Leatherback Sea Turtle

Dermochelys coriacea

Description

The leatherback sea turtle, Dermochelys coriacea, is the largest of all the sea turtles. It is also unique among sea turtles, because instead of a bony carapace, it has leatherlike outer skin in which is embedded a mosaic of small bones. The carapace of the leatherback sea turtle is also different from that of other sea turtles. Other sea turtles have bony plates covered with horny scutes on the carapace, while the slightly flexible carapace of the leatherback is distinguished by a rubberlike texture. The oval carapace of the leatherback is about 1.6 in (4 cm) thick and is made primarily of tough, oil-saturated connective tissue raised into seven prominent longitudinal ridges and tapered to a blunt point posteriorly. A nearly continuous layer of small dermal bones lies just below the leathery outer skin of the carapace. No sharp angle is formed between the carapace and the plastron, resulting in a barrel-shaped appearance. The front flippers are proportionally longer than in other sea turtles and may span more than 8 ft (2 m) in an adult. The leatherback's mean curved carapace length for adult females nesting in the U. S. Caribbean is 5.1 ft (1.6 m).

On Sandy Point National Wildlife Refuge (St. Croix, U. S. Virgin Islands), weights of 578-1,116 lb (262-506 kg) have been recorded. Adults and near adults captured in Virginia waters had curved carapace lengths of 54-72 in (137-183 cm). Size and weight relationships calculated from adult females in St. Croix, suggest the Virginia leatherbacks weigh 450-1,534 lb (204-696 kg). The largest leatherback on record weighed 2,019 lb (916 kg). Leatherback hatchlings are mostly black dorsally and covered with tiny polygonal or beadlike scales; the flippers are margined in white and rows of white scales appear as stripes along the length of the back. In the U. S. Virgin Islands hatchlings average 2.4 in (6.1 cm) in straightline carapace length and 1.6 oz (45.8 g) in weight. Both front and rear flippers lack claws. In the adult, the epidermis is black and scaleless. This scaleless condition is unique among sea turtles. The undersurface is mottled, pinkish-white and black, the proportion of light to dark pigment being highly variable. In both adults and hatchlings, the upper jaw bears two toothlike projections, each flanked by deep cusps, at the premaxillary-maxillary sutures.

The crawl of the nesting leatherback is very deep and broad, with symmetrical diagonal marks left by the front flippers usually with a deep incised median groove formed by the dragging of the relatively long tail. The internal anatomy and physiology of the leatherback are also distinctive. The core body temperature, at least for adults in cold water, has been shown to be several degrees Celsius above the ambient temperature. This may be due to several features, including the thermal inertia of a large body mass, an insulating layer of subepidermal fat, countercurrent heat exchangers in the flippers, potentially heat-generating brown adipose tissue, and a relatively low freezing point for lipids. The skeleton of the leatherback remains extensively cartilaginous, even in adult animals, and the species is unique among turtles in showing an extensive cartilage canal vascular system in the epiphyseal regions.

Behavior

There is some indirect evidence that mating typically occurs prior to migration to the nesting ground. Nesting behavior is similar to that of other marine turtle species. Gravid females emerge from the sea nocturnally; daytime nesting occurs only occasionally. Because of a proclivity for nesting in high-energy and thus frequently unpredictable environments, it is not uncommon for large numbers of eggs to be lost to erosion, though this is not always the case. While the majority of females return to the same nesting beach repeatedly throughout the nesting season, some females are known to nest on separate beaches less than 62 mi (100 km) apart within a season. In the U. S. and wider Caribbean, nesting commences in March and continues into July. The most systematic data available on reproductive output has been gathered at Sandy Point National Wildlife Refuge and Isla Culebra, Puerto Rico. Data from these projects reveal that females arrive at the nesting beach asynchronously, renest an average of every nine to ten days, deposit five to seven nests per year, and remigrate predominantly at two-to three-year intervals. Clutch size averages 116 eggs, including 80 yolked eggs, on Sandy Point National Wildlife Refuge. Eggs incubate 55-75 days, consistently averaging 63 days on both Sandy Point and Culebra and 64 days on Hutchinson Island, Florida.

