(Linnaeus, 1758)
Green turtle
Recognition
This medium-sized to large (to 153 cm) sea turtle has a single pair of prefrontal scales on the head and a serrated cutting edge on the lower jaw. Its broad, low, heart-shaped carapace lacks a vertebral keel and is smooth-rimed posteriorly. All vertebrals are broader than long. There are four pairs of pleurals; the 1st does not touch the cervical. Carapacial scutes are olive to brown and may contain a mottled, radiating, or wavy pattern. The bridge has four inframarginal scutes that lack pores. The formula of the immaculate white or yellow plastron is: abd >< fem > an >< gul > pect > hum > intergul. All skin is brown or, sometimes, gray to black, and many head scales may have yellow margins. The horny inner surface of the upper jaw has well-developed vertical ridges, and the cutting edge of the lower jaw is strongly serrated. Four postocular scales are present.
The chromosomes total 56: 14 metacentric and submetacentric macrochromosomes, 10 telocentric and subtelocentric macrochromosomes, and 32 microchromosomes (Bickham et al., 1980; Bachère, 1981).
The male carapace is more tapering posteriorly and the plastral hindlobe is narrower than in females. In males the tail is strongly prehensile in a vertical plane, is tipped with a heavy flattened nail, and extends far beyond the posterior carapacial margin; in females the tail barely reaches the margin. The male forelimb has a large single, curved claw.
Distribution
Chelonia mydas ranges through the Atlantic, Pacific, and Indian oceans, chiefly in the tropics. It occurs as far north in the Pacific as Alaska (Hodge, 1981) and in the Atlantic to Great Britain (Brongersma, 1972).
C. m. mydas is known to nest on the beaches of Costa Rica, French Guiana, Guyana, Surinam, Veracruz and Quintana Roo (Mexico), Ascension Island, Mona Island, Aves Island, Trinidad, the Dry Tortugas, Mujeres Island, and the Cape Verde Islands. It also may occasionally nest on the coasts of Florida, Georgia, Bermuda, Alta Vela, the Cayman Islands, Cuba, and Brazil. On the beaches of the Caribbean, the Gulf of Mexico, and the Cape Verde Islands the nesting season extends from March to October, with the greatest activity in May and June. On Ascension Island the season extends from December to July with the period from February to April most important. C. m. agassizii nests on the western coast of Africa and on the beaches of Ethiopia, Madagascar, the Seychelles Islands, Mauritius, the Maldive Islands, Yemen, Sri Lanka, Pakistan, Myanmar, Malaysia, Thailand, Indonesia, Borneo, the islands of the Strait of Malacca, Talang Islands, northern Australia, the Caroline Islands, the Marshall Islands, the Hawaiian Islands and other central Pacific islands, the Galápagos Islands, and the Pacific coasts of Central America and Mexico.
Geographic Variation
Two subspecies are recognized. Chelonia mydas mydas (Linnaeus, 1758), the Atlantic green turtle, ranges in the Atlantic Ocean from New England and the British Isles to Argentina and extreme South Africa. It is often abundant about Ascension Island, in the Cape Verde Islands and the Cayman Islands, and off Bermuda. This subspecies is predominantly brown and has an elongated, shallow carapace that is not markedly indented above the hindlimbs. It attains a greater carapace length than its Pacific counterpart. Chelonia mydas agassizii Bocourt, 1868, the Pacific green turtle, ranges from Ethiopia and Yemen around the Cape of Good Hope to western Africa, and east through the Indian and Pacific oceans to the western coast of the New World, from Alaska to Chile. This turtle is greenish or olive brown and has a broad, deep shell that is commonly markedly indented above the hindlimbs. Some individuals are melanistic, becoming slate gray to black in overall color. Caldwell (1962a) described C. m. carrinegra from the Gulf of California on the basis of this dark pigment; however, inasmuch as black turtles also occur along the coast of Central America and in the Galápagos Islands and paler individuals can be found in the Gulf of California, there seems to be no geographic distinction between the two color morphs and all eastern Pacific green turtles should be considered C. m. agassizii. Western Pacific green turtles may yet prove to be a third subspecies, C. m. japonica (Thunberg, 1787).
