BIRGUS LATRO - (LINNAEUS, 1767)
Arthropoda (Phylum) > Crustacea (Subphylum) > Malacostraca (Class) > Eumalacostraca (Subclass) > Eucarida (Superorder) > Decapoda (Order) > Pleocyemata (Suborder) > Anomura (Infraorder) > Paguroidea (Superfamily) > Coenobitidae (Family) > Birgus (Genus)
Crabe des cocotiers, Crabe voleur, pagure larron, crabe de cocotier, Coconut crab, robber crab, palm thief, Granchio del cocco, Cangrejo de los cocoteros, Palmendieb, Caranguejo-dos-coqueiros,
Identification
Bernard-l'ermite terrestre de très grande taille (30 cm). Rostre proéminent et branchiostégites très développés latéralement. Abdomen calcifié et partiellement replié sous le céphalothorax. Pinces fortes et légèrement asymétriques. Quatrième paire de pattes terminée par une petite pince.
Distribution
Le crabe des cocotiers est surtout abondant dans les îles isolées de la zone inter-tropicale de l'océan Indien et dans les îles et atolls du Pacifique ouest à centre. Il est aussi présent sur les côtes de quelques pays continentaux de cette même zone.
Afrique orientale, Tanzanie, Seychelles, Territoire britannique de l'océan Indien, Inde, Thaïlande, îles Cocos (Keeling), île Christmas, Malaisie, Indonésie, Philippines, province de Chine, Taïwan, Japon, Australie, Palau, Papouasie Nouvelle Guinée, Guam, îles Mariannes du Nord, Micronésie, îles Salomon, Nauru, îles Marshall, Nouvelle-Calédonie, Vanuatu, îles Fidji, Wallis et Futuna, Tuvalu, Tokelau, Samoa, Niue, Tonga, îles Cook, Kiribati, Polynésie française (Tuamotu et Gambier), Madagascar (sud ouest).
Biotope
Le crabe des cocotiers est une espèce de bernard-l'ermite supra-littorale qui vit presque exclusivement sur les petites îles tropicales. Avec l'âge, il s'éloigne de la mer et peut se rencontrer jusqu'à 4 km dans les terres. Les mâles habitent généralement plus loin de la côte que les femelles. Ils vivent dans des zones de forêt dense, mais aussi dans divers milieux arborés, dont des cocoteraies sableuses. De manière générale, il se cache la journée dans une faille, une anfractuosité ou un trou, qu'il peut creuser dans le sol pour se protéger de la prédation et du dessèchement. Il se réfugie aussi dans son trou pendant les périodes de mue qui durent environ un mois. Il est généralement actif la nuit, mais peut aussi être vu le jour selon les conditions météorologiques et d'ensoleillement.
Description
Birgus latro est un bernard-l'ermite et le plus grand arthropode terrestre au monde avec une taille moyenne de 30 cm. Les plus gros spécimens peuvent peser plus de 5 kg avec une envergure des pinces qui peut atteindre 1 m.
La couleur du corps varie d'un bleu-violet à un rouge-orange selon les individus et les îles. Le rostre est proéminent et les yeux sub-cylindriques sont rouge grenat. Les branchiostégites sont très développés. Seuls les juvéniles utilisent une coquille de gastéropode pour se protéger. Chez les adultes, les plaques tergales (plaques situées sur la face dorsale) de l'abdomen sont rigidifiées et l'abdomen est droit et partiellement replié sous le céphalothorax.
Comme tous les décapodes, Birgus latro possède cinq paires de pattes, de l'avant vers l'arrière : deux pinces puissantes, légèrement asymétriques, utilisées pour décortiquer les noix de coco ; Trois paires de pattes locomotrices, la troisième paire de pattes ayant la particularité de posséder une petite pince ; Une paire de toutes petites pattes, cachées sous la carapace, non visibles dorsalement. Les mâles sont généralement plus grands que les femelles.
Espèces ressemblantes
Le crabe des cocotiers adulte a une morphologie très particulière qui ne permet pas de le confondre avec une autre espèce. Cependant, les formes jeunes du Birgus latro ressemblent beaucoup à des Coenobita, et il est facile de les confondre. Son nom commun "crabe des cocotiers" entraîne souvent des confusions. En effet, ce n'est pas un crabe à proprement parler, mais un bernard-l'ermite, et la même dénomination est couramment utilisée dans les Caraïbes pour désigner l'espèce Cardisoma guanhumi (Latreille, 1828) qui est fréquemment observée sur les palmes des cocotiers.
Synonymes
Birgus laticauda (Latreille, 1829)
Cancer crumenatus (Rumphius, 1705)
Cancer crumenatus orientalis (Seba, 1759)
Cancer latro (Linnaeus, 1767)
Origine du nom français
Le nom "crabe des cocotiers" fait référence au milieu des cocoteraies dans lequel il vit. Ce crustacé peut grimper facilement dans les cocotiers. Les autres noms communs "crabe voleur" et "pagure larron" font référence au latin latro, signifiant voleur. Pagure est un synonyme de bernard-l'ermite (s'écrivant aussi bernard-l'hermite), en référence à la supra-famille des Paguridés.
Etymologie
Birgus : le genre créé par Leach en 1815 n'est pas expliqué par l'auteur. Ce n'est pas une racine grecque ou latine.
latro : du latin [latro] = voleur, brigand. Le crabe des cocotiers est en effet capable de voler des objets comportant des traces de nourriture à proximité des habitations, voire même à l'intérieur.
