Green Sea Turtle

Chelonia mydas

Description

The green turtle is the largest hard-shelled sea turtle; adults commonly reach 3.3 ft (1 m) in carapace length and 330 lb (150 kg) in mass—and can weigh up to 450 lb (205 kg). The mean size of female green turtles nesting in Florida is 3.3 ft (1 m) standard straight carapace length and 300 lb (136.1 kg) body mass. Hatchling green turtles weigh approximately 0.9 oz (25 g), and the carapace is about 2 in (5 cm) long. The dorsal surface is black, and the ventral surface is white. The plastron of Atlantic green turtles remains a yellowish-white throughout life, but the carapace changes in color from solid black to a variety of shades of gray, green, brown, and black in starburst or irregular patterns.

Characteristics that distinguish the green turtle from other sea turtle species are a smooth carapace with four pairs of lateral scutes and a single pair of elongated prefrontal scales between the eyes. The nuchal scute does not touch the first costal scute and the inframarginal scutes do not have pores. Each flipper has a single claw and the carapace is ovalshaped and depressed. The crawls of nesting green turtles are deeply cut, with symmetrical diagonal marks made by the front flippers.

Behavior

Female green turtles emerge on nesting beaches at night to deposit eggs; the process takes an average of two hours. From one to seven clutches are deposited within a breeding season at 12-14 day intervals. The average number is usually given as two to three clutches, but accurate data on the number of clutches deposited per season are difficult to obtain. Mean clutch size is usually 110-115 eggs, but this average varies among populations. Average clutch size reported for Florida was 136 eggs in 130 clutches. Only occasionally do females produce clutches in successive years. Usually two, three, four, or more years intervene between breeding seasons. Mating occurs in the water off the nesting beaches. Very little is known about the reproductive biology of males, but it is thought that males may migrate to the nesting beach every year. 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. Temperature-dependent sex determination has been demonstrated for green turtles. Eggs incubated below a pivotal temperature, which may vary among populations, produce primarily males, and eggs incubated above the pivotal temperature produce primarily females. Green turtles grow slowly. In the southern Bahamas, green turtles grew 12-29.5 in (30-75 cm) in 17 years, and growth rate decreased with increasing carapace length.

Growth rates measured in green turtles from Florida, the U. S. Virgin Islands, 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 sexual maturity range is 20-50 years of age.

The navigation feats of the green turtle are well known, but poorly understood. It is known that hatchlings and adult females on the nesting beach orient toward the ocean using photic cues. We do not know what cues are employed in pelagic stage movements, in movements among foraging grounds, or in migrations between foraging grounds and nesting beach. Because green turtles feed in marine pastures in quiet, low-energy areas and 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 620 mi (1,000 km) away. It has been generally accepted, but not proven, that green turtles return to nest on their natal beach. Green turtles do exhibit strong site-fidelity in successive nesting seasons.

It is assumed that post-hatchling, pelagic-stage green turtles are omnivorous, but there are no data on diet from this age class. It is known that once green turtles shift to benthic feeding grounds they are herbivores. They feed on both seagrasses and algae. The location of the foraging grounds of green turtles that nest in Florida is not known.

A population of juvenile green turtles forage as herbivores in the central Indian River Lagoon, near Sebastian, Florida, and along the near-shore Sabelleriid worm reefs off Indian River County, Florida.Postnesting females have been tracked by satellite telemetry from the beaches of the Archie Carr National Wildlife Refuge to the shallow, benthic habitats of the Florida Keys.

Habitat

Green turtles occupy three habitat types: high-energy oceanic beaches, convergence zones in the pelagic habitat, and benthic feeding grounds in relatively shallow, protected waters. 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. When turtles reach a carapace length of approximately 8-10 in (20-25 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 grounds support only certain size classes of green turtles; the turtles apparently move among these foraging areas—called developmental feeding grounds—as they grow. Other feeding areas, such as Miskito Cays, Nicaragua, support a complete size range of green turtles from 8 in (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.

In South Florida, the green sea turtle shares nesting beaches with the threatened loggerhead sea turtle, in every county where it nests, and with the endangered leatherback sea turtle, most commonly in Martin and Palm Beach Counties. Other federally listed species that occur in coastal dune and coastal strand habitat, and that need to be considered when managing nesting beaches, are the southeastern beach mouse and the beach jacquemontia.

Distribution

The green sea turtle is a circumglobal species in tropical and subtropical waters. In U. S. Atlantic waters, green turtles are found around the U. S. Virgin Islands, Puerto Rico, and the continental United States from Texas to Massachusetts. Areas that are known as important feeding areas for green turtles in Florida include Indian River Lagoon, Florida Keys, Florida Bay, Homosassa, Crystal River, and Cedar Key.

Major green turtle nesting colonies in the Atlantic occur on Ascension Island in the south Atlantic, Aves Island in the eastern Caribbean, and on the beaches of Costa Rica and Suriname. In U. S. Atlantic waters, green turtles nest in small numbers in the U. S. Virgin Islands and in Puerto Rico. Although they nest in all coastal counties in south Florida, the largest nesting actually occurs along the east coast of Florida, particularly in Brevard, Indian River, St. Lucie, Martin, Palm Beach, and Broward Counties. Nesting along the southwest coast of Florida was documented for the first time in 1994.

Statewide, green sea turtle nests amounted to 0.2% of total sea turtle nesting during 1979 to 1992. From 1988 to 1992, while survey efforts remained relatively constant, the total number of reported green sea turtle nests statewide fluctuated between 455 and 2,509. In addition, it appears that green sea turtle nesting exhibits a two-year cycle in activity.

Although the majority of green turtle nesting occurred in Brevard County, just outside of the South Florida Ecosystem, Palm Beach County supported the second highest percentage of green turtle nests during that period with 23.1% of nests.

