Hawksbill sea turtle

The hawksbill sea turtle (Eretmochelys imbricata) is a critically endangered sea turtle belonging to the family Cheloniidae. It is the only extant species in the genus Eretmochelys. The species has a worldwide distribution, with Atlantic and Indo-Pacific subspecies—E. i. imbricata and E. i. bissa, respectively.[2]

Hawksbill sea turtle
Hawksbill sea turtle (Eretmochelys imbricata) off the coast of Saba

Critically Endangered  (IUCN 3.1)[1]
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Order: Testudines
Suborder: Cryptodira
Superfamily: Chelonioidea
Family: Cheloniidae
Subfamily: Cheloniinae
Genus: Eretmochelys
Fitzinger, 1843
Species:
E. imbricata
Binomial name
Eretmochelys imbricata
Subspecies

E. imbricata bissa (Rüppell, 1835)
E. imbricata imbricata (Linnaeus, 1766)

Range of the hawksbill sea turtle
Synonyms
  • Testudo imbricata Linnaeus, 1766
  • E. imbricata squamata junior synonym

The hawksbill's appearance is similar to that of other marine turtles. In general, it has a flattened body shape, a protective carapace, and flipper-like limbs, adapted for swimming in the open ocean. E. imbricata is easily distinguished from other sea turtles by its sharp, curving beak with prominent tomium, and the saw-like appearance of its shell margins. Hawksbill shells slightly change colors, depending on water temperature. While this turtle lives part of its life in the open ocean, it spends more time in shallow lagoons and coral reefs. The World Conservation Union, primarily as a result of human fishing practices, classifies E. imbricata as critically endangered.[1] Hawksbill shells were the primary source of tortoiseshell material used for decorative purposes. The Convention on International Trade in Endangered Species outlaws the capture and trade of hawksbill sea turtles and products derived from them.[3]

Anatomy and morphology
Carapace's serrated margin and overlapping scutes are evident in this individual

Adult hawksbill sea turtles typically grow to 1 m (3 ft) in length, weighing around 80 kg (180 lb) on average. The heaviest hawksbill ever captured weighed 127 kg (280 lb).[4] The turtle's shell, or carapace, has an amber background patterned with an irregular combination of light and dark streaks, with predominantly black and mottled-brown colors radiating to the sides.[5]

Several characteristics of the hawksbill sea turtle distinguish it from other sea turtle species. Its elongated, tapered head ends in a beak-like mouth (from which its common name is derived), and its beak is more sharply pronounced and hooked than others. The hawksbill's forelimbs have two visible claws on each flipper.

Close-up of the hawksbill's distinctive beak

One of the hawksbill's more easily distinguished characteristics is the pattern of thick scutes that make up its carapace. While its carapace has five central scutes and four pairs of lateral scutes like several members of its family, E. imbricata's posterior scutes overlap in such a way as to give the rear margin of its carapace a serrated look, similar to the edge of a saw or a steak knife. The turtle's carapace has been known to reach almost 1 m (3 ft) in length.[6] The hawksbill appears to frequently employ its sturdy shell to insert its body into tight spaces in reefs.[7]

Crawling with an alternating gait, hawksbill tracks left in the sand are asymmetrical. In contrast, the green sea turtle and the leatherback turtle have a more symmetrical gait.[8][9]

Due to its consumption of venomous cnidarians, hawksbill sea turtle flesh can become toxic.[10]

The hawksbill sea turtle has been shown to be biofluorescent and is the first reptile recorded with this characteristic. It is unknown if this is derived from the turtle's diet, which includes biofluorescent organisms like the hard coral Physogyra lichtensteini. Males have more intense pigmentation than females, and a behavioural role of these differences is speculated.[11][12]

Distribution

Hawksbill sea turtles have a wide range, found predominantly in tropical reefs of the Indian, Pacific, and Atlantic Oceans. Of all the sea turtle species, E. imbricata is the one most associated with warm tropical waters. Two major subpopulations are known, in the Atlantic and Indo-Pacific.[13]

Another model of the possible distribution of E. imbricata: Red circles represent known major nesting sites. Yellow circles are minor nesting sites.