Alimentation
Birgus latro est omnivore, nécrophage, voire cannibale. Il est opportuniste, mais ne se nourrit que d'aliments à fort niveau énergétique, tels que des fruits et des graines, comme les noix de coco, très riches en lipides. Il chasse aussi des espèces de crabes sympatriques, comme le crabe rouge Gecarcoidea natalis et Discoplax hirtipes. Il a aussi été observé se nourrissant de charognes de crabes et de bernard-l'ermite, dont des Birgus latro, d'oiseaux, de poissons et de tortues. Après la mue, il mange son exuvie pour restaurer ses réserves calciques.
Reproduction - Multiplication
La maturité sexuelle du crabe des cocotiers est atteinte vers 5 ans et il pourrait vivre 30 à 50 ans. La période de reproduction est estivale, mais semble varier selon les îles et leur climat. L'accouplement se déroule exclusivement à terre, principalement en forêt et en phase d'intermue. Le mâle dépose son spermatophore sur les orifices génitaux de la femelle permettant la fécondation externe des ovocytes lors de leur expulsion, par des mouvements continus des pléopodes de la femelle. Les œufs fécondés (50 000 environ par ponte) sont portés par la femelle sous son abdomen pendant 1 mois environ. Seules les femelles grainées migrent la nuit vers la mer à marée haute. Elles y relâchent leurs œufs en se faisant balayer par les vagues. Une fois relâchés en mer, les œufs éclosent immédiatement. Le développement larvaire se déroule en 2 à 3 semaines avec 4 stades larvaires de type zoé. Les larves restent dans les couches superficielles de l'océan par géotactisme (réaction dûe à la gravité, ici, elles se dirigent dans le sens inverse de cette force vers les couches superficielles) et phototactisme (réaction à une variation d'intensité lumineuse). Le dernier stade larvaire est le stade glaucothoé. Ce stade amphibie dure 3 semaines avec récupération d'une coquille de gastéropode pour la protection de l'abdomen. Les larves glaucothoés remontent sur l'estran et se métamorphosent en bernard-l'ermite juvéniles. Après plusieurs mues, elles abandonnent leur coquille de gastéropode qui leur servait de protection.
Vie associée
Birgus latro vit seul, mais est observé fréquemment en sympatrie avec des espèces de crabes terrestres (Gecarcoidea natalis et Discoplax hirtipes) et d'autres bernard-l'ermite du genre Coenobita (Coenobita brevimanus, Coenobita rugosus et Coenobita perlatus).
Divers biologie
Comme tous les crustacés, le crabe des cocotiers grandit par mues successives. Elles durent environ 1 mois. La fréquence des mues, ainsi que l'augmentation de taille qu'elles permettent, deviennent moindres avec l'âge. On estime qu'il faut environ 40 ans pour atteindre une taille de 40 cm.
Les branchies du crabe des cocotiers constituent un poumon rudimentaire. Malgré une vie aérienne, il a néanmoins régulièrement besoin de points d'eau pour hydrater ses branchies et pour réguler la balance hydrominérale de son organisme. Il n'a cependant pas besoin de s'immerger complètement et se noie s'il est maintenu trop longtemps sous l'eau.
Des espèces allochtones introduites telles que les cochons, les rats et les lézards, peuvent constituer une menace pour les stades juvéniles.
Le crabe des cocotiers ne se nourrit pas exlusivement de noix de coco. Il peut mettre au menu des fruits toxiques pour l'homme, comme ceux du faux manguier. Il y a donc un risque d'intoxication grave notamment en mangeant l'abdomen, et même des décès de temps en temps.
Espèce réglementée
Espèce protégée en Polynésie française. Elle est inscrite en Déficience de Données (DD) dans la liste rouge de l'UICN. Même si elle n'est pas classée comme espèce en danger par l'UICN, cette espèce est très vulnérable, en voie d'extinction dans toutes les zones où elle est soumise à une chasse intensive. Plusieurs pays ont mis en place des mesures de protection : taille limite de capture, protection des individus de couleur orange...
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The coconut crab, Birgus latro, is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest land-living arthropod in the world, and is probably at the upper size limit for terrestrial animals with exoskeletons in recent Earth atmosphere, with a weight of up to 4.1 kg (9.0 lb). It can grow to up to 1 m (3 ft) in length from leg to leg. It is found on islands across the Indian Ocean and parts of the Pacific Ocean.
Description
Birgus latro is the largest land-living arthropod in the world; reports about its size vary, but most sources give a body length of up to 40 cm (16 in), a weight of up to 4.1 kg (9.0 lb), and a leg span of more than 0.91 m (3.0 ft), with males generally being larger than females. The carapace may reach a length of 78 mm (3.1 in), and a width of up to 200 mm (7.9 in). The body of the coconut crab, like that of all decapods, is divided into a front section (cephalothorax), which has 10 legs, and an abdomen. The front-most pair of legs has large chelae (claws), with the left being larger than the right. The next two pairs, as with other hermit crabs, are large, powerful walking legs with pointed tips, which allow coconut crabs to climb vertical or overhanging surfaces. The fourth pair of legs is smaller with tweezer-like chelae at the end, allowing young coconut crabs to grip the inside of a shell or coconut husk to carry for protection; adults use this pair for walking and climbing. The last pair, very small, is used by females to tend their eggs, and by the males in mating. This last pair of legs is usually held inside the carapace, in the cavity containing the breathing organs. Some difference in colour is seen between the animals found on different islands, ranging from orange-red to purplish-blue; in most regions, blue is the predominant colour, but in some places, including the Seychelles, most individuals are red.Although Birgus latro is a derived type of hermit crab, only the juveniles use salvaged snail shells to protect their soft abdomens, and adolescents sometimes use broken coconut shells to protect their abdomens. Unlike other hermit crabs, the adult coconut crabs do not carry shells, but instead have hardened abdominal terga from deposited chitin and chalk. Not being constrained by the physical confines of living in a shell allows this species to grow much larger than other hermit crabs in the family Coenobitidae. Like most true crabs, Birgus latro bends its tail underneath its body for protection. The hardened abdomen protects the coconut crab and reduces water loss on land, but must be moulted periodically. Adults moult annually, and dig a burrow up to 1 m (3 ft 3 in) long in which to hide while vulnerable. It remains in the burrow for three to 16 weeks, depending on the size of the animal. After moulting, it takes one to three weeks for the exoskeleton to harden, depending on the animal's size, during which time the animal's body is soft and vulnerable, and it stays hidden for protection.