No data on the growth rate of juvenile leather-back turtles in the wild are available. This situation arises from the unfortunate fact that the distribution of juvenile leatherback turtles is unknown, and thus specimens are unavailable for capture-recapture methodologies designed to measure growth. The problem is exacerbated by poor survivability in captivity, which further limits opportunities for study. Nonetheless, some investigators have been successful in raising leatherbacks and publishing data on captive growth rates. Captive growth data are widely disparate, but the very rapid growth reported by some investigators has led to speculations that leatherbacks may reach sexual maturity in two to three years. Leatherbacks apparently grow to sexual maturity at an earlier age than other sea turtles.

The leatherback migrates farther and ventures into colder water more than any other marine reptile. The evidence currently available from tag returns and strandings in the western Atlantic suggests that adults engage in routine migrations between boreal, temperate, and tropical waters, presumably to optimize both foraging and nesting opportunities. The composition of barnacle communities on Caribbean-nesting leatherbacks provides indirect evidence that gravid females embark from and subsequently return to temperate latitudes. Direct evidence of long-distance movement is scarce, but is available from leatherbacks tagged while nesting in the Caribbean and subsequently stranding in northern latitudes; and also from a turtle tagged in Chesapeake Bay in 1985 and killed in Cuba in 1986. In addition, a nester tagged at Jupiter Beach, Florida, was recaptured near Cayo Arcas, Gulf of Campeche, Mexico, while a nester tagged at Sandy Point National Wildlife Refuge, St. Croix, was recaptured near Cayos Triangulos, also in the Gulf of Campeche, two years later and some 1,860 mi (3,000 km) from the tagging site. The longest known movement is that of an adult female who traveled 3,670 mi (5,900 km) to Ghana, West Africa, after nesting in Suriname. An adult female tagged with a satellite transmitter while nesting in French Guiana in 1986 traveled 510 mi (820 km) in three weeks at an average speed of 24 mi/day (40 km/day). A nester tagged with a satellite transmitter on Sandy Point National Wildlife Refuge in 1989 traveled 320 mi (515 km) and ventured some 125 mi (200 km) south of St. Croix before the transmitter was removed 18 days later when the turtle emerged to nest on Isla Culebra.

Food habits are known primarily from the stomach samples of slaughtered animals. Leatherbacks feed on jellyfish, siphonophores, and salpae in temperate and boreal latitudes. Aerial surveys document leatherbacks in Virginia waters, especially from May to July during peak jellyfish abundance. Foraging has most often been observed at the surface, but foraging may occur at depths. A leatherback was reported feeding on octopus bait on a handline at 164 ft (50 m) in depth off western Australia. Dives may reach a maximum depth of 4,265 ft (1,300 m), but 95% of all dives last less than 20 minutes in length and are less than 655 ft (200 m) in depth.

Habitat

Adult leatherback sea turtles are highly migratory and believed to be the most pelagic of all sea turtles. Habitat requirements for juvenile and post-hatchling leatherbacks, however, are virtually unknown. Nesting females prefer high-energy beaches with deep, unobstructed access—beaches that occur most frequently along continental shorelines. Leatherbacks stranding on U. S. shores are generally of adult or near adult size, demonstrating the importance of pelagic habitat under U. S. jurisdiction to turtles breeding in tropical and subtropical latitudes. Direct evidence of this is available from Caribbean and South American tagged turtles stranding on U. S. shores. Nesters tagged in French Guiana subsequently stranded in Georgia, as well as in New York, New Jersey, South Carolina, and Texas. Nesters tagged in Trinidad and St. Croix subsequently stranded in New York and New Jersey, respectively. Conversely, an individual tagged in Virginia waters in 1985 was killed a year later in Cuba. Additional evidence of the importance of U. S. coastal waters for leatherbacks is provided by the Sea Turtle Stranding and Salvage Network. During the period 1980-91, 816 leatherback strandings were recorded along the continental U. S. coastline. During this same period, 161 leatherbacks were recovered dead along Florida's coast. Curved carapace lengths for the Florida strandings ranged from 43-77 in (110-195 cm). Eighty-four percent of all leatherback strandings in Florida occurred between January to April and October to December. Strandings were lowest during summer months, May to September.

Distribution

The wide-ranging leatherback sea turtle nests on shores of the Atlantic, Pacific, and Indian Oceans. Nonbreeding animals have been recorded as far north as the British Isles and the maritime provinces of Canada and as far south as Argentina and the Cape of Good Hope. Efforts to determine the distribution and numbers of leatherback sea turtles in the marine environment have met with varying degrees of success.