Atlantic subpopulation
Hawksbill sea turtle in Saba, Netherlands Antilles

In the Atlantic, hawksbill populations range as far west as the Gulf of Mexico and as far southeast as the Cape of Good Hope in South Africa.[13][14][15] They live off the Brazilian coast (specifically Bahia, Fernando de Noronha) through southern Florida and the waters off Virginia.[4]

In the Caribbean, the main nesting beaches are in the Lesser Antilles, Barbados,[16] Guadeloupe,[17] Tortuguero in Costa Rica,[18] and in the Yucatan. They feed in the waters off Cuba[19] and around Mona Island near Puerto Rico[20] among other places.

Indo-Pacific subpopulation

In the Indian Ocean, hawksbills are a common sight along the east coast of Africa, including the seas surrounding Madagascar and nearby island groups, and all along the southern Asian coast, including the Persian Gulf, the Red Sea, and the coasts of the Indian subcontinent and Southeast Asia. They are present across the Malay Archipelago and northern Australia. Their Pacific range is limited to the ocean's tropical and subtropical regions. In the west, it extends from the southwestern tips of the Korean Peninsula and the Japanese Archipelago south to northern New Zealand.

The Philippines hosts several nesting sites, including the island of Boracay and Punta Dumalag in Davao City. Dahican Beach in Mati City, Davao Oriental hosts one of the most important hatcheries of its kind along with olive ridley sea turtles in the archipelagic country of the Philippines.[21] A small group of islands in the southwest of the archipelago has been named the "Turtle Islands" because two species of sea turtles nest there: the hawksbill and the green sea turtle.[22] In January 2016, a juvenile was seen in Gulf of Thailand.[23] A 2018 article by The Straits Times reported that around 120 hawksbill juvenile turtles recently hatched at Pulau Satumu, Singapore.[24] Commonly found in Singapore waters, hawksbill turtles have returned to areas such East Coast Park and Palau Satumu to nest.[25] In Hawaii, hawksbills mostly nest on the "main" islands of Oahu, Maui, Molokai, and Hawaii.[26] In Australia, hawksbills are known to nest on Milman Island in the Great Barrier Reef.[27] Hawksbill sea turtles nest as far west as Cousine Island in the Seychelles, where the species has been legally protected since 1994, and the population is showing some recovery.[28] The Seychelles' inner islands and islets, such as Aldabra, are popular feeding grounds for immature hawksbills.[9][29]

Eastern Pacific subpopulation

In the eastern Pacific, hawksbills are known to occur from the Baja Peninsula in Mexico south along the coast to southern Peru.[13] Nonetheless, as recently as 2007, the species had been considered largely extirpated in the region.[30] Important remnant nesting and foraging sites have since been discovered in Mexico, El Salvador, Nicaragua, and Ecuador, providing new opportunities for research and conservation. In contrast to their traditional roles in other parts of the world, where hawksbills primarily inhabit coral reefs and rocky substrate areas, in the eastern Pacific, hawksbills tend to forage and nest principally in mangrove estuaries, such as those present in the Bahia de Jiquilisco (El Salvador), Gulf of Fonseca (Nicaragua, El Salvador, and Honduras), Estero Padre Ramos (Nicaragua), and the Gulf of Guayaquil (Ecuador).[31] Multi-national initiatives, such as the Eastern Pacific Hawksbill Initiative, are currently pushing efforts to research and conserve the population, which remains poorly understood.

Habitat and feeding

Habitat

Adult hawksbill sea turtles are primarily found in tropical coral reefs. They are usually seen resting in caves and ledges in and around these reefs throughout the day. As a highly migratory species, they inhabit a wide range of habitats, from the open ocean to lagoons and even mangrove swamps in estuaries.[6][32] Little is known about the habitat preferences of early life-stage E. imbricata; like other young sea turtles, they are assumed to be completely pelagic, remaining at sea until they mature.[33]

Feeding
E. imbricata in a coral reef in Venezuela

While they are omnivorous, sea sponges are their principal food; they constitute 70–95% of the turtles' diets.[34] However, like many spongivores, they feed only on select species, ignoring many others. Caribbean populations feed primarily on the orders Astrophorida, Spirophorida, and Hadromerida in the class Demospongiae.[35] Aside from sponges, hawksbills feed on algae, cnidarians, comb jellies and other jellyfish, and sea anemones.[6] They also feed on the dangerous jellyfish-like hydrozoan, the Portuguese man o' war (Physalia physalis). Hawksbills close their unprotected eyes when they feed on these cnidarians. The man o' war's stinging cells cannot penetrate the turtles' armored heads.[4]