Respiration
Except as larvae, coconut crabs cannot swim, and they will drown if left in water for more than an hour. They use a special organ called a branchiostegal lung to breathe. This organ can be interpreted as a developmental stage between gills and lungs, and is one of the most significant adaptations of the coconut crab to its habitat. The branchiostegal lung contains a tissue similar to that found in gills, but suited to the absorption of oxygen from air, rather than water. This organ is expanded laterally and is evaginated to increase its surface area; located in the cephalothorax, it is optimally placed to reduce both the blood/gas diffusion distance and the return distance of oxygenated blood to the pericardium. Coconut crabs use their hindmost, smallest pair of legs to clean these breathing organs and to moisten them with water. The organs require water to function properly, and the coconut crab provides this by stroking its wet legs over the spongy tissues nearby. Coconut crabs may drink water from small puddles by transferring it from their chelipeds to their maxillipeds. In addition to the branchiostegal lung, the coconut crab has an additional rudimentary set of gills. Although these gills are comparable in number to aquatic species from the families Paguridae and the Diogenidae, they are reduced in size and have comparatively less surface area.
Sense of smell
The coconut crab has a well-developed sense of smell, which it uses to locate its food. The process of smelling works very differently depending on whether the smelled molecules are hydrophilic molecules in water or hydrophobic molecules in air. As most crabs live in the water, they have specialised organs called aesthetascs on their antennae to determine both the concentration and the direction of a smell. However, as coconut crabs live on the land, the aesthetascs on their antennae are shorter and blunter than those of other crabs and look more like those of insects. While insects and the coconut crabs originate from different paths, the same need to detect smells in the air led to the development of remarkably similar organs. Coconut crabs flick their antennae as insects do to enhance their reception. They have an excellent sense of smell and can detect interesting odours over large distances. The smells of rotting meat, bananas, and coconuts, all potential food sources, catch their attention especially. The olfactory system in the coconut crab's brain is well-developed compared to other areas of the brain.
Lifecycle
Coconut crabs mate frequently and quickly on dry land from May to September, especially between early June and late August. Male coconut crabs deposit a mass of spermatophores on the abdomen of the female; the abdomen opens at the base of the third pereiopods, and fertilisation is thought to occur on the external surface of the abdomen as the eggs pass through the spermatophore mass. The extrusion of eggs occurs on land in crevices or burrows near the shore. Shortly thereafter, the female lays her eggs and glues them to the underside of her abdomen, carrying the fertilised eggs underneath her body for a few months. At the time of hatching, the female coconut crab releases the eggs into the ocean. This usually takes place on rocky shores at dusk, especially when this coincides with high tide. The empty egg cases remain on the female's body after the larvae have been released, and the female eats the egg cases within a few days.
The larvae float in the pelagic zone of the ocean with other plankton for three to four weeks, during which a large number of them are eaten by predators. The larvae pass through three to five zoea stages before moulting into the postlarval glaucothoe stage; this process takes from 25 to 33 days. Upon reaching the glaucothoe stage of development, they settle to the bottom, find and wear a suitably sized gastropod shell, and migrate to the shoreline with other terrestrial hermit crabs. At that time, they sometimes visit dry land. Afterwards, they leave the ocean permanently and lose the ability to breathe in water. As with all hermit crabs, they change their shells as they grow. Young coconut crabs that cannot find a seashell of the right size often use broken coconut pieces. When they outgrow their shells, they develop a hardened abdomen. The coconut crab reaches sexual maturity around five years after hatching. They reach their maximum size only after 40 to 60 years.
Distribution
Coconut crabs live in the Indian Ocean and the central Pacific Ocean; their distribution closely matches that of the coconut palm. The western limit of the range of Birgus latro is Zanzibar, off the coast of Tanzania, while the Tropic of Cancer and Tropic of Capricorn mark the northern and southern limits, respectively, with very few population in the subtropics, such as the Ryukyu Islands. The coconut crab once lived on the mainlands of Australia and Madagascar and on the island of Mauritius, but it no longer occurs in any of these places. As they cannot swim as adults, coconut crabs must have colonised the islands as planktonic larvae.
Christmas Island in the Indian Ocean has the largest and densest population of coconut crabs in the world, although it is outnumbered there by more than 50 times by the Christmas Island red crab, Gecarcoidea natalis. Other Indian Ocean populations exist on the Seychelles, including Aldabra and Cosmoledo, but the coconut crab is extinct on the central islands. Coconut crabs occur on several of the Andaman and Nicobar Islands in the Bay of Bengal. They occur on most of the islands, and the northern atolls, of the Chagos Archipelago.
