(Vandelli, 1761)
Leatherback turtle
Recognition
The black, smooth carapace (to 291 cm but usually less than 200 cm) is elongated, lyre shaped, and tapers to a supracaudal point above the tail. It lacks horny scutes, but is covered with a ridged, leathery skin. Shell bones, except the nuchal and those at the rim of the plastron, are lost; their place is taken by a mosaic of small bony plates embedded in the leathery skin. The largest of these polygonal bones form seven prominent longitudinal keels, which divide the carapacial surface into eight sections. The nuchal is attached to the neural arch of the 8th cervical vertebra. A series of 10 dorsal vertebrae runs medially along the carapace; the free dorsal ribs, except the last, articulate with the neural arch and two adjacent centra and are embedded in a layer of cartilage. Ribs one and ten are short. The hingeless plastron is cream to white with five longitudinal ridges underlined by small polygonal bones. An entoplastron is absent, but the bones at the rim are apparently homologous to the epiplastra, hyoplastra, hypoplastra and xiphiplastra of other turtles. Head and neck are black or dark brown, with white to light pink blotches, the snout is blunt and nonprojecting, and there is a toothlike cusp on each side of the gray upper jaw. The skull roof is complete. The vomer meets the premaxillae, separating both the nares and palatines, and the choanae lie posterior to the alveolar surfaces of the vomer. No basisphenoid-palatine contact occurs, and the maxilla is separated from both the pterygoid and quadratojugal. Neither a secondary palate nor a palatine fenestra is present. The quadrate is open posteriorly and does not enclose the stapes. The supraoccipital process is very short. Both jaws are very short behind the coronoid process, and the anterior maxillary ridge is cusped. The neck is short and incompletely retractile; its 4th cervical vertebra is biconvex. Limbs are paddlelike, clawless, and black with some white blotches.
A full chromosomal complement consists of 56 chromosomes: 24 macrochromosomes (14 metacentric and submetacentric, 10 telocentric and subtelocentric) and 32 microchromosomes; chromosomal numbers and morphology are similar to those of cheloniid turtles (Medrano et al., 1987).
Males have concave plastra and are rather depressed in profile. They are more tapering posteriorly and have tails longer than their hindlimbs; the tails of females are barely half as long.
Distribution
Dermochelys ranges throughout the waters of the Atlantic, Pacific, and Indian oceans from Labrador, Iceland, the British Isles, Norway, Alaska, and Japan south to Argentina, Chile, Australia, and the Cape of Good Hope (Pritchard, 1980).
Western Atlantic nesting beaches include northern South America (Brazil, French Guiana, Surinam, Guyana, Venezuela, Trinidad, Tobago, and Colombia), Central America (Panama, Costa Rica, Nicaragua, Belize, and Mexico), Caribbean Islands (Antigua, Barbados, Culebra, Dominica, Dominican Republic, Grenada, Guadeloupe, Jamaica, Martinique, Nevis, St. Lucia, St. Kitts, St. Maarten, St. Vincent, and St. Thomas (Márquez, 1990); of these only the beaches of French Guiana, Surinam, and Costa Rica can be considered major nesting areas, all other beaches produce few nests annually). In North America, rare nesting has been reported on Padre Island, Texas (but not recently), and along the Atlantic Coast less than 50 leatherbacks nest each year on beaches ranging from Dade to St. Johns counties, Florida, and on Cumberland and Blackbeard islands, Georgia (Ernst et al., 1994). In the eastern Atlantic, only minor and solitary nesting occurs in Mauritania, Senegal, Liberia, Ghana, Togo, Zaire, and Angola in June-August (Márquez, 1990); no nesting has been reported on Mediterranean beaches in recent times. Eastern Pacific nesting beaches are located in Ecuador, Panama, Costa Rica, Guatemala, and Mexico (Márquez, 1990). Nesting populations in Oaxaca, Guerrero, and Michoacán, Mexico are apparently large (Pritchard, 1982). Nesting in the Indo-Pacific region is known from KwaZulu-Natal and Mozambique, although minor; occasional nesting occurs in Tanzania, Zanzibar, Kenya and the Seychelles, Somalia, South Yemen, Oman, and the Laccadive Islands; along the western coast of India in Goa and Kerala; in Sri Lanka; Malaysia (a major nesting site is at Terengganu), Irian Java, Western Sumatra, and China (Márquez, 1990). Only sporadic nesting occurs on eastern and northeastern Australia beaches (Márquez, 1990). No nesting is known from the Central Pacific islands, but in the South Pacific leatherbacks nest on Papua New Guinea, the Solomon Islands, Vanuatu and Fiji (Márquez, 1990).
