The Atlantic Green Turtle (Chelonia mydas)
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Captive Atlantic green turtles in a tank at the Cayman Island Turtle Farm. 91K JPEG |
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Captive Atlantic green turtles in a tank at the Cayman Island Turtle Farm. 91K JPEG |
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The following is based on information from the Recovery Plan for U.S. Population of Atlantic Green Turtles, U.S Fish and Wildlife Service and National Marine Fisheries Service, 1991. Obtained from the U.S. National Marine Fisheries Service, and used with their kind permission.
The green turtle is listed as endangered by the International Union for the Conservation of Nature. Under the U.S. Endangered Species Act of 1973, the green turtle was listed as threatened except for the breeding populations in Florida and on the Pacific coast of Mexico, where it is listed as Endangered.
Green turtles continue to be heavily exploited by humans, and the destruction and loss of nesting and foraging sites is a serious problem. Humans have already caused the extinction of large green turtle populations, including those that once nested in Bermuda and Cayman Islands. The status of green turtle populations is difficult to determine because of our lack of knowledge about their life cycles. The number of nests deposited in Florida appears to be increasing, but we don't know whether this is due to an increase in the number of nests or because we have started to monitor nesting beaches more closely.
The green sea turtle is the largest hard-shelled sea turtle. Adults of this species commonly reach 100 cm in carapace length and 150 kg in mass. The average size of a female nesting in Florida is 101.5 cm straight carapace length, with an average body mass of 136 kg.
Hatchling green turtles weigh about 25 g (about a handful of Smarties) and have a carapace about 50 mm long. Hatchlings are black on top and white underneath. The plastron of Atlantic green turtles remains a yellowish white throughout life, but the carapace changes colour from black to various shades of gray, green, brown and black, forming swirls and irregular patterns on their shells.
Growth rates of pelagic-stage green turtles have not been measured under natural conditions; however, growth rates of green turtles have been measured at their feeding grounds. Green turtles grow slowly.
In the southern Bahamas, green turtles grew from 30 to 75 cm in 17 years, and according to Bjorndal and Bolten, growth rate decreased with increasing carapace length. Growth rates measured in green turtles from Florida and Puerto Rico fall within the range of growth rates measured in the southern Bahamas. Based on growth rate studies of wild green turtles, the researchers Balazs, Frazer and Ehrhart estimate the age at sexual maturity range anywhere from 20 to 50 years.
Green turtles occupy three habitat types:
Females deposit egg clutches on high energy beaches, usually on islands, where a deep nest cavity can be dug above the high water line. Hatchlings leave the beach and apparently move into convergence zones in the open ocean where they spend an undetermined length of time (Carr, 1986). When turtles reach a carapace length of approximately 20 to 25 cm, they leave the pelagic habitat and enter benthic feeding grounds. Most commonly these foraging habitats are pastures of seagrasses and/or algae, but small green turtles can also be found over coral reefs, worm reefs and rocky bottoms. Some feeding areas, such as Miskito Cays, Nicaragua, support a complete size range of green turtles from 20 cm to breeding adults. Coral reefs or rocky outcrops near feeding pastures are often used as resting areas, both at night and during the day.
Scientists assume that post-hatchling, pelagic-stage green turtles are omnivorous, but there are no data on diet from this age class. Our personal experience with a juvenile Hawaiian green confirms this theory. Our first turtle encounter resulted when a juvenile swam up to us and attempted to take some of the cuttlefish we were using to attract eels for photographic purposes.
Scientists do know that once green turtles shift to benthic feeding grounds, they are herbivores. They feed on both seagrasses and algae.
The green turtle can be found throughout the world in all tropical and sub-tropical oceans. In the U.S., Atlantic green turtles can be found around the U.S. Virgin Islands, Puerto Rico, and continental U.S. from Texas to Massachusetts. Important feeding areas for green turtles in Florida include Indian River Lagoon, Florida Keys, Florida Bay, Homosassa, Crystal River and Cedar Key.
Major green nesting colonies in the Atlantic are located on Ascension Island, Aves Island, Costa Rica and Suriname. In the U.S., green turtles nest in small numbers in the United States Virgin Islands, and in Puerto Rico. Greens nest in larger numbers in Florida but actual data is presently not helpful in assessing trends in nesting.
Female green turtle emerge at night to deposit eggs, the process taking an average of two hours. Up to seven clutches are deposited at 12 to 14 day intervals, but the average is probably two or three clutches. Accurate counts of the number of clutches per season are difficult to get. The average clutch size is usually 110-115 eggs, but this varies among populations.
It is uncommon for females to produce clutches in successive years. Usually 2, 3, 4 or more years intervene between breeding seasons. Mating occurs in the water off the nesting beaches. Little is known about the reproductive biology of males, but evidence is accumulating that males migrate to the nesting beach every year.
The hatching success of undisturbed nests is usually high, but on some beaches, predators destroy a high percentage of nests. Large numbers of nests are also destroyed by inundation and erosion.
One interesting discovery in recent years is that incubation temperatures determine the sex of hatchling turtles. In 1985, Standora and Spotila reported this effect on green turtles. Eggs incubated below a pivotal temperature--which might vary among populations--produce primarily males, and eggs incubated above this temperature produce primarily females.
The navigation feats of the green turtle are well known, but poorly understood. We know that hatchlings and adult females on the nesting beach orient toward the ocean using light cues. For a long time, no one knew what cues were employed in pelagic movements, in movements among foraging grounds, or in migrations between foraging grounds and nesting beaches. Recently published work, however, has suggested that the earth's magnetic field plays a role in these feats.
Because green turtles feed in marine pastures in quiet, low-energy areas, but nest on high energy beaches, their feeding and nesting habitats are, of necessity, located some distance apart. Green turtles that nest on Ascension Island forage along the coast of Brazil, some 1,000 km away! The location of the foraging grounds of green turtles that nest in Florida is still unknown.
It has been generally accepted, but not proven, that green turtles return to nest on the beach where they were born. Green turtles do exhibit strong site-fidelity in successive nesting seasons. Our personal experience with Hawaiian green turtles is that they also exhibit strong site fidelity for their foraging grounds.
Green sea turtles are listed as threatened or endangered throughout their habitat.
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