades, sea turtles have moved from unrestricted exploitation to global protection, with individual countries providing additional protection, although serious threats remain unabated.
Human action presents both intentional and unintentional threats to the species' survival. Intentional threats include continued hunting and egg harvesting. More dangerous are unintentional threats, including boat strikes, fishermen's nets that lack turtle excluder devices, pollution and habitat destruction. Chemical pollution may create tumors;[52] effluent from harbors near nesting sites may create disturbances; and light pollution may disorient hatchlings. Habitat loss usually occurs due to human development of nesting areas. Beach-front construction, land "reclamation" and increased tourism are examples of such development.[5][6] An infectious tumor-causing disease, fibropapillomatosis, is also a problem in some populations. The disease kills a sizeable fraction of those it infects, though some individuals seem to resist the disease.[15][53][54] Because of these threats, many populations are in a vulnerable state.
Pacific green turtle’s foraging habitats are poorly understood and mostly unknown.[55] These foraging grounds are most likely along the coast of Baja California, Mexico and southern California,[56] in which these turtle’s have a high risk of incidental capture by coastal fisheries. The main mortality factor for these turtles is the shrimp trawlers in Mexico, in which many of these turtles go undocumented.[56] The only foraging area that has been identified is the San Diego Bay, but it is heavily polluted with metals and PCBs.[56] These contaminants have a negative effect on the ocean environment, and have been shown to cause lesions and sometimes mortality.[56] Green turtles also are threatened by entanglement and ingestion of plastic.[56] In San Diego Bay, an adult green turtle was found dead with monofilament netting tightly packed in its esophagus.[56]
The International Union for the Conservation of Nature (IUCN) has repeatedly listed green sea turtles in its Red List under differing criteria. In 1982, they officially classified it as an endangered species.[57] The 1986,[58] 1988,[59] 1990,[60] 1994,[61] and the landmark 1996 edition of the IUCN Red List, retained the listing.[62]
In 2001, Nicholas Mrosovsky filed a delisting petition, claiming some green turtle populations were large, stable and in some cases, increasing. At the time, the species was listed under the strict EN A1abd criteria. The IUCN Standards and Petitions Subcommittee ruled that visual counts of nesting females could not be considered "direct observation" and thus downgraded the species' status to EN A1bd—retaining the turtle's endangered status.[63]
In 2004, the IUCN reclassified C. mydas as endangered under the EN A2bd criteria, which essentially states the wild populations face a high risk of extinction because of several factors. These factors include a probable population reduction of more than 50% over the past decade as estimated from abundance indices and by projecting exploitation levels.[64]
On 3 May 2007, C. mydas was listed on Appendix I of the Convention on International Trade in Endangered Species (CITES) as a member of the family Cheloniidae.[65] The species was originally listed on Appendix II in 1975. The entire family was moved to Appendix I in 1977, with the exception of the Australian population of C. mydas. In 1981, the Australian population joined the rest. It is therefore illegal to import, export, kill, capture or harass green turtles.[66]
The Mediterranean population is listed as critically endangered.[5][14] The eastern Pacific, Hawaiian and Southern California subpopulations are designated threatened. Specific Mexican subpopulations are listed as endangered. The Florida population is listed as endangered. The World Wide Fund for Nature has labeled populations in Pakistan as "rare and declining".[37]
In addition to management by global entities such as the IUCN and CITES, specific countries around the world have undertaken conservation efforts.
The traditional uses of turtle on Bali were once deemed sustainable, but have been questioned considering greater demand from the larger and wealthier human population. The harvest was the most intensive in the world.[49] In 1999, Indonesia restricted turtle trade and consumption because of the decreasing population and threat of a tourist boycott. It rejected a request made by Bali Governor I Made Mangku Pastika in November 2009 to set a quota of 1,000 turtles to be killed in Hindu religious ceremonies. While conservationists respect the need for turtles in rituals, they wanted a smaller quota.[67]
Ecotourism is one initiative in Sabah, Malaysia. The island of Pulau Selingan is home to a turtle hatchery. Staff people place some of the eggs laid each night in a hatchery to protect them from predators. Incubation takes around sixty days. When the eggs hatch, tourists assist in the release of the baby turtles into the sea.[68] In the United States, the U.S. Fish and Wildlife Services and National Marine Fisheries Service classify C. mydas as a threatened species under the Endangered Species Act,[69] rendering it a federal offense to capture or kill an individual turtle. The Hawaiian subpopulation has made a remarkable comeback and is now one focus of ecotourism and has become something of a state mascot. Students of Hawaii Preparatory Academy on the Big Island have tagged thousands of specimens since the early 1990s.[15] In the United Kingdom the species is protected by a Biodiversity Action Plan, due to excess harvesting and marine pollution.[70] The Pakistani-branch of the World Wide Fund for Nature has been initiating projects for secure turtle hatching since the 1980s. However, the population has continued to decline.[6]
In the Atlantic, conservation initiatives have centered around Caribbean nesting sites. The Tortuguero nesting beaches in Costa Rica have been the subject of egg-collection limits since the 1950s. The Tortuguero National Park was formally established in 1976, in part, to protect that region's nesting grounds.[21] On Ascension Island, which contains some of the most important nesting beaches, an active conservation program has been implemented.,[71] Karumbé has been monitoring foraging and developmental areas of juvenile green turtles in Uruguay since 1999.[72]
In the Pacific, green sea turtles nest on the motu (islets) in the Funafuti Conservation Area, a marine conservation area covering 33 square kilometers (12.74 square miles) of reef, lagoon and motu on the western side of Funafuti atoll in Tuvalu.[73]
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