Geographic Variation
Two subspecies have been described. Dermochelys coriacea coriacea (Vandelli, 1761), the Atlantic leatherback, ranges through the Atlantic Ocean, the Gulf of Mexico, and the Caribbean Sea, from Labrador to Norway southward to Argentina and the Cape of Good Hope. It supposedly has longer forelimbs in comparison to total body length, a shorter head, and is darker with less light mottling on the back, lower jaw and throat. Dermochelys c. schlegelii (Garman, 1884), the Pacific leatherback, occurs in the Pacific and Indian oceans from Alaska to Chile and west to Japan and eastern Africa. It has shorter forelimbs, a longer head, and is paler, with more light mottling on the back, lower jaw, and throat.
These supposed races are poorly differentiated, and a detailed study will be necessary to determine their validity. Also an additional small third subspecies may occur in the eastern Pacific Ocean off the western Americas; if it proves valid, the name angusta (Philippi, 1899) is available.
Habitat
The leatherback is pelagic, but occasionally it enters the shallow waters of bays and estuaries. It shares its major nesting beaches with most of the other species of marine turtles.
Natural History
The size and age of maturity in males is unknown, but nesting females average 148.7-162.2 cm in carapace length (Van Buskirk and Crowder, 1994; Hughes, 1996). The annual gametic cycles of both sexes are practically unknown. Ultrasonography indicated that 82% of nesting females at Las Baulas National Park, Costa Rica had mature pre-ovulatory ovaries in November, 40% in December, and 23% in January (Rostal et al., 1996). Mean follicular diameter was 3.33 cm and did not vary significantly through the nesting season.
Mating was thought to occur in tropical waters at the time of nesting, but may actually take place prior to or during the migration from temperate to tropical waters (Eckert and Eckert, 1988). The only recorded observation of copulating Dermochelys coriacea took place in 44 m deep water on 5 April off Culebra Island, Puerto Rico (Carr and Carr, 1986). The two turtles surfaced about 5 m apart, and the female remained stationary while the male swam quickly to her with head held high above the water. When about a meter behind her, he lunged forward, lodging the center of his plastron on the posterior dorsal surface of her carapace. She immediately dove, but he took a breath and followed. The two leatherbacks emerged a second time about 2 m apart, and the male lunged onto her back again, forcing her to once again dive, only more slowly. When they surfaced for the third time, the turtles were very close together and she did not resist his subsequent lunge. He positioned the center of his plastron just posterior to the center of her carapace, his weight forcing her beneath the water. Both turtles began to list, and then roll laterally back and forth. At this time the male's tail and semi-erect penis were visible. The turtles then separated, submerged and surfaced a fourth time. The male repeated his behavior, curving his tail beneath the base of her tail. Her forelimbs became rigid and slightly elevated above the water, while his forelimbs and neck became rigid. The turtles quivered slightly and slowly sank beneath the surface until lost from view. Dermochelys nests on Culebra Island, so this may have been a mating involving a female about to oviposit.
Nesting by western Atlantic Dermochelys may occur from February to August, but mostly from April into July. Nesting on eastern Pacific beaches occurs from September to March, and possibly to May, but most is concentrated from November into January. Nesting in the Indo-Pacific region is known from November-September. In Australia nesting occurs in December-February, and in the South Pacific leatherbacks nest from November-January (Márquez, 1990). Typical nesting beaches have a slope of 8-12° and are free of much abrasive material, such as coral or rock.
Leatherbacks usually land singly or in small groups at night. The nest may be flask shaped or not, depending on whether the turtle digs down to damp sand. A body pit is scraped out first, and then the actual nest cavity. The egg chamber may be 47 x 30 cm and 61-103 cm below the beach surface. Typically, a female leatherback nests every 2-3 years (average, 2.28 years; Van Buskirk and Crowder, 1994). During a nesting season, leatherbacks usually oviposit at 8-12 day intervals, but the internesting period may be much longer.
