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Hawksbill sea turtle

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Hawksbill Turtle
Eretmochelys imbricata in Útila.
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Reptilia
Order:
Testudines
Family:
Cheloniidae
Genus:
Eretmochelys
Species:
E. imbricata
Binomial name
Eretmochelys imbricata
subspecies

Eretmochelys imbricata bissa (Rüppell, 1835)
Eretmochelys imbricata imbricata (Linnaeus, 1766)

Range of the Hawksbill turtle
Synonyms

Eretmochelys imbricata squamata junior synonym

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

The hawksbill's appearance is similar to that of other marine turtles. It has a generally flattened body shape, a protective carapace, and its flipper-like arms are 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. While the turtle lives a part of its life in the open ocean, it is most often encountered in shallow lagoons and coral reefs where it feeds on its chosen prey, sea sponges. Some of the sponges eaten by E. imbricata are known to be highly toxic and lethal when eaten by other organisms. In addition, the sponges that hawksbills eat are usually those with high silica content, making the turtles one of few animals capable of eating siliceous organisms. They also feed on other invertebrates, such as comb jellies and jellyfish.[3]

Because of human fishing practices, Eretmochelys imbricata populations around the world are threatened with extinction and the turtle has been classified as critically endangered by the World Conservation Union.[1] Several countries, such as China and Japan, have valued hunting hawksbill turtles for their flesh, which is considered good eating. Hawksbill turtle shells are the primary source of tortoise shell material, used for decorative purposes. By the Convention on International Trade in Endangered Species, it is illegal to capture and to trade in hawksbill turtles and products derived from them in many nations.[4]

Anatomy and morphology

Eretmochelys imbricata. The carapace's serrated margin and overlapping scutes are evident in this individual.

Eretmochelys imbricata has the typical appearance of a marine turtle. Like the other members of its family, it has a depressed body form and flipper-like limbs adapted for swimming. Adult hawksbill turtles have been known to grow up to a meter in length, weighing around 80 kilograms on average. The heaviest hawksbill ever captured was measured to be 127 kilograms.[3] 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]

The hawksbill turtle has several characteristics that distinguish it from other, closely-related species. Its elongated, tapered head ends in a beak-like mouth (from which its common name is derived), its beak more sharply pronounced and hooked than other sea turtles. The hawksbill's arms have two visible claws on each flipper.

A close-up of the hawksbill's distinct beak.

One of the hawksbill's more-easily distinguished characteristics is the pattern of the thick scutes that make up its carapace. While its carapace has five central scutes and four pairs of lateral scutes like several members of the same 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 itself has been known to reach almost a meter in length.[6]

The sand tracks of hawksbill turtles are asymmetrical, as they crawl on land with an alternating gait. This is opposed to the green sea turtle and the leatherback turtle, which crawl rather symmetrically.[7][8]

Due to its consumption of venomous cnidarians hawksbill turtle flesh can reach certain levels of toxicity.[9]



Distribution

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

Hawksbill 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 tropical waters. Two major subpopulations are acknowledged to exist, the Atlantic and Indo-Pacific subpopulations.[10]

Atlantic subpopulation

A hawksbill turtle in Saba, Netherlands Antilles.

In the Atlantic, E. imbricata populations can be seen as far west as the Gulf of Mexico and the species' eastern range reaches up to the southern tip of the African continent.[11] The northern limits of the species' range can go as far north as Long Island Sound[12] along the northern border of the United States. On the other side of the Atlantic, hawksbills have been sighted in the frigid waters of the English Channel, the species' northernmost occurrence to date. Their southern reach is known all the way to the Cape of Good Hope in Africa.[13][10]

In the Caribbean, they are known from the Brazilian coast (specifically Bahia), southern Florida and Hawaii. They have also been seen on the beaches of Antigua and Barbuda.[6] Costa Rica has its share of E. imbricata nesting sites, specifically in the vicinity of Tortuguero.[13] The island of Cuba is a known feeding ground for the Caribbean hawksbill turtle population.[14] In Puerto Rico, the waters around Mona Island serve as feeding grounds for Caribbean E. imbricata.[15] While a tropical species, E. imbricata has been found in areas in the United States within higher latitudes, such as Massachusetts and Long Island Sound.[12] They have also been seen in the waters off Virginia.[3]

Indo-Pacific subpopulation

The species' Indo-Pacific population is widespread throughout the entire region. In the Indian Ocean, hawksbills are a common sight all along the east coast of the African continent, including the seas surrounding Madagascar and nearby island groups. The species' Indian Ocean range stretches all the way along the coast of Asia, including the Persian Gulf and the Red Sea, along the entire coast of the Indian subcontinent, across the entire Indonesian archipelago and the northwestern coast of Australia. The Pacific range of E. imbricata is somewhat limited to the ocean's tropical and subtropical regions. Its northernmost reach in the region are the waters off the southwestern tip of the Korean peninsula and the Japanese archipelago. The range continues, enveloping the entire region of Southeast Asia, the entire northern coast of Australia all the way south to the northern part of New Zealand. Across the Pacific, hawksbills are known as far north as the Baja peninsula in Mexico, along the waters off the Central American and South American coast to the northern tip of Chile.[10]