Hirth (1980b) suggested that C. mydas not be split into subspecies until more precise definitions and demarcation of their ranges are available. Others studying marine turtles would elevate C. m. agassizii to specific status. Atlantic and Pacific green turtles differ in size, carapace shape, and coloration, but these differences are no more striking than those between subspecies of some emydid turtles, such as Trachemys scripta and Terrapene carolina. Examination of karyotype morphology reveals little difference (Bachère, 1981), although myoglobin and protein electrophoretic studies indicate that some genetic differentiation exists between breeding aggregations of Chelonia in both the Atlantic and Pacific oceans (Williams and Brown, 1976; Bickham et al., 1980; Bowen, 1988; Meylan et al., 1990; Bowen et al., 1992). This is to be expected because the green turtle has great nest site tenacity which acts as a reproductive barrier between nesting populations.
Overall similarity of mitochondrial DNA genotypes within Atlantic nesting populations indicates either recent divergence or that gene exchange only occurs at very low levels (Bowen, 1988; Bowen et al., 1992). Hawaiian C. m. agassizii are also very similar to some Atlantic populations, but differ by having five restriction enzymes (Bowen et al., 1989; Avise et al., 1992). The Atlantic and Pacific green turtles have presumably been separated for at least three million years, over which time a nucleotide sequence divergence of only 0.2% per million years has occurred, or about 10% the conventional pace. Even at this decreased rate, many restriction site changes should have taken place if these two populations have speciated. This is supported by an electrophoretic survey of 23 gene loci in populations of C. mydas from the Atlantic, Pacific, and Indian oceans by Bonhomme et al. (1987) which revealed virtually no genetic heterozygosity or geographical differentiation.
The above studies of nesting females show only contemporary restriction of gene flow between the Atlantic and Pacific populations (such as in Atlantic and Pacific populations of Caretta and Lepidochelys; Bowen et al., 1991). In addition, study of male DNA from several breeding populations in the Atlantic, Pacific, and Indian oceans by Karl et al. (1992) revealed moderate rates of male-mediated gene flow. Positive relationships between genetic similarity and geographic proximity suggest the existence of historical connections and/or contemporary gene flow between particular rookery populations, likely via matings on overlapping feeding grounds, migration corridors, or nonnatal rookeries.
Kamezaki and Matsui (1995) took 20 cranial and four mandibular measurements on skulls of green turtles from six separate nesting populations: Comoros, Seychelles, Ogasawara (Japan), Galápagos, Tortuguero (Costa Rica), and Guyana. Turtles from Comoros, Seychelles, and Guyana had longer skulls than those from Ogasawara, Tortuguero, and Galapagos. Discriminant analyses showed that the green turtles from the Comoros and Seychelles could not be discriminated, but those from the other four populations could be completely or nearly completely separated by the skull measurements. The Galápagos C. mydas were completely separated from the other populations by a canonical discriminant analysis, but they were not differentiated from the other nesting populations by any character relative to skull length. Kamezaki and Matsui (1995) thought these results supported recognition of the eastern Pacific population as a distinct subspecies, but not as a separate species.
Although these studies support continuing to regard the Atlantic and Pacific populations of green turtles as subspecies of one species rather than separate species, there is still disagreement. Parham and Zug (1996) believed it inappropriate to use the name agassizii for a single geographic morphotype and that subspecific recognition of the various populations is unjustified.
Habitat
The green turtle migrates across the open seas but feeds in shallow water supporting an abundance of submerged vegetation. It shares its nesting beaches with all species of sea turtles (including the Australian flatback, N. depressus).
Natural History
Sexual maturity in both sexes is reached sometimes between 6-13 years in captivity (Wood and Wood, 1980), but most probably mature at an older age in the wild. Ehrhardt and Witham (1992) estimated the age at maturity of free living Atlantic green turtles to be 19-24 years. Males begin to show the lengthened mature tail at 64-65 cm and those 75 cm long have fully developed tails (Caldwell, 1962; Balazs, 1980). Female carapace length at maturity varies between populations, 65-83 cm (Carr and Ogren, 1960; Hirth, 1980c). Nesting females average 99.1 cm in carapace length (Van Buskirk and Crowder, 1994). The annual physical changes in the male testes have not been adequately documented, although mature sperm is present at the time of spring mating and testis mass and spermatogenic activity are greater in January than in September (Licht et al., 1985b). Two mature males examined by Owens (1980) 3-4 months before the nesting season had loose packed sperm in the epididymides and testicular spermiogenesis. During a non-reproductive year adult female ovaries weigh 43-100 g and follicles of several size classes are present (Owens, 1980). Ovarian weight in a reproductive year is high prior to the first mating, rises until ovulation of the first clutch, drops below the premating weight, and then rises again until the first clutch is laid and the second clutch is ovulated. This cycle is repeated after each successive oviposition with ovarian weight decreasing after each ovulation until all mature eggs have been deposited and ovarian weight reaches its lowest level for the cycle (Owens and Morris, 1985).