Dutton et al. (1992) reported that a female may lay as many as 11 clutches a year in the Virgin Islands, but probably 5-7 clutches a season, mean 6.17 (Van Buskirk and Crowder, 1994), are normal. Individual clutches contain 50-166 eggs (average, 81.5; Van Buskirk and Crowder, 1994). Subsequent clutches usually contain less eggs than the clutch immediately preceding them, and the last clutch of the season is composed of substantially fewer eggs that the first clutch of the year (Tucker and Frazer, 1994). Differences in the clutch size occur between nesting populations, as well. Nests from Atlantic beaches average 80-90 yolked eggs, but those from the eastern Pacific average only 60-66 yolked eggs (Pritchard, 1971). Normal eggs are spherical, have soft, white shells, and are 49-65 mm in diameter. Abnormal eggs may be ellipsoidal, small and spherical, or somewhat dumbbell-shaped, and 13-45 mm in diameter. Incubation lasts 50-78 days (Márquez, 1990), but most undisturbed clutches hatch in 60-65 days.
Hatchlings have 51-68 mm carapaces (average, 60.1; Van Buskirk and Crowder, 1994), and are dark brown or black with white or yellow carapacial keels and flipper margins. Their skin is covered with small scales and the tail dorsally keeled; both scales and keel soon disappear.
Dermochelys is highly carnivorous, particularly on invertebrates, but it may sometimes ingest algae and vertebrates, probably secondarily. Preferred prey are the numerous ocean jellyfish (particularly Scyphomedusae). Prey reported from wild leatherbacks include: jellyfish (at least nine genera), sea urchins, octopi, squid, snails (eight genera), bivalves, amphipods, crabs, tunicates, small fish, a hatchling ridley turtle (Lepidochelys), small bird feathers, blue-green algae, green algae, and floating kelps (Carr, 1952; Pritchard, 1971; Pritchard and Trebbau, 1984; Márquez, 1990; Ernst et al., 1994). Several dead, beached Dermochelys have had plastic bags in their stomachs; apparently these were mistaken for jellyfish. The jaw apparatus of leatherbacks lacks the massive construction, crushing plates, and musculature for taking hard-shelled prey found in some cheloniid sea turtles, but the jaw rims are sharp and well-adapted for a jellyfish diet. In the relatively long esophagus are numerous keratinized, posteriorly pointing spines which probably prevent soft prey, such as jellyfish, from being regurgitated and escaping from the mouth; these are illustrated in Den Hartog and Van Nierop (1984).
Dermochelys consistently ranges more poleward than other sea turtles; it survives the cold waters off Alaska, Labrador, and Iceland. Frair et al. (1972) discovered that the deep body temperature of a leatherback taken from cold water was 18°C above the water temperature, and thought its large size favored heat retention from muscular activity. Their data show that leatherbacks can maintain a considerable differential between their body temperatures and the ambient level in situations where behavioral regulation is not possible. Greer et al. (1973) demonstrated that a countercurrent circulatory system exists in the fore and hindlimbs that permits homeothermy. Perhaps the high oil content of leatherback flesh also retards heat loss.
Remarks
Brongersma (1996) thought the previous restriction of the type locality of Dermochelys coriacea schlegelii "Guaymas, Mexico" by Smith and Taylor (1950) is erroneous and that the actual source of the type material is more likely Japan; so, even if the eastern Pacific populations are distinctive, the name schlegelii is not available to them.
There has been some controversy regarding the authorship of the specific name coriacea; see Fretey and Bour (1980), Rhodin and Smith (1982), Bour and Dubois (1983a), and Smith and Rhodin (1986) for a discussion about authorship and type specimen.
IUCN Red List Status (1996)
Endangered (A1abd). Pritchard (1982) reported the breeding population of Dermochelys from the Mexican Pacific states of Michoacán, Guerrero, and Oaxaca is the largest known. He estimated the world population of mature females to be 115,000, but, due to severe stress on all major populations, still thought the species endangered. Spotila et al. (1996) estimated the number of female leatherbacks nesting on 28 beaches throughout the world, and calculated the world's total population of female Dermochelys coriacea was between 26,200 and 42,900 in 1995, less than 33.3% of that in 1980. This drastic decline since Pritchard's 1982 estimate strengthens Pritchard's conclusion that the species is endangered. However, leatherback populations in the various oceans show different trends; those in the Indian Ocean are declining, while those in the Atlantic have remained relatively stable (Pritchard, 1996b), so the concern may be more in one area than in another.