A 1987 aerial survey of shallow Gulf of Mexico waters described leatherbacks as uncommon in all study areas, the highest density being 0.07 leatherbacks per 100 sq mi (0.027 per 100 sq km) off the shore of Louisiana in October. Earlier surveys in the Atlantic revealed leatherbacks in the study area year-round, but no density estimates were given. There is a significant negative correlation between leatherbacks and water temperature in the spring, fall, and winter, suggesting that the species is not dependent upon warm temperatures and is likely to be associated with cooler, perhaps more productive waters. The same study reported that leatherbacks appeared to prefer water of about 68°F (20°C) and were rarely sighted in the Gulf Stream sampling areas. In another study that summarized incidental catch and interview data, as well as atsea observations recorded during shore to Gulf Stream summer transacts, researchers concluded that, at least off North Carolina, leatherbacks were rarely seen in the Gulf Stream and were most often seen in waters of less than 3,000 ft (915 m) in depth. Studies have concluded that leatherbacks were observed more frequently in colder waters at higher latitudes during the summer than were other sea turtle species.

Nesting grounds for the leatherback are distributed circumglobally, with the Pacific coast of Mexico supporting the world's largest known concentration of nesting turtles. It is estimated that 115,000 adult female leatherbacks remain worldwide and that some 50% of them may nest in western Mexico. The largest nesting colony in the wider Caribbean region is found at Yalimapo-Les Hattes, French Guiana, where the total number of adult females is estimated to be 14,700-15,300. Lower density Caribbean nesting is also reported from Suriname, Guyana, Colombia, Venezuela, Panama, and Costa Rica. In 1970, on the islands of the eastern Caribbean, it was estimated that 150-200 leatherbacks nested annually in Trinidad, primarily at Matura and Paria Bays. In 1971, perhaps 200-250 leatherbacks nested annually in Trinidad; more recent population estimates are not available. Nesting north of Trinidad in the Lesser and Greater Antilles is predictable, but occurs nowhere in large numbers. The largest subregional nesting colony is in the Dominican Republic, where an estimated 300 leatherbacks nest annually. Declines in the number of nesting females have been documented in Malaysia, India, Thailand, and the West Indies.

The U. S. Caribbean supports relatively minor nesting colonies of 150-200 adult females a year, but represents the most significant nesting activity within the U. S. territories. Leatherback nesting in the U. S. Caribbean is reported from the U. S. Virgin Islands and Puerto Rico, including Islas Culebra, Vieques, and Mona. The total number of nests deposited annually on Sandy Point National Wildlife Refuge, St. Croix, has ranged from 82-355. On Isla Culebra, the colony is smaller. Playas Resaca and Brava receive 91-100% of all leatherback nesting on Culebra. Throughout the southeastern United States, the geography of beach coverage is more or less complete, but the timing is often inadequate to gain a complete picture of leatherback nesting. Beach patrols are designed to maximize observations of leatherback sea turtle nests and generally commence in May, whereas leatherbacks start nesting as early as late February or March. Thus, current data slightly underestimate actual nesting activity.

Leatherback nests reported from Florida and Georgia are probably deposited by 10-25 females annually. Leatherback turtles have been known to nest in Georgia and South Carolina, but only on rare occasions. A researcher was informed by a resident of Padre Island, Texas, that a few nesting individuals had been seen on the island in the 1930s, but none in recent years.

Leatherback nesting in Florida was once considered extremely rare. The leatherback, however, is now known to nest regularly in small numbers on Florida's east coast. Leatherback nesting has also been reported on the west coast of Florida on St. Vincent National Wildlife Refuge, St. Joseph Peninsula State Park, and St. George Island. In south Florida, leatherbacks have been observed on nesting beaches in Indian River, St. Lucie, Martin, Palm Beach, Broward, and Dade Counties on the east coast. It is not known whether leatherback populations within the United States are stable, increasing or declining, but there is no question that some nesting populations have been virtually exterminated. The number of leatherbacks nesting in the past at what is now Sandy Point National Wildlife Refuge is unknown, but studies of the population since 1981 show annual fluctuations that do not project a long-term decline. As of the late 1990s, most beaches in Florida were monitored for sea turtle nesting. In Florida, leatherback nesting fluctuated widely during the survey period of 1979-94. Between 1988 and 1992, annual reported leatherback sea turtle nests varied between 98 and 188 statewide. The distribution of these nests differs from the loggerhead and green sea turtle nests. Leatherback nests have a center of distribution at Palm Beach County that supports half of the total nests reported throughout Florida. Martin and St. Lucie County beaches have been the site of 27.7% and 13.2% of leatherback nests, respectively. South of Palm Beach County, the number of leatherback nests declines more sharply. Broward County supported 3% of leatherback nesting and Dade County supported 1.6%.

