(Linnaeus, 1766)
Hawksbill turtle
Recognition
This is a small to medium-sized (to 114 cm) sea turtle with four pairs of pleurals (the 1st not touching the cervical) and two pairs of prefrontal scales. The carapace is shield shaped (heart-shaped in the young and more elongated and straight sided in the adult), has four pairs of pleural scutes, a keel on the last four vertebrals, and posterior serrations. No lateral fontanelles occur in the adult. All vertebrals are broader than long; neurals 9-11 are six sided and shortest anteriorly. There are 11 pairs of peripherals; the 9th pair does not touch ribs. In the young, the carapacial scutes strongly overlap the next most posterior, but as the turtle matures the overlapping becomes progressively less, until finally the scutes lie side by side. The carapace is dark greenish brown; in the young it shows a tortoise-shell pattern. There are four poreless inframarginal scutes on the yellow bridge. The plastron is hingeless and yellow; in juveniles it may have two longitudinal ridges and a few dark blotches, especially on the anterior scutes. The plastral formula is: an > fem > abd >< hum >< pect > gul > intergul. Head scales are black to chestnut brown at the center and lighter at their margins; the jaws are yellow with some brown streaks or bars. There are two pairs of prefrontals and three postoculars. The chin and throat are yellow, and the neck dark above. The triturating surfaces of the premaxillae and maxillae bear a ridge; the lower jaw is only slightly serrated. The vomer touches the premaxillae separating the maxillae; the descending processes of the prefrontal only touch the vomer. The frontal enters the orbit, and there is a low, blunt ridge on the vomer and palatines. The snout is elongated and narrow, rather like a hawk's beak, and is not notched at the tip. The floor of the mouth is deeply excavated at the mandibular symphysis. Two claws occur on the forelimbs.
The karyotype is 2n = 56, with 10 metacentric, 2 submetacentric, 2 subtelocentric, and 8 acrocentric macrochromosomes, and 34 microchromosomes (Bickham, 1981; Kamezaki, 1990).
Males have somewhat concave plastra; long thick tails that extend beyond the posterior carapacial margin; and long, heavy claws.
Distribution
The hawksbill ranges in the Atlantic, Pacific, and Indian oceans from California, Japan, the Red Sea, the British Isles, and Massachusetts south to Peru, Australia, Madagascar, northwestern Africa, and southern Brazil.
The Atlantic hawksbill nests on beaches in Florida, Jamaica (Pedro and Morant cays), the Cayman Islands, Aves Island, the Virgin Islands, Grenada, Tobago, Trinidad, Guyana, Surinam, French Guiana, Venezuela, Panama, Costa Rica, Nicaragua, Mexico, and the islands and keys off Central America. The Pacific hawksbill nests on beaches on the eastern coast of Africa, the Seychelles Islands, islands off Madagascar, Arabia and in the Persian Gulf and Gulf of Aden, Sri Lanka, Indonesia, northern Australia, the Ryukyu Islands, Samoa, Fiji, Hawaii, and in Honduras, Nicaragua, El Salvador, and Mexico.
Geographic Variation
Two subspecies have been described. Eretmochelys imbricata imbricata (Linnaeus, 1766), the Atlantic hawksbill turtle, ranges through the warmer parts of the western Atlantic Ocean, from Massachusetts through the Gulf of Mexico to southern Brazil. It also has been recorded from Ireland, Scotland, Morocco, and St. Helena. This subspecies has a nearly straight-sided carapace that tapers posteriorly; a keel that is continuous on only the last four vertebrals; ridges that converge posteriorly on only the last two vertebrals; and the upper surfaces of the head and flippers with less black, and little black plastral blotching in juveniles. Eretmochelys imbricata bissa (Rüppell, 1835), the Pacific hawksbill, ranges through the tropical portions of the Indian and Pacific oceans, from Madagascar to the Red Sea on the east coast of Africa and east to Australia and Japan in the western Pacific, to the Hawaiian Islands in the central Pacific, and from Peru to Baja California in the eastern Pacific. Stragglers occasionally reach California. It has a more heart-shaped carapace, a fully continuous vertebral keel, all vertebrals with ridges that converge posteriorly, the head and flippers almost solid black, and, in the young, black plastral pigmentation.
The characters used to separate these supposed races may be of little value. Carapace shape changes with age and growth, as probably also does the extent of carapacial keeling, and the degree of melanism is quite variable in every population (see discussions in Witzell, 1983; and Pritchard and Trebbau, 1984). A quantitative study is needed to determine the true relationship between the Atlantic and Pacific populations.