In the Pacific, the coconut crab's range became known gradually. Charles Darwin believed it was only found on "a single coral island north of the Society group". The coconut crab is far more widespread, though it is not abundant on every Pacific island it inhabits. Large populations exist on the Cook Islands, especially Pukapuka, Suwarrow, Mangaia, Takutea, Mauke, Atiu, and Palmerston Island. These are close to the eastern limit of its range, as are the Line Islands of Kiribati, where the coconut crab is especially frequent on Teraina (Washington Island), with its abundant coconut palm forest. The Gambier Islands marks the species' eastern limit.
Ecology
Diet
The diet of coconut crabs consists primarily of fleshy fruits (particularly Ochrosia ackeringae, Arenga listeri, Pandanus elatus, P. christmatensis), nuts (coconuts Cocos nucifera, Aleurites moluccana), and seeds (Annona reticulata), and on the pith of fallen trees. However, as they are omnivores, they will consume other organic materials such as tortoise hatchlings and dead animals. They have been observed to prey upon crabs, such as Gecarcoidea natalis and Discoplax hirtipes, and scavenge on the carcasses of other coconut crabs. During a tagging experiment, one coconut crab was observed killing and eating a Polynesian rat (Rattus exulans). Coconut crabs may be responsible for the disappearance of Amelia Earhart's remains, consuming them after her death and hoarding her skeletal remnants in their burrows.
The coconut crab can take a coconut from the ground and cut it to a husk nut, take it with its claw, climb up a tree 10 m (33 ft) high and drop the husk nut, to access the content inside. They often descend from the trees by falling, and can survive a fall of at least 4.5 m (15 ft) unhurt. Coconut crabs cut holes into coconuts with their strong claws and eat the contents, although it can take several days before the coconut is opened.
Thomas Hale Streets discussed their behaviour in 1877, doubting that the animal would climb trees to get at the nuts. In the 1980s, Holger Rumpff was able to confirm Streets's report, observing and studying how they open coconuts in the wild. The animal has developed a special technique to do so: if the coconut is still covered with husk, it will use its claws to rip off strips, always starting from the side with the three germination pores, the group of three small circles found on the outside of the coconut. Once the pores are visible, the coconut crab will bang its pincers on one of them until they break. Afterwards, it will turn around and use the smaller pincers on its other legs to pull out the white flesh of the coconut. Using their strong claws, larger individuals can even break the hard coconut into smaller pieces for easier consumption.
Habitat
Coconut crabs are considered one of the most terrestrial decapods, with most aspects of its life linked to a terrestrial existence; they will drown in sea water in less than a day. Coconut crabs live alone in underground burrows and rock crevices, depending on the local terrain. They dig their own burrows in sand or loose soil. During the day, the animal stays hidden to reduce water loss from heat. The coconut crab's burrows contain very fine yet strong fibres of the coconut husk which the animal uses as bedding. While resting in its burrow, it closes the entrances with one of its claws to create the moist microclimate within the burrow necessary for its breathing organs. In areas with a large coconut crab population, some may come out during the day, perhaps to gain an advantage in the search for food. Other times they will emerge if it is moist or raining, since these conditions allow them to breathe more easily. They live almost exclusively on land, returning to the sea only to release their eggs; on Christmas Island, for instance, Birgus latro is abundant 6 km (3.7 mi) from the sea.
Relationship with human beings
Adult coconut crabs have no known predators apart from other coconut crabs and people. Its large size and the quality of its meat mean the coconut crab is extensively hunted and is rare on islands with a human population. The coconut crab is eaten by Southeast Asians and Pacific Islanders and is considered a delicacy and an aphrodisiac, and intensive hunting has threatened the species' survival in some areas. While the coconut crab itself is not innately poisonous, it may become so depending on its diet, and cases of coconut crab poisoning have occurred. For instance, consumption of the sea mango Cerbera manghas by the coconut crab may make the coconut crab toxic due to the presence of cardiac cardenolides.
The pincers of the coconut crab are powerful enough to cause noticeable pain to a human; furthermore, the coconut crab will often keep its hold for extended periods of time. Thomas Hale Streets reported a trick used by Micronesians of the Line Islands to get a coconut crab to loosen its grip: "It may be interesting to know that in such a dilemma a gentle titillation of the under soft parts of the body with any light material will cause the crab to loosen its hold."
In the Cook Islands, the coconut crab is known as unga or kaveu, and in the Mariana Islands it is called ayuyu, and is sometimes associated with taotaomo'na because of the traditional belief that ancestral spirits can return in the form of animals such as the coconut crab.
Conservation
Coconut crab populations in several areas have declined or become locally extinct due to both habitat loss and human predation. In 1981, it was listed on the IUCN Red List as a vulnerable species, but a lack of biological data caused its assessment to be amended to Data Deficient in 1996.
Conservation management strategies have been put in place in some regions, such as minimum legal size limit restrictions in Guam and Vanuatu, and a ban on the capture of egg-bearing females in Guam and the Federated States of Micronesia. In the Northern Mariana Islands, hunting of adults not bearing eggs above a carapace length of 30 mm (1.2 in) may take place in September, October, and November, and only under licence. A bag limit of five coconut crabs on any given day, and 15 across the whole season has been set.