Virtually nothing is known of the pelagic distribution of hatchling or juvenile leatherback turtles. The paths taken by hatchlings leaving their natal beaches are uncharted. Discussions of the "lost year" (the early pelagic stage of sea turtle development) that include tabulated summaries of neonate and juvenile sea turtles associated with sargassum weed or taken from pelagic habitats have not mentioned sightings of young leatherback sea turtles.

Threats

A variety of natural and introduced predators such as raccoons, feral hogs, armadillos, opossums, foxes, ghost crabs, and ants prey on incubating eggs and hatchling sea turtles. The principal predator of leatherback sea turtle eggs is the raccoon. Raccoons are particularly destructive and may take up to 96% of all nests deposited on a beach. In 1996, Hobe Sound National Wildlife Refuge (located north of West Palm Beach in Martin County) experienced depredation in 23% of the nests enumerated. In addition to the destruction of eggs, certain predators may take considerable numbers of hatchlings just prior to or upon emergence from the sand.

Commercial fishing is also a threat. Many scientists believe that gill nets are inadvertently catching sea turtles with fish, and the sea turtles are being killed or disabled instead of being returned to the ocean alive. According to a team of scientists at Drexel University in Philadelphia, Pennsylvania, commercial fishing is the biggest threat to these turtles, and those practices must be changed in order to sustain the population.

Conservation and Recovery

Conservation efforts for the leatherback have greatly improved since it was federally listed as endangered on June 2, 1970. During the 1970s, nest survey and protection efforts were generally sporadic and did little to reduce the widespread egg poaching on U. S. Caribbean beaches. Beginning in 1981, however, intensive nest survey and protection efforts were initiated on the single most important leatherback nesting beach in the U. S. Caribbean, Sandy Point, St. Croix. Prior to this, the majority of the 150-350 nests deposited annually were lost to poaching or erosion. Now overall hatch success exceeds 50-60% in most years. The U. S. Fish and Wildlife Service (FWS), in cooperation with Earth-watch, initiated similar measures on the other main U. S. Caribbean leatherback nesting beaches on Isla Culebra, Puerto Rico, in 1984. Prior to the intensive nighttime patrolling, a high percentage of the nests on this island were poached. Overall hatch success is now greater than 75% in most years. Nest survey and protection efforts occur on several other U. S. Caribbean beaches of lesser but still significant importance such as Manchenil, St. Croix, and Pinones, Humacao, and Luquillo beaches in Puerto Rico. In Florida, leatherback nesting data are collected in conjunction with loggerhead nesting surveys that generally begin in early to mid-May. While a portion of the leatherback nesting season is missed by the systematic loggerhead and green turtle surveys, most nests are observed by someone and probably reported because of intensive public use of the main leatherback nesting beaches in Florida. Along with the basic information on nest numbers, clutch size, and hatching success, the Sandy Point and Culebra projects have included additional studies of the nesting females and provided information on intra-and internesting frequency, movements, survivor-ship, turtle size and weight, diving behavior, prereproductive migrations, nest temperature and expected hatchling sex ratio, depredation rates, nest site selection, and embryonic deformities. In 1982, 765 acres (310 hectares) of land on Isla Culebra, including Playas Resaca and Brava, were transferred to Culebra National Wildlife Refuge. In 1984 the FWS purchased the 1.5 mi (2.4 km) long leatherback nesting beach at Sandy Point, St. Croix, establishing Sandy Point National Wildlife Refuge. These actions ensure the long-term protection of the most important leatherback nesting beaches in the U. S. Virgin Islands and Puerto Rico although neither area is immune from external threats such as light pollution.

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
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

References

Fountain, Henry. 2000. "Leatherback Turtles Are Near Extinction." New York Times 1 June, sec. A, p. 20.

Towle, E. L. 1978. "Report on Sea Turtle Nesting…(with Specific Reference to Leatherback Nests at Sandy Point, St. Croix)." Report. Island Resources Foundation, St. Thomas, U.S. Virgin Islands.

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

U. S. Fish and Wildlife Service. 1981. "Recovery Plan for the St. Croix Population of the Leatherback Turtle." U. S. Fish and Wildlife Service, Atlanta.