Habitat
This marine turtle is characteristically an inhabitant of rocky places and coral reefs. It also occurs in shallow coastal waters, such as mangrove-bordered bays, estuaries, and lagoons with mud bottoms and little or no vegetation, and in small narrow creeks and passes. It is occasionally found in deep waters and has been taken from floating patches of Sargassum weed. Eretmochelys shares its water habitat and nesting beaches with all other marine turtles.
Natural History
Mature females nesting on Atlantic Ocean beaches have straight-line carapace lengths of 62.5-94.0 cm, with an average of 78.6 cm (Witzell, 1983; Bjorndal et al., 1985; Van Buskirk and Crowder, 1994). Carr (1952) thought Costa Rican Eretmochelys mature sexually at three years of age; Harrison (1963) believed that Sarawak hawksbills are mature at a mean carapace length of 49.5 cm; and Witzell (1980) thought Samoan Eretmochelys mature at 50 cm and 3.5 years of age (based on captive growth). The gonadal cycles are unknown.
Mating occurs in the shallow water off the nesting beaches. According to Hornell (1927), copulation begins soon after the spent females return to the sea, but males have been observed to follow a female onto the beach. Whether mating takes place at any other time or place is not known.
The nesting season occurs mostly toward the end of spring and throughout summer, usually before the peak of the green turtle season; sometimes another peak for the hawksbill appears after that of the green turtle (Márquez, 1990). Nesting occurs at night and takes about an hour. Females apparently oviposit in 3-year cycles. We refer the reader to Carr et al. (1966) for a lucid description of nesting behavior.
The white, calcareous eggs usually are spherical, averaging about 38 mm (35-42) in diameter, but a few in each clutch may be slightly elongated. Eggs examined by Deraniyagala (1939) were thinly covered with a mucilaginous secretion which appeared to absorb water and was found to remain moist for 48 h. The average number of eggs in 57 nests examined by Carr et al. (1966) was 161 (53-206). The average incubation period of 13 clutches moved to artificial nests by Carr et al. (1966) was 58.6 days (52-74). These clutches contained a total of 2193 eggs that were artificially buried in 9 nests, and 46.7% hatched (range in the individual nests, 12-80%). Probably mortality was influenced by the moving of eggs. Van Buskirk and Crowder (1994) report females lay an average of 2.74 clutches per year, and clutches average 130 eggs.
Hatching usually occurs at night or in early morning. Eggs hatch almost simultaneously and the hatchlings follow one another to the surface in quick succession. The hatchling carapace (39-50 mm) is heart-shaped with a vertebral keel; the plastron has two longitudinal ridges. Hatchlings are black or very dark brown except for the keels, the shell edge, and areas on the neck and flippers, which are light brown.
Eretmochelys is omnivorous but seems to prefer invertebrates, particularly sponges. In fact it feeds almost exclusively on sponges in the Caribbean (Meylan, 1988b). Its diet is taxonomically narrow and highly uniform geographically. It includes sponges that are toxic to other vertebrates and contains more silica than that of other vertebrates (Meylan, 1988b). It is also known to consume coelenterates (Portuguese man-of-war, anemones, hydroids, coral), bryozoans, flat worms, polychaetes, sea urchins, mollusks (gastropods, bivalves, cephalopods, scaphopods), tunicates, and fish; plants consumed are algae (brown, green, red), various sea grasses, and mangrove (wood, bark, leaves, fruits). Captives eat fish, meat, bread, octopi, squid, crabs, mussels, and oysters. Hatchlings seem to be herbivorous, but become more omnivorous as they age.
The translucent carapacial scutes form the tortoise-shell of commerce. These are clear amber, streaked with red, white, green, brown, and black. Usually the turtle is killed before the scutes are removed by the application of heat. There is evidence that if the scutes are removed carefully and the turtle is returned to the sea it can regenerate lost scutes. This may be possible if the epidermal Malpighian cells are not damaged. A single turtle can yield 10-12 lb of tortoise-shell (called "carey" by the Caribbeans). The flesh and eggs are eaten in many parts of the range. This requires caution, for Eretmochelys tends to store in its tissues the toxins of various poisonous organisms that it eats.
Remark
Immunoprecipitation tests on the serum of marine turtles have shown that Eretmochelys is more closely related to Caretta and Lepidochelys than to Chelonia (Frair, 1979).
Females from various Australian and Caribbean nesting beaches exhibit different frequences of mtDNA haplotypes, which distinguish them and indicate different breeding populations exist that should be considered separate management units (Broderick et al., 1994; Bowen et al., 1996; Bowen and Bass, 1997; Mrosovsky, 1997).
IUCN Red List Status (1996)
Critically endangered (A1abd+2bcd). Survival of the hawksbill is questionable. Existing conservation laws must be enforced, and egg collecting stopped.