In the Philippines, there are several known nesting sites for the species. Hawksbill turtle hatchlings have been found on the island of Boracay.[16] A small group of islands in the southwest of the archipelago have been named the "Turtle Islands" precisely because they are known nesting grounds for two species of sea turtle, including Eretmochelys imbricata. (The other being the green sea turtle, Chelonia mydas.)[17] In Australia, E. imbricata are known to nest on Milman Island in the Great Barrier Reef.[18] In the Indian Ocean, hawksbill turtles have been found to nest as far west as Cousine Island in the Seychelles, where the species has been legally protected since 1994. The Seychelles' inner islands and islets, such as Aldabra Island, are ripe feeding grounds for immature hawksbills.[8][19]

Ecology and life history

Habitat

Adult hawksbill 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 have also been encountered in a wide range of habitats, from the open ocean to lagoons and even mangrove swamps in estuaries.[6][20] While much is not known about the habitat preferences of early-life stage E. imbricata, like other sea turtles' young, they are assumed to be completely pelagic and thus make the open sea their home until they mature.[21]

Feeding ecology

E. imbricata in a coral reef in Tortuga.

While they are known to be omnivorous, the principal food of hawkbill turtles are sponges. Sponges constitute 70 – 95% of the diets of E. imbricata populations in the Caribbean. However like many spongivores, E. imbricata feed only on a few select species, and will ignore many others. The Caribbean hawksbill populations were found to feed primarily on sponges from the class Demospongiae, specifically ones belonging to the orders Astrophorida, Spirophorida and Hadromerida.[22] Select sponge species known to be fed on by these turtles include Geodia gibberosa.[3] Aside from sponges, hawksbills also feed on algae and cnidarians like jellyfish and sea anemones.[6] The hawksbill is also known to 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, for Man o' War's stinging cells cannot penetrate the turtles' armoured heads.[3]


Eretmochelys imbricata have shown themselves to be highly resilient and resistant to their prey. Some of the sponges known to be eaten by hawksbills, such as Aaptos aaptos, Chondrilla nucula, Tethya actinia, Spheciospongia vesparum and Suberites domuncula, are highly (often lethally) toxic to other organisms. In addition, hawksbills are known to choose sponge species that have a significant amount of siliceous spicules, such as Ancorina, Geodia, Ecionemia and Placospongia.[22]

Life history

Young E. imbricata from Réunion Island.

Much is not known about the life history of Eretmochelys imbricata.[23] Hawksbills are known to mate biyearly in secluded lagoons in remote islands throughout their range. Mating season for Atlantic hawksbills usually takes place from April to November. For Indian Ocean populations such as the Seychelles hawksbill population, the mating season is from September to February.[8] As with other sea turtles, hawksbills mate in shallow lagoons off the shores of their prospective nesting beaches. After mating, the females drag their heavy bodies high onto the beach during the night. They will then clear out an area and dig a nesting hole using their rear flippers. The female then lays a clutch of eggs in the nest and then covers them with sand. Caribbean and Florida nests of E. imbricata normally contain around 140 eggs. After the several-hour-long process, the female then returns to the sea. This is the only time when hawksbill turtles are known to leave the ocean.[6][11]

The baby turtles, usually weighing less than two dozen grams, hatch during the night after around two months. These newly-emergent hatchlings are darkly-colored, with heart-shaped carapaces measuring around 2.5 centimeters (1 in) long. They instinctually head for the sea, attracted by the reflection of the moon on the water (a mechanism which can be disrupted by anthropogenic 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 predators such as shorebirds and shore crabs.[6]

E. imbricata hatchling in Grenada.

The early life history of juvenile hawksbill turtles is unknown. Upon reaching the sea, the hatchlings are assumed to enter a pelagic life stage (like other marine turtles) for an undetermined amount of time. While hawksbill turtle growth rates are not known, when E. imbricata juveniles reach around 35 cm, they switch from a pelagic life style to a coral reef-associated one. Hawksbill turtles are hypothesized to reach maturity after thirty years.[11]

While there is no clear consensus because of a lack of data, hawksbill turtles are believed to live from thirty to fifty years in the wild.[24] Like other sea turtles, hawksbill turtles are solitary for most of their lives, they only group together to mate. They were once thought to be habitual, but they are now known to be highly migratory.[23] Because of their tough carapaces, hawksbill turtles have no major predators as there are few creatures that are capable of biting through their protective shell. Sharks and estuarine crocodiles are a few of their natural predators. Octopuses and some species of pelagic fish have also been known to prey on the adult turtles.[23]

Evolutionary history

Within the sea turtles, Eretmochelys imbricata has several unique anatomical and ecological traits, including being the only primarily spongivorous reptile known. Because of this, its evolutionary position has been somewhat unclear. Molecular analyses supports the probability that the Eretmochelydae evolved from carnivorous ancestors rather than herbivorous ones. As the taxonomic tribe Carettini is composed of carnivorous species (such as the loggerhead turtle), the hawksbill most probably evolved from them instead of the herbivorous Chelonini, which includes the green turtle.[25]

Etymology and taxonomic history

Hawksbill turtle (top right) in a 1904 plate by Ernst Haeckel.