Mating occurs in the water off the nesting beaches during the laying season. Although mated pairs have been seen more than 1 km from shore, the greatest concentration is close to shore. Copulation occurs at any hour before or after the first nesting emergence and may last several hours. Several males (1-5) may simultaneously court and attempt to mate with a single female. Copulating pairs float at the surface, with the male atop the female. The enlarged claws on the front flippers and the strong nail at the tip of the prehensile tail provide a firm three-point attachment; moreover, the male's claws cut into the female's carapace at the third intermarginal seam and leave deep, bleeding wounds.
Carr (1952) reported that the nesting season in Australia extends from late October to mid-February; in the Gulf of Siam the year around, with a July-to-November peak; on Sri Lanka from July to November; in the Malacca Straits from December through January; in the Seychelles Islands the year around with a March-to-May peak; and in Borneo from May to September. Loveridge and Williams (1957) reported that on the western coast of Africa the laying season is from September to January. Hendrickson (1958) found that the nesting beaches on islands usually were on the leeward side, and in Sarawak the principal ones had fringing coral reefs below mean low tide. Hirth and Carr (1970) compared the characteristics of the nesting beaches at South Yemen, Aldabra Island, Ascension Island, Aves Island, and Tortuguero, Costa Rica. They found that the sand varied from fine to coarse in texture and from olive gray to white in color. The pH ranged from 6.9 to 8.0 and the carbonate content was high in all beaches except at Tortuguero. Organic content was 0.30-1.18%.
Nesting is nocturnal; Carr and Giovannoli (1957) described the nesting process of a female found at Tortuguero, Costa Rica, and we refer the reader to that paper for details. Females nest in 2- or 3-year cycles, with the latter more frequent. Female C. mydas nest several times each season, with an average of 2.93 clutches per year (Van Buskirk and Crowder, 1994).
The average interval between oviposition of successive clutches is 3.16 days (Johnson and Ehrhardt, 1996), and 12-238 (X = 112.8; Van Buskirk and Crowder, 1994) eggs are deposited in each nest depending on the size of the female. The nearly spherical eggs are 35-58 mm in diameter. Incubation usually takes 50-55 days.
Hatchlings emerge after dark. Their carapace (35-59 mm, average 50.1 mm; Van Buskirk and Crowder, 1994) is keeled and dark green to brown; it may have a mottled pattern and light border. The plastron is white or yellow with two longitudinal ridges. Skin is blackish, flippers have white borders, and the upper beak is light. Eggs incubated in warm nests produce a preponderance of females; clutches from cool nests hatch out mostly males (Morreale et al., 1982; Mrosovsky, 1982).
Chelonia mydas is omnivorous, but the juvenile is more carnivorous than the adult. Plants consumed include green, brown, and red algae, mangrove (roots and leaves), and Zostera, Cymodocea, Thallasia, Halophila, Posidonia, Halodule, and Portulaca. Roots seem to be preferred. Animal food includes small mollusks, crustaceans, sponges, and jellyfish. Deraniyagala (1939) reported that green turtles scavenged near a house, feeding on the kitchen refuse thrown into the water. Captives feed readily on liver, kidney, beef, and fish. The serrated lower jaw probably is an adaptation to grazing.
Green turtles are the only marine turtles subsisting mainly on plants—a diet poor in vitamin D. They are also the only marine turtles that come to the shore to bask—perhaps a means of producing the needed vitamin D through the action of the sunlight on skin sterols. Green turtles overwinter on the sea bottom of the Infiernillo Channel in the Gulf of California (Felger et al., 1976), and perhaps this behavior also occurs elsewhere.
Remark
The common name of this turtle is derived from the greenish color of its body fat.
IUCN Red List Status (1996)
Mediterranean and Black Sea, Cyprus, Turkey: Critically endangered (A1a, B1+2ce, E); elsewhere: Endangered (A1abd). Exploitation of the green turtle on both the nesting beaches and the feeding grounds makes its future bleak. It has already disappeared from many beaches where it was once numerous. Collection of eggs on the nesting beaches is particularly injurious. If Chelonia mydas is to survive, conservation methods must be applied wherever the turtles breed or feed. In particular, the nesting beaches must be rigorously protected by governmental regulation and must be adequately patrolled during the breeding season.