Chelonia mydas
TAXONOMY
Testudo mydas Linnaeus, 1758, Insulas Pelagi: Insulam Adscensionis ("Oceanic Islands: Ascension Island"). No subspecies are currently recognized. Nevertheless, some authors recognize the Pacific green turtle (also called the black turtle) as a distinct species; others consider the Pacific green turtle a subspecies of Chelonia mydas. Results of DNA studies do not support recognition of the Pacific turtle as distinct from other green seaturtles.
OTHER COMMON NAMES
English: Green turtle; French: Tortue verte; German: Suppenschildkröte; Spanish: Tortuga verde, Tortuga blanca.
PHYSICAL CHARACTERISTICS
Large, reaching 60 in (152 cm) carapace length and 750 lb (340 kg) body mass. The head is small and rounded anteriorly, and only one pair of elongated prefrontal scales is present on the top of the head forward of the eyes. The heart-shaped carapace is only weakly serrate posteriorly and has four pairs of pleural scutes, the first pair of which does not contact the nuchal scute. Twelve marginal scutes are typically present along each side of the shell. Four inframarginal scutes (all lacking pores) are present on each bridge between the marginal and the plastral scutes. The greenish color of the fat of this turtle is the source of its common name.
DISTRIBUTION
All tropical and temperate seas.
HABITAT
Although green seaturtles venture into temperate seas, adults are found primarily in the tropics. These turtles can be found in the open sea, but they are most commonly seen in areas of shallow water with an abundance of submerged vegetation, especially sea grass flats. Hatchlings are more pelagic and often are found in mats of sargassum.
BEHAVIOR
Green seaturtles nest primarily on tropical beaches and may migrate more than 1,900 mi (3,000 km) between feeding and nesting areas. These turtles are known to thermoregulate by basking at the water's surface, but they are the only marine turtle known to leave the water to bask on land. One population in the Gulf of California is known to hibernate under water by partially burying itself in the substrate.
FEEDING ECOLOGY AND DIET
Although it is assumed that hatchlings and juveniles are primarily carnivorous, few data are available. Adults are well known to be almost completely herbivorous, feeding primarily on several genera of sea grasses as well as on algae. Animal matter, such as sponges, jellyfish, mollusks, crustaceans, sea urchins, and sea squirts, is occasionally ingested, some perhaps secondarily while the turtle feeds on sea grasses. Feeding generally occurs during the day.
REPRODUCTIVE BIOLOGY
The age at maturity of green seaturtles is not definitively known but is speculated to be between 20 and 30 years. Courtship and mating take place off the nesting beaches, and females may mate with several males. Courtship involves chasing, nuzzling, rubbing, sniffing, and biting the female. If the female is receptive, the male mounts her shell from behind and swings his much larger tail under hers for intromission. Copulation may last several hours; one report describes a 52-hour mating. Mating may occur at or below the water's surface. Females can store sperm, perhaps for several years, so individual eggs in the same clutch may have different fathers.
There is considerable variation in the timing of the green seaturtle nesting season, both locally and globally. For example, in the western Atlantic, nesting is typically from March to October, with a peak from May to September. In the eastern Pacific, nesting may occur any time between February and January. Females exhibit considerable fidelity to their nesting beaches, and this trait accounts for slight genetic differences between separate nesting colonies. Females nest anywhere from the open sand above the high tide line to fully shaded areas just inland from the beach. Most nesting occurs at night. Nest construction is similar to that of the loggerhead seaturtle, except that the green seaturtle digs the deepest body pit of any cheloniid sea turtle (up to 20 in [50 cm]). Females lay as many as seven clutches in a season, usually at 12–14-day intervals, but laying two to five clutches is most common. Clutch size is generally positively related to the female's size. Clutches range from three to 238 eggs, although clutches of 100–120 eggs are typical. The leathery eggs are nearly spherical and 1–2 in (2.5–5.8 cm), usually 1.4–1.6 in (3.5–4 cm) in diameter and weigh 1–2 oz (28–65 g, usually 35–50 g). Most females nest only every two years, but cycles of one to four years are known. Incubation typically requires 50–70 days but may take between 30 and 90 days, depending on nest temperature. Hatchlings emerge from their nests at night and move immediately to the sea. Green seaturtles exhibit temperature-dependent sex determination. High temperatures produce mostly females, and low temperatures produce mostly males.
CONSERVATION STATUS
Green seaturtles are classified as Endangered on the IUCN Red List. The Mediterranean subpopulation is listed as Critically Endangered.
SIGNIFICANCE TO HUMANS
Despite international protection, green seaturtles and their eggs are still consumed by local peoples in many parts of the world. Many other human activities also increase mortality. Turtles are killed for sport, drown in shrimp or fish nets, and are wounded by boat propellers. Other turtles are killed by predators whose numbers have increased because of human activities (e.g., raccoons, pigs, and dogs). Nesting grounds are destroyed by hotel and housing developments.