In Tuvalu, coconut crabs live on the motu (islets) in the Funafuti Conservation Area, a marine conservation area covering 33 km2 (12.74 mi2) of reef, lagoon and motu on the western side of Funafuti atoll.
Crabe des cocotiers, Crabe voleur, pagure larron, crabe de cocotier, Coconut crab, robber crab, palm thief, Granchio del cocco, Cangrejo de los cocoteros, Palmendieb, Caranguejo-dos-coqueiros,
Identification
Bernard-l'ermite terrestre de très grande taille (30 cm). Rostre proéminent et branchiostégites très développés latéralement. Abdomen calcifié et partiellement replié sous le céphalothorax. Pinces fortes et légèrement asymétriques. Quatrième paire de pattes terminée par une petite pince.
Distribution
Le crabe des cocotiers est surtout abondant dans les îles isolées de la zone inter-tropicale de l'océan Indien et dans les îles et atolls du Pacifique ouest à centre. Il est aussi présent sur les côtes de quelques pays continentaux de cette même zone.
Afrique orientale, Tanzanie, Seychelles, Territoire britannique de l'océan Indien, Inde, Thaïlande, îles Cocos (Keeling), île Christmas, Malaisie, Indonésie, Philippines, province de Chine, Taïwan, Japon, Australie, Palau, Papouasie Nouvelle Guinée, Guam, îles Mariannes du Nord, Micronésie, îles Salomon, Nauru, îles Marshall, Nouvelle-Calédonie, Vanuatu, îles Fidji, Wallis et Futuna, Tuvalu, Tokelau, Samoa, Niue, Tonga, îles Cook, Kiribati, Polynésie française (Tuamotu et Gambier), Madagascar (sud ouest).
Biotope
Le crabe des cocotiers est une espèce de bernard-l'ermite supra-littorale qui vit presque exclusivement sur les petites îles tropicales. Avec l'âge, il s'éloigne de la mer et peut se rencontrer jusqu'à 4 km dans les terres. Les mâles habitent généralement plus loin de la côte que les femelles. Ils vivent dans des zones de forêt dense, mais aussi dans divers milieux arborés, dont des cocoteraies sableuses. De manière générale, il se cache la journée dans une faille, une anfractuosité ou un trou, qu'il peut creuser dans le sol pour se protéger de la prédation et du dessèchement. Il se réfugie aussi dans son trou pendant les périodes de mue qui durent environ un mois. Il est généralement actif la nuit, mais peut aussi être vu le jour selon les conditions météorologiques et d'ensoleillement.
Description
Birgus latro est un bernard-l'ermite et le plus grand arthropode terrestre au monde avec une taille moyenne de 30 cm. Les plus gros spécimens peuvent peser plus de 5 kg avec une envergure des pinces qui peut atteindre 1 m.
La couleur du corps varie d'un bleu-violet à un rouge-orange selon les individus et les îles. Le rostre est proéminent et les yeux sub-cylindriques sont rouge grenat. Les branchiostégites sont très développés. Seuls les juvéniles utilisent une coquille de gastéropode pour se protéger. Chez les adultes, les plaques tergales (plaques situées sur la face dorsale) de l'abdomen sont rigidifiées et l'abdomen est droit et partiellement replié sous le céphalothorax.
Comme tous les décapodes, Birgus latro possède cinq paires de pattes, de l'avant vers l'arrière : deux pinces puissantes, légèrement asymétriques, utilisées pour décortiquer les noix de coco ; Trois paires de pattes locomotrices, la troisième paire de pattes ayant la particularité de posséder une petite pince ; Une paire de toutes petites pattes, cachées sous la carapace, non visibles dorsalement. Les mâles sont généralement plus grands que les femelles.
Espèces ressemblantes
Le crabe des cocotiers adulte a une morphologie très particulière qui ne permet pas de le confondre avec une autre espèce. Cependant, les formes jeunes du Birgus latro ressemblent beaucoup à des Coenobita, et il est facile de les confondre. Son nom commun "crabe des cocotiers" entraîne souvent des confusions. En effet, ce n'est pas un crabe à proprement parler, mais un bernard-l'ermite, et la même dénomination est couramment utilisée dans les Caraïbes pour désigner l'espèce Cardisoma guanhumi (Latreille, 1828) qui est fréquemment observée sur les palmes des cocotiers.
Synonymes
Birgus laticauda (Latreille, 1829)
Cancer crumenatus (Rumphius, 1705)
Cancer crumenatus orientalis (Seba, 1759)
Cancer latro (Linnaeus, 1767)
Origine du nom français
Le nom "crabe des cocotiers" fait référence au milieu des cocoteraies dans lequel il vit. Ce crustacé peut grimper facilement dans les cocotiers. Les autres noms communs "crabe voleur" et "pagure larron" font référence au latin latro, signifiant voleur. Pagure est un synonyme de bernard-l'ermite (s'écrivant aussi bernard-l'hermite), en référence à la supra-famille des Paguridés.
Etymologie
Birgus : le genre créé par Leach en 1815 n'est pas expliqué par l'auteur. Ce n'est pas une racine grecque ou latine.
latro : du latin [latro] = voleur, brigand. Le crabe des cocotiers est en effet capable de voler des objets comportant des traces de nourriture à proximité des habitations, voire même à l'intérieur.