Hawksbills are highly resilient and resistant to their prey. Some of the sponges they eat, such as Aaptos aaptos, Chondrilla nucula, Tethya actinia, Spheciospongia vesparium, and Suberites domuncula, are highly (often lethally) toxic to other organisms. In addition, hawksbills choose sponge species with significant numbers of siliceous spicules, such as Ancorina, Geodia (G. gibberosa[4]), Ecionemia, and Placospongia.[35]

Life history
Young E. imbricata from Réunion Island
A female hawksbill turtle laying eggs on beach at Mona island, Puerto Rico

Not much is known about the life history of hawksbills.[36] Their life history can be divided into three phases, namely the pelagic phase, from hatching to about 20 cm, the benthic phase, when the immature turtles recruit to foraging areas, and the reproductive phase, when they reach sexual maturity.[37][38] The pelagic phase possibly lasts until the turtles reach around 20 cm in length in 1–3 years,[39] reaching sexual maturity at around 40 cm (2–4 years).[40] Hawksbills show a degree of fidelity after recruiting to the benthic phase,[41] however movement to other similar habitats is possible.[42]

Breeding
Hawksbill baby turtle in Paulista, Pernambuco, Brazil

Hawksbills mate biannually in secluded lagoons off their nesting beaches in remote islands throughout their range. The most significant nesting beaches are in Mexico, the Seychelles, Indonesia, Sri Lanka and Australia. Mating season for Atlantic hawksbills usually spans April to November. Indian Ocean populations, such as the Seychelles hawksbill population, mate from September to February.[9] After mating, females drag their heavy bodies high onto the beach during the night. They clear an area of debris and dig a nesting hole using their rear flippers, then lay clutches of eggs and cover them with sand. Caribbean and Florida nests of E. imbricata normally contain around 140 eggs. After the hours-long process, the female returns to the sea.[6][14] Their nests can be found throughout beaches in about 60 countries.

The baby turtles, usually weighing less than 24 g (0.85 oz) hatch at night after around two months. These newly emergent hatchlings are dark-colored, with heart-shaped carapaces measuring around 2.5 cm (0.98 in) long. They instinctively crawl into the sea, attracted by the reflection of the moon on the water (possibly disrupted by light sources such as street lamps and lights). While they emerge under the cover of darkness, baby turtles that do not reach the water by daybreak are preyed upon by shorebirds, shore crabs, and other predators.[6]

Hawksbill hatchling in Puerto Rico

Maturity

Hawksbills evidently reach maturity after 20 years.[14] Their lifespan is unknown.[43] Like other sea turtles, hawksbills are solitary for most of their lives; they meet only to mate. They are highly migratory.[36] Because of their tough carapaces, adults' only predators are sharks, estuarine crocodiles, octopuses, and some species of pelagic fish.[36]

A series of biotic and abiotic cues, such as individual genetics, foraging quantity and quality,[44] or population density, may trigger the maturation of the reproductive organs and the production of gametes and thus determine sexual maturity. Like many reptiles, all marine turtles of a same aggregation are highly unlikely to reach sexual maturity at the same size and thus age.[45] Age at maturity has been estimated to occur between 10[46] and 25 years of age[47] for Caribbean hawksbills. Turtles nesting in the Indo-Pacific region may reach maturity at a minimum of 30 to 35 years.[48][49]

Evolutionary history

Within the sea turtles, E. imbricata has several unique anatomical and ecological traits. It is the only primarily spongivorous reptile. Because of this, its evolutionary position is somewhat unclear. Molecular analyses support placement of Eretmochelys within the taxonomic tribe Carettini, which includes the carnivorous loggerhead and ridley sea turtles, rather than in the tribe Chelonini, which includes the herbivorous green turtle. The hawksbill probably evolved from carnivorous ancestors.[50]

Etymology and taxonomic history
Hawksbill sea turtle (top right) in a 1904 plate by Ernst Haeckel

Linnaeus originally described the hawksbill sea turtle as Testudo imbricata in 1766, in the 12th edition of his Systema Naturae.[51] In 1843, Austrian zoologist Leopold Fitzinger moved it into genus Eretmochelys.[52] In 1857, the species was temporarily misdescribed as Eretmochelys imbricata squamata.[53]

Two subspecies are accepted in E. imbricata's taxon. E. i. bissa (Rüppell, 1835) refers to populations that reside in the Pacific Ocean.[54] The Atlantic population is a separate subspecies, E. i. imbricata (Linnaeus, 1766). The nominate subspecies is the Atlantic taxon, because Linnaeus' type specimen was from the Atlantic.[55]

Fitzinger derived the genus name, Eretmochelys, from the Greek roots eretmo and chelys, corresponding to "oar" and "turtle", respectively. The name refers to the turtles' oar-like front flippers. The species name imbricata is Latin, corresponding to the English term imbricate. This appropriately describes the turtles' overlapping posterior scutes. The Pacific hawksbill's subspecies name, bissa, is Latin for "double". The subspecies was originally described as Caretta bissa; the term referred to the then-species being the second species in the genus.[56] Caretta is the genus of the hawksbill's much larger relative, the loggerhead sea turtle.