The hawksbill turtle was originally described by Carolus Linnaeus as Testudo imbricata in 1766.[2] It was moved into the genus Eretmochelys by the Austrian zoologist Leopold Fitzinger in 1843.[26] In 1857, the species was redescribed as Eretmochelys imbricata squamata, a designation that is now invalid.[27]

There are two accepted subspecies in the E. imbricata taxon. Eretmochelys imbricata bissa (Rüppell, 1835) refers to all known populations of the Eretmochelys imbricata that reside in the Pacific Ocean.[28] The Atlantic population has been found to be a separate subspecies, Eretmochelys imbricata imbricata (Linnaeus, 1766). The subspecies name of imbricata was retained because the type specimen that Linnaeus used to initially describe the species was from the Atlantic.[29]

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 and the term referred to the then-species' being the second species in the genus.[30] Caretta is the genus of the hawksbill's much larger relative, the loggerhead turtle.

Importance to humans

A Japanese tortoiseshell ornament.

Throughout the world, hawksbill turtles are taken by humans even though it is illegal to hunt them in many countries.[31] In some parts of the world, hawksbill turtles are taken and eaten as a delicacy. As far back as the fifth century B.C., sea turtles including the hawksbill were eaten as delicacies in China.[32]

Many cultures also use the turtles' shells for decoration. In China where it was known as tai mei, it is called the tortoise-shell turtle, named primarily for its shell which was used for decoration.[32] In Japan, the turtles are also harvested for their shell scutes, which are called bekko in the local Nihongo. It is used in various personal implements, such as eyeglass frames. In 1994, Japan stopped importing hawksbill shells from other nations. Prior to this, the Japanese hawksbill shell trade was around 30,000 kilograms of raw shells per year.[14][33] In the west, hawksbill turtle shells have been harvested by the ancient Greeks and ancient Romans for jewelry, such as combs, brushes and rings.[34] A bulk of the world's hawksbill turtle shell trade is harvested from the Caribbean. In 2006, it was found that processed shells of the turtles are regularly available, often in large amounts in countries in the region, including the Dominican Republic and Colombia.[35]

Conservation

General consensus has determined sea turtles, including Eretmochelys imbricata to be at the very least, threatened species because of their long lifespans, slow growth and maturity, and slow reproductive rates. Many adult turtles have been killed by humans both deliberately and incidentally. In addition, the nesting sites of the turtles are also threatened by human and animal encroachment. Small mammals have been known to raid the nesting sites and dig up the turtles' eggs.[6] In the U.S. Virgin Islands, Eretmochelys imbricata nests (along with the nests of other sea turtles like Dermochelys coriacea) are often raided by mongooses right after being laid.[36]

File:Hawksbill Turtle 1.JPG.jpg
A hawksbill turtle in Tobago.

In 1996, the IUCN Red List of Threatened Species classified Eretmochelys imbricata as critically endangered.[1] Its status as an endangered species was challenged prior to this, with two petitions claiming that the turtle (along with three other species) had several significant stable populations worldwide. These petitions were rejected by the IUCN based on their analysis of data submitted by the Marine Turtle Specialist Group (MTSG). The data given by the MTSG showed that the worldwide population of hawksbill turtles had been reduced by 80% in the last three of the species' generations, and that was no significant increase in the turtles' populations as of 1996. In light of this data, the IUCN applied the critically endangered (A1) status upon the species. CR A2 status was denied however, because the IUCN believed that there was insufficient data to show that the population of hawksbill turtles were due to decrease by a further 80% in the future.[37]



Historically, Eretmochelys imbricata was first listed as endangered by the IUCN in 1982.[38] This endangered status continued all the way through several reassessments in 1986,[39] 1988,[40] 1990[41] and 1994[42] until it was upgraded in status to critically endangered in 1996 (see above).

The species (along with the entire family Cheloniidae) has been listed on Appendix I of the Convention on International Trade in Endangered Species.[4] It is illegal to import or export turtle products, kill, capture or harass hawksbill turtles.[31]

Local involvement in the conservation efforts for the species have also increased in the past few years. The United States Fish and Wildlife Service has classified the hawksbill turtle as endangered since 1970. The U.S. government has several recovery plans in place for protecting its populations of E. imbricata.[43]

References

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  9. ^ "The Hawksbill Turtle: Eretmochelys imbricata". Auckland Zooaccessdate = 2007-07-14.
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See also

External links

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