Alimentation
Birgus latro est omnivore, nécrophage, voire cannibale. Il est opportuniste, mais ne se nourrit que d'aliments à fort niveau énergétique, tels que des fruits et des graines, comme les noix de coco, très riches en lipides. Il chasse aussi des espèces de crabes sympatriques, comme le crabe rouge Gecarcoidea natalis et Discoplax hirtipes. Il a aussi été observé se nourrissant de charognes de crabes et de bernard-l'ermite, dont des Birgus latro, d'oiseaux, de poissons et de tortues. Après la mue, il mange son exuvie pour restaurer ses réserves calciques.
Reproduction - Multiplication
La maturité sexuelle du crabe des cocotiers est atteinte vers 5 ans et il pourrait vivre 30 à 50 ans. La période de reproduction est estivale, mais semble varier selon les îles et leur climat. L'accouplement se déroule exclusivement à terre, principalement en forêt et en phase d'intermue. Le mâle dépose son spermatophore sur les orifices génitaux de la femelle permettant la fécondation externe des ovocytes lors de leur expulsion, par des mouvements continus des pléopodes de la femelle. Les œufs fécondés (50 000 environ par ponte) sont portés par la femelle sous son abdomen pendant 1 mois environ. Seules les femelles grainées migrent la nuit vers la mer à marée haute. Elles y relâchent leurs œufs en se faisant balayer par les vagues. Une fois relâchés en mer, les œufs éclosent immédiatement. Le développement larvaire se déroule en 2 à 3 semaines avec 4 stades larvaires de type zoé. Les larves restent dans les couches superficielles de l'océan par géotactisme (réaction dûe à la gravité, ici, elles se dirigent dans le sens inverse de cette force vers les couches superficielles) et phototactisme (réaction à une variation d'intensité lumineuse). Le dernier stade larvaire est le stade glaucothoé. Ce stade amphibie dure 3 semaines avec récupération d'une coquille de gastéropode pour la protection de l'abdomen. Les larves glaucothoés remontent sur l'estran et se métamorphosent en bernard-l'ermite juvéniles. Après plusieurs mues, elles abandonnent leur coquille de gastéropode qui leur servait de protection.
Vie associée
Birgus latro vit seul, mais est observé fréquemment en sympatrie avec des espèces de crabes terrestres (Gecarcoidea natalis et Discoplax hirtipes) et d'autres bernard-l'ermite du genre Coenobita (Coenobita brevimanus, Coenobita rugosus et Coenobita perlatus).
Divers biologie
Comme tous les crustacés, le crabe des cocotiers grandit par mues successives. Elles durent environ 1 mois. La fréquence des mues, ainsi que l'augmentation de taille qu'elles permettent, deviennent moindres avec l'âge. On estime qu'il faut environ 40 ans pour atteindre une taille de 40 cm.
Les branchies du crabe des cocotiers constituent un poumon rudimentaire. Malgré une vie aérienne, il a néanmoins régulièrement besoin de points d'eau pour hydrater ses branchies et pour réguler la balance hydrominérale de son organisme. Il n'a cependant pas besoin de s'immerger complètement et se noie s'il est maintenu trop longtemps sous l'eau.
Des espèces allochtones introduites telles que les cochons, les rats et les lézards, peuvent constituer une menace pour les stades juvéniles.
Le crabe des cocotiers ne se nourrit pas exlusivement de noix de coco. Il peut mettre au menu des fruits toxiques pour l'homme, comme ceux du faux manguier. Il y a donc un risque d'intoxication grave notamment en mangeant l'abdomen, et même des décès de temps en temps.
Espèce réglementée
Espèce protégée en Polynésie française. Elle est inscrite en Déficience de Données (DD) dans la liste rouge de l'UICN. Même si elle n'est pas classée comme espèce en danger par l'UICN, cette espèce est très vulnérable, en voie d'extinction dans toutes les zones où elle est soumise à une chasse intensive. Plusieurs pays ont mis en place des mesures de protection : taille limite de capture, protection des individus de couleur orange...
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The coconut crab, Birgus latro, is a species of terrestrial hermit crab, also known as the robber crab or palm thief. It is the largest land-living arthropod in the world, and is probably at the upper size limit for terrestrial animals with exoskeletons in recent Earth atmosphere, with a weight of up to 4.1 kg (9.0 lb). It can grow to up to 1 m (3 ft) in length from leg to leg. It is found on islands across the Indian Ocean and parts of the Pacific Ocean.
Description
Birgus latro is the largest land-living arthropod in the world; reports about its size vary, but most sources give a body length of up to 40 cm (16 in), a weight of up to 4.1 kg (9.0 lb), and a leg span of more than 0.91 m (3.0 ft), with males generally being larger than females. The carapace may reach a length of 78 mm (3.1 in), and a width of up to 200 mm (7.9 in). The body of the coconut crab, like that of all decapods, is divided into a front section (cephalothorax), which has 10 legs, and an abdomen. The front-most pair of legs has large chelae (claws), with the left being larger than the right. The next two pairs, as with other hermit crabs, are large, powerful walking legs with pointed tips, which allow coconut crabs to climb vertical or overhanging surfaces. The fourth pair of legs is smaller with tweezer-like chelae at the end, allowing young coconut crabs to grip the inside of a shell or coconut husk to carry for protection; adults use this pair for walking and climbing. The last pair, very small, is used by females to tend their eggs, and by the males in mating. This last pair of legs is usually held inside the carapace, in the cavity containing the breathing organs. Some difference in colour is seen between the animals found on different islands, ranging from orange-red to purplish-blue; in most regions, blue is the predominant colour, but in some places, including the Seychelles, most individuals are red.Although Birgus latro is a derived type of hermit crab, only the juveniles use salvaged snail shells to protect their soft abdomens, and adolescents sometimes use broken coconut shells to protect their abdomens. Unlike other hermit crabs, the adult coconut crabs do not carry shells, but instead have hardened abdominal terga from deposited chitin and chalk. Not being constrained by the physical confines of living in a shell allows this species to grow much larger than other hermit crabs in the family Coenobitidae. Like most true crabs, Birgus latro bends its tail underneath its body for protection. The hardened abdomen protects the coconut crab and reduces water loss on land, but must be moulted periodically. Adults moult annually, and dig a burrow up to 1 m (3 ft 3 in) long in which to hide while vulnerable. It remains in the burrow for three to 16 weeks, depending on the size of the animal. After moulting, it takes one to three weeks for the exoskeleton to harden, depending on the animal's size, during which time the animal's body is soft and vulnerable, and it stays hidden for protection.