The status of green turtle populations is difficult to determine because of the long generation time and inaccessibility of the early life stages. The number of nests deposited in Florida appears to be increasing, but whether this upward trend is due to an increase in the number of nests or is a result of more thorough monitoring of the nesting beaches is uncertain.

Threats

There is little information available to assess the comprehensive effects of disease and/or parasites on wild populations of green turtles. The vast majority of diseases and conditions that have been identified or diagnosed in sea turtles are described from captive stock, either turtles in experimental head-start programs or mariculture facilities. One notable exception is the occurrence of fibropapillomatosis in the green turtle. Fibropapillomatosis is a disease characterized by one or more noncancerous fibrous tumors, commonly located on areas of soft skin. The tumors can be debilitating and, in severe cases, fatal. They can result in reduced vision, disorientation, blindness, physical obstruction to normal swimming and feeding, an apparent increased susceptibility to parasitism by marine leeches, and an increased susceptibility to entanglement in monofilament fishing line. Blood counts and serum profiles of green turtles inflicted with fibropapillomas indicate marked debilitation. Fibropapillomas are now common on immature green turtles in the central Indian River system of Florida, Florida Bay, and in the Florida Keys. In the central Indian River Lagoon, approximately half of all green turtles captured have been found to bear papillomas of varying degree. Fibropapillomas are also commonly found on Hawaiian green turtles. Since 1989, incidence of this disease at Kaneohe Bay, Oahu, Hawaii, has ranged from 49% to 92%. Recent reports from Puerto Rico and the U. S. Virgin Islands, however, indicate a very low occurrence of fibropapillomas on green turtles collected in those areas.

Conservation and Recovery

There are a number of management activities ongoing in south Florida to benefit the green sea turtle. In addition to management of coastal habitats, the National Marine Fisheries Service (NMFS) and the U. S. Fish and Wildlife Service (FWS) have discussed additional conservation measures for the green turtle in the marine environment.

Conservation of sea turtle nesting habitat is continuing on several national wildlife refuges in South Florida, including Archie Carr, Hobe Sound, Ten-Thousand Islands, and the complex of satellite refuges in the Florida Keys. Acquisition of high-density nesting beaches between Melbourne Beach and Wabasso Beach, Florida, is continuing, in order to complete the Archie Carr National Wildlife Refuge. Approximately 35% of green turtle nesting in the United States occurs along this 20 mi (33 km) stretch of beach. The State of Florida purchased the first parcel specifically for the refuge in July 1990. Federal acquisition began in 1991. When completed, the refuge will protect up to 10 mi (16 km) of nesting beach. Since the initial acquisition, Brevard County and the Richard King Mellon Foundation have joined in as acquisition partners. Hobe Sound National Wildlife Refuge, located north of West Palm Beach in Martin County, contains 3.25 mi (5.25 km) of Atlantic coast shoreline for nesting habitat. In addition to providing some of the most productive sea turtle nesting habitat in the United States, the refuge is also home to scrub-jays and gopher tortoises. The beach management program of longest duration has been the effort to reduce destruction of nests by natural predators, such as raccoons and armadillos. Control of numerous exotic plants such as Australian pine and Brazilian pepper are also major issues in managing Hobe Sound National Wildlife Refuge.

Information on the status and distribution of the green turtle is critical to its conservation. Monitoring the various life stages of the turtles on nesting beaches is being conducted to evaluate current and past management practices. Data are collected on the number of nests laid, the number of nests that successfully hatch, and the production of hatchlings reaching the ocean. Standardized ground surveys on index beaches are underway throughout Florida by the FWS, the State of Florida's Department of Environmental Protection, and private groups and universities. Index beaches include 80% of the nesting activity in Florida. Because of slow growth rates and subsequent delayed sexual maturity, all monitoring will need to be conducted over a long period of time to establish population trends.

Mortality of green sea turtles has been monitored since 1980 through the implementation of a regional data collection effort. This voluntary stranding network from Maine to Texas is coordinated by the NMFS and serves to document the geographic and seasonal distribution of sea turtle mortality. Since 1987, four index zones have been systematically surveyed. It is clear that strandings represent an absolute minimum mortality. They can be used, however, as an annual index to mortality and are an indication of the size and distribution of turtles being killed. They can also provide valuable biological information on food habits, reproductive condition, and sex ratios.

Research is underway at the University of Florida to determine the cause of the fibropapillomatosis disease affecting the green turtle. Evidence of a herpes-like virus was found, but it is unclear whether this is a primary or secondary infection. Management recommendations to reduce the incidence and impact of this disease include improving habitat quality in areas where occurrence is high, using strict hygiene techniques when handling affected turtles, and minimizing translocations of affected turtles.

Contacts

National Marine Fisheries Service
Office of Protected Resources
1315 East-West Highway
Silver Spring, Maryland 20910
Fax: (301) 713-0376
http://www.nmfs.gov/prot_res/prot_res.html

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

References

Bjorndal, K. A. 1980. "Demography of the Breeding Population of the Green Turtle, Chelonia mydas, at Tortuguero, Costa Rica." Copeia 1980 (3): 525-530.

Carr, A. F. 1975. "The Ascension Island Green Turtle Colony." Copeia 1975 (3): 547-555.

Dodd, C. K., Jr. 1982. "Nesting of the Green Turtle Chelonia mydas in Florida: Historic Review and Present Trends." Brimleyana 7: 39-54.

U. S. Fish and Wildlife Service. 1984. "Recovery Plan for Marine Turtles." U. S. Fish and Wildlife Service, Atlanta.

Wood, J. R., and F. E. Wood. 1980. "Reproductive Biology of Captive Green Sea Turtles Chelonia mydas. " American Zoology 20: 499-505.