Exploitation by humans
Palauan women's money (toluk)

Throughout the world, hawksbill sea turtles are taken by humans, though it is illegal to hunt them in many countries.[57] In some parts of the world, hawksbill sea turtles are eaten as a delicacy. As far back as the fifth century BC, sea turtles, including the hawksbill, were eaten as delicacies in China.[58]

Captured hawksbill sea turtle from Mauritius at Orly Airport customs 1995

Many cultures also use turtle shells for decoration. These turtles have been harvested for their beautiful shell since Egyptian times, and the material known as tortoiseshell is normally from the hawksbill.[59] In China, where it was known as tai mei, the hawksbill is called the "tortoise-shell turtle", named primarily for its shell, which was used for making and decorating a variety of small items, as it was in the West.[58] Along the south coast of Java, stuffed hawksbill turtles are sold in souvenir shops, though numbers have decreased in the last two decades.[60] In Japan, the turtles are also harvested for their shell scutes, which are called bekko in Japanese. It is used in various personal implements, such as eyeglass frames and the shamisen (Japanese traditional three-stringed instrument) picks.[59] In 1994, Japan stopped importing hawksbill shells from other nations. Prior to this, the Japanese hawksbill shell trade was around 30,000 kg (66,000 lb) of raw shells per year.[19][61] In the West, hawksbill sea turtle shells were harvested by the ancient Greeks and ancient Romans for jewellery, such as combs, brushes, and rings.[62] The bulk of the world's hawksbill shell trade originates in the Caribbean. In 2006, processed shells were regularly available, often in large amounts, in countries including the Dominican Republic and Colombia.[63]

The hawksbill sea turtle appears on the reverse side of the Venezuelan 20-bolivar and the Brazilian 2-reais banknotes. A much-beloved fountain sculpture of a boy riding a hawksbill, affectionately known as Turtle Boy, stands in Worcester, Massachusetts.

Conservation
A hawksbill sea turtle in Tobago

Consensus has determined sea turtles, including E. imbricata to be, at the very least, threatened species because of their slow growth and maturity, and slow reproductive rates. Many adult turtles have been killed by humans, both accidentally and deliberately. Their existence is threatened due to pollution and loss of nesting areas because of coastal development. Biologists estimate that the hawksbill population has declined 80 percent in the past 100–135 years.[1] Human and animal encroachment threatens nesting sites, and small mammals dig up the eggs to eat.[6] In the US Virgin Islands, mongooses raid hawksbill nests (along with those of other sea turtles, such as Dermochelys coriacea) right after they are laid.[64]

In 1982, the IUCN Red List of Threatened Species first listed E. imbricata as endangered.[65] This endangered status continued through several reassessments in 1986,[66] 1988,[67] 1990,[68] and 1994[69] until it was upgraded in status to critically endangered in 1996.[1] Two petitions challenged its status as an endangered species prior to this, claiming the turtle (along with three other species) had several significant stable populations worldwide. These petitions were rejected based on their analysis of data submitted by the Marine Turtle Specialist Group (MTSG). The data given by the MTSG showed the worldwide hawksbill sea turtle population had declined by 80% in the three most recent generations, and no significant population increase occurred as of 1996. CR A2 status was denied, however, because the IUCN did not find sufficient data to show the population likely to decrease by a further 80% in the future.[70]

The species (along with the entire family Cheloniidae) has been listed on Appendix I of the Convention on International Trade in Endangered Species.[3] It is prohibited to import or export turtle products, or to kill, capture, or harass hawksbill sea turtles.[57]

Local involvement in conservation efforts has also increased in the past few years.

The United States Fish and Wildlife Service and National Marine Fisheries Service have classified hawksbills as endangered under the Endangered Species Act[71] since 1970. The US government established several recovery plans[72] for protecting E. imbricata.[73] The Zoological Society of London has inscribed the reptile as an EDGE species.[74]

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