Respiration
Except as larvae, coconut crabs cannot swim, and they will drown if left in water for more than an hour. They use a special organ called a branchiostegal lung to breathe. This organ can be interpreted as a developmental stage between gills and lungs, and is one of the most significant adaptations of the coconut crab to its habitat. The branchiostegal lung contains a tissue similar to that found in gills, but suited to the absorption of oxygen from air, rather than water. This organ is expanded laterally and is evaginated to increase its surface area; located in the cephalothorax, it is optimally placed to reduce both the blood/gas diffusion distance and the return distance of oxygenated blood to the pericardium. Coconut crabs use their hindmost, smallest pair of legs to clean these breathing organs and to moisten them with water. The organs require water to function properly, and the coconut crab provides this by stroking its wet legs over the spongy tissues nearby. Coconut crabs may drink water from small puddles by transferring it from their chelipeds to their maxillipeds. In addition to the branchiostegal lung, the coconut crab has an additional rudimentary set of gills. Although these gills are comparable in number to aquatic species from the families Paguridae and the Diogenidae, they are reduced in size and have comparatively less surface area.
Sense of smell
The coconut crab has a well-developed sense of smell, which it uses to locate its food. The process of smelling works very differently depending on whether the smelled molecules are hydrophilic molecules in water or hydrophobic molecules in air. As most crabs live in the water, they have specialised organs called aesthetascs on their antennae to determine both the concentration and the direction of a smell. However, as coconut crabs live on the land, the aesthetascs on their antennae are shorter and blunter than those of other crabs and look more like those of insects. While insects and the coconut crabs originate from different paths, the same need to detect smells in the air led to the development of remarkably similar organs. Coconut crabs flick their antennae as insects do to enhance their reception. They have an excellent sense of smell and can detect interesting odours over large distances. The smells of rotting meat, bananas, and coconuts, all potential food sources, catch their attention especially. The olfactory system in the coconut crab's brain is well-developed compared to other areas of the brain.
Lifecycle
Coconut crabs mate frequently and quickly on dry land from May to September, especially between early June and late August. Male coconut crabs deposit a mass of spermatophores on the abdomen of the female; the abdomen opens at the base of the third pereiopods, and fertilisation is thought to occur on the external surface of the abdomen as the eggs pass through the spermatophore mass. The extrusion of eggs occurs on land in crevices or burrows near the shore. Shortly thereafter, the female lays her eggs and glues them to the underside of her abdomen, carrying the fertilised eggs underneath her body for a few months. At the time of hatching, the female coconut crab releases the eggs into the ocean. This usually takes place on rocky shores at dusk, especially when this coincides with high tide. The empty egg cases remain on the female's body after the larvae have been released, and the female eats the egg cases within a few days.
The larvae float in the pelagic zone of the ocean with other plankton for three to four weeks, during which a large number of them are eaten by predators. The larvae pass through three to five zoea stages before moulting into the postlarval glaucothoe stage; this process takes from 25 to 33 days. Upon reaching the glaucothoe stage of development, they settle to the bottom, find and wear a suitably sized gastropod shell, and migrate to the shoreline with other terrestrial hermit crabs. At that time, they sometimes visit dry land. Afterwards, they leave the ocean permanently and lose the ability to breathe in water. As with all hermit crabs, they change their shells as they grow. Young coconut crabs that cannot find a seashell of the right size often use broken coconut pieces. When they outgrow their shells, they develop a hardened abdomen. The coconut crab reaches sexual maturity around five years after hatching. They reach their maximum size only after 40 to 60 years.
Distribution
Coconut crabs live in the Indian Ocean and the central Pacific Ocean; their distribution closely matches that of the coconut palm. The western limit of the range of Birgus latro is Zanzibar, off the coast of Tanzania, while the Tropic of Cancer and Tropic of Capricorn mark the northern and southern limits, respectively, with very few population in the subtropics, such as the Ryukyu Islands. The coconut crab once lived on the mainlands of Australia and Madagascar and on the island of Mauritius, but it no longer occurs in any of these places. As they cannot swim as adults, coconut crabs must have colonised the islands as planktonic larvae.
Christmas Island in the Indian Ocean has the largest and densest population of coconut crabs in the world, although it is outnumbered there by more than 50 times by the Christmas Island red crab, Gecarcoidea natalis. Other Indian Ocean populations exist on the Seychelles, including Aldabra and Cosmoledo, but the coconut crab is extinct on the central islands. Coconut crabs occur on several of the Andaman and Nicobar Islands in the Bay of Bengal. They occur on most of the islands, and the northern atolls, of the Chagos Archipelago.
In the Pacific, the coconut crab's range became known gradually. Charles Darwin believed it was only found on "a single coral island north of the Society group". The coconut crab is far more widespread, though it is not abundant on every Pacific island it inhabits. Large populations exist on the Cook Islands, especially Pukapuka, Suwarrow, Mangaia, Takutea, Mauke, Atiu, and Palmerston Island. These are close to the eastern limit of its range, as are the Line Islands of Kiribati, where the coconut crab is especially frequent on Teraina (Washington Island), with its abundant coconut palm forest. The Gambier Islands marks the species' eastern limit.
Ecology
Diet
The diet of coconut crabs consists primarily of fleshy fruits (particularly Ochrosia ackeringae, Arenga listeri, Pandanus elatus, P. christmatensis), nuts (coconuts Cocos nucifera, Aleurites moluccana), and seeds (Annona reticulata), and on the pith of fallen trees. However, as they are omnivores, they will consume other organic materials such as tortoise hatchlings and dead animals. They have been observed to prey upon crabs, such as Gecarcoidea natalis and Discoplax hirtipes, and scavenge on the carcasses of other coconut crabs. During a tagging experiment, one coconut crab was observed killing and eating a Polynesian rat (Rattus exulans). Coconut crabs may be responsible for the disappearance of Amelia Earhart's remains, consuming them after her death and hoarding her skeletal remnants in their burrows.
The coconut crab can take a coconut from the ground and cut it to a husk nut, take it with its claw, climb up a tree 10 m (33 ft) high and drop the husk nut, to access the content inside. They often descend from the trees by falling, and can survive a fall of at least 4.5 m (15 ft) unhurt. Coconut crabs cut holes into coconuts with their strong claws and eat the contents, although it can take several days before the coconut is opened.
Thomas Hale Streets discussed their behaviour in 1877, doubting that the animal would climb trees to get at the nuts. In the 1980s, Holger Rumpff was able to confirm Streets's report, observing and studying how they open coconuts in the wild. The animal has developed a special technique to do so: if the coconut is still covered with husk, it will use its claws to rip off strips, always starting from the side with the three germination pores, the group of three small circles found on the outside of the coconut. Once the pores are visible, the coconut crab will bang its pincers on one of them until they break. Afterwards, it will turn around and use the smaller pincers on its other legs to pull out the white flesh of the coconut. Using their strong claws, larger individuals can even break the hard coconut into smaller pieces for easier consumption.
Habitat
Coconut crabs are considered one of the most terrestrial decapods, with most aspects of its life linked to a terrestrial existence; they will drown in sea water in less than a day. Coconut crabs live alone in underground burrows and rock crevices, depending on the local terrain. They dig their own burrows in sand or loose soil. During the day, the animal stays hidden to reduce water loss from heat. The coconut crab's burrows contain very fine yet strong fibres of the coconut husk which the animal uses as bedding. While resting in its burrow, it closes the entrances with one of its claws to create the moist microclimate within the burrow necessary for its breathing organs. In areas with a large coconut crab population, some may come out during the day, perhaps to gain an advantage in the search for food. Other times they will emerge if it is moist or raining, since these conditions allow them to breathe more easily. They live almost exclusively on land, returning to the sea only to release their eggs; on Christmas Island, for instance, Birgus latro is abundant 6 km (3.7 mi) from the sea.
Relationship with human beings
Adult coconut crabs have no known predators apart from other coconut crabs and people. Its large size and the quality of its meat mean the coconut crab is extensively hunted and is rare on islands with a human population. The coconut crab is eaten by Southeast Asians and Pacific Islanders and is considered a delicacy and an aphrodisiac, and intensive hunting has threatened the species' survival in some areas. While the coconut crab itself is not innately poisonous, it may become so depending on its diet, and cases of coconut crab poisoning have occurred. For instance, consumption of the sea mango Cerbera manghas by the coconut crab may make the coconut crab toxic due to the presence of cardiac cardenolides.
The pincers of the coconut crab are powerful enough to cause noticeable pain to a human; furthermore, the coconut crab will often keep its hold for extended periods of time. Thomas Hale Streets reported a trick used by Micronesians of the Line Islands to get a coconut crab to loosen its grip: "It may be interesting to know that in such a dilemma a gentle titillation of the under soft parts of the body with any light material will cause the crab to loosen its hold."
In the Cook Islands, the coconut crab is known as unga or kaveu, and in the Mariana Islands it is called ayuyu, and is sometimes associated with taotaomo'na because of the traditional belief that ancestral spirits can return in the form of animals such as the coconut crab.
Conservation
Coconut crab populations in several areas have declined or become locally extinct due to both habitat loss and human predation. In 1981, it was listed on the IUCN Red List as a vulnerable species, but a lack of biological data caused its assessment to be amended to Data Deficient in 1996.
Conservation management strategies have been put in place in some regions, such as minimum legal size limit restrictions in Guam and Vanuatu, and a ban on the capture of egg-bearing females in Guam and the Federated States of Micronesia. In the Northern Mariana Islands, hunting of adults not bearing eggs above a carapace length of 30 mm (1.2 in) may take place in September, October, and November, and only under licence. A bag limit of five coconut crabs on any given day, and 15 across the whole season has been set.
In Tuvalu, coconut crabs live on the motu (islets) in the Funafuti Conservation Area, a marine conservation area covering 33 km2 (12.74 mi2) of reef, lagoon and motu on the western side of Funafuti atoll.