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Coconut Crab
Coconut Crab
Information
Common Name Robber Crab and Palm Thief.
Range Indian Ocean and parts of the Pacific Ocean.
Scientific Classification
Kingdom Animalia
Phylum Arthropoda
Class Malacostraca
Order Decapoda
Family Coenobitidae
Genus Birgus
Species Birgus latro
Conservation Status
VUSpecies
Vulnerable

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 times[citation needed], with a weight up to 4.1 kg (9.0 lb). It can grow to up to 1 m (3 ft 3 in) in length from leg to leg. It is found on islands across the Indian Ocean, and parts of the Pacific Ocean as far east as the Gambier Islands and Pitcairn Islands, mirroring the distribution of the coconut palm; it has been extirpated from most areas with a significant human population, including mainland Australia and Madagascar.

The coconut crab is the only species of the genus Birgus, and is related to the terrestrial hermit crabs of the genus Coenobita. It shows a number of adaptations to life on land. Like other hermit crabs, juvenile coconut crabs use empty gastropod shells for protection, but the adults develop a tough exoskeleton on their abdomens and stop carrying a shell. Coconut crabs have organs known as branchiostegal lungs, which are used instead of the vestigial gills for breathing, and after the juvenile stage they will drown if immersed in water for long. They have an acute sense of smell, which has developed convergently with that of insects, and which they use to find potential food sources.

Adult coconut crabs feed primarily on fruits, nuts, seeds, and the pith of fallen trees, but they will eat carrion and other organic matter opportunistically. Anything left unattended on the ground is a potential source of food, which they will investigate and may carry away - thereby getting the alternative name of "robber crab". The species is popularly associated with the coconut palm, yet coconuts are not a significant part of its diet. Although it lives in a burrow, the crab has been filmed climbing coconut and pandanus trees. No film shows a crab selectively picking coconut fruit, though they might dislodge ripe fruit that otherwise would fall naturally. Climbing is an immediate escape route (if too far from the burrow) to avoid predation (when young) by large sea birds, or cannibalism (at any age) by bigger, older crabs.

Mating occurs on dry land, but the females return to the edge of the sea to release their fertilised eggs, and then retreat back up the beach. The larvae that hatch are planktonic for 3–4 weeks, before settling to the sea floor, entering a gastropod shell and returning to dry land. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years. In the 3–4 weeks that the larvae remain at sea, their chances of reaching another suitable location is enhanced if a floating life support system avails itself to them. Examples of the systems that provide such opportunities include floating logs and rafts of marine, or terrestrial, vegetation. Similarly, floating coconuts can be a very significant part of the crab's dispersal options.[4]

Appearance[]

B. latro is the largest terrestrial arthropod, and indeed terrestrial invertebrate, in the world;[5][6] reports about its size vary, but most sources give a body length up to 40 cm (16 in),[7] a weight up to 4.1 kg (9.0 lb), and a leg span more than 0.91 m (3.0 ft),[8] with males generally being larger than females.[9] The carapace may reach a length of 78 mm (3.1 in), and a width up to 200 mm (7.9 in).[6]

The body of the coconut crab is, like that of all decapods, 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.[10] 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.[11] 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 of legs is very small and is used by females to tend their eggs, and by the males in mating.[10] This last pair of legs is usually held inside the carapace, in the cavity containing the breathing organs. Some difference in color occurs between the animals found on different islands, ranging from orange-red to purplish blue;[12] in most regions, blue is the predominant color, but in some places, including the Seychelles, most individuals are red.[10]

Although B. 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 for that purpose. Unlike other hermit crabs, the adult coconut crabs do not carry shells, but instead harden their abdominal terga by depositing 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.[13] Like most true crabs, B. latro bends its tail underneath its body for protection.[10] 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.[11] It remains in the burrow for 3 to 16 weeks, depending on the size of the animal.[11] After moulting, 1 to 3 weeks are needed 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.[14]

Behavior[]

Adult coconut crabs have no known predators apart from other coconut crabs and humans. Its large size and the quality of its meat means that the coconut crab is extensively hunted and is very rare on islands with a human population.[46] 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.[12] 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.[46][47] 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.[48]

The pincers of the coconut crab are powerful enough to cause noticeable pain to a human; furthermore, the coconut crab often keeps its hold for extended periods of time. Thomas Hale Streets reports 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".[35]

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.[49]

Diet[]

The diet of coconut crabs consists primarily of fleshy fruits (particularly Ochrosia ackeringae, Arenga listeri, Pandanus elatus, P. christmatensis); nuts (Aleurites moluccanus), drupes (Cocos nucifera,) and seeds (Annona reticulata);[36] and the pith of fallen trees.[37] However, as they are omnivores, they will consume other organic materials such as tortoise hatchlings and dead animals.[11][38] They have been observed to prey upon crabs such as Gecarcoidea natalis and Discoplax hirtipes, as well as scavenge on the carcasses of other coconut crabs.[36] During a tagging experiment, one coconut crab was observed killing and eating a Polynesian rat (Rattus exulans).[39] In 2016, a large coconut crab was observed climbing a tree to disable and consume a red-footed booby on the Chagos Archipelago.[40]

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 coconut flesh inside.[41] They often descend from the trees by falling, and can survive a fall of at least 4.5 m (15 ft) unhurt.[42] 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.[37]

Thomas Hale Streets discussed the behaviour in 1877, doubting that the animal would climb trees to get at the coconuts.[35] In the 1980s, Holger Rumpf was able to confirm Streets' report, observing and studying how they open coconuts in the wild.[37] 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 bangs its pincers on one of them until it breaks. Afterwards, it turns around and uses 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.[43]

Lifespan[]

Coconut crabs are hunted extensively for food, which can be problematic due to their long lifespan and slow growth rate. They can live to be over 60 years old.

Gallery[]

Facts[]

  • The coconut crab has been known to western scientists since the voyages of Francis Drake around 1580[55] and William Dampier around 1688.[56] Based on an account by Georg Eberhard Rumphius (1705), who had called the animal "'Cancer crumenatus'", Carl Linnaeus (1767) named the species Cancer latro,[57] from the Latin 'latro', meaning "robber". The genus Birgus was erected in 1816 by William Elford Leach, containing only Linnaeus' Cancer latro, which was thus renamed Birgus latro.[3] Birgus is classified in the family Coenobitidae, alongside one other genus, Coenobita, which contains the terrestrial hermit crabs.[3][29]Common names for the species include coconut crab, robber crab, and palm thief,[1] which mirrors the animal's name in other European languages (e.g. German: Palmendieb).[58] In Japan (the species lives on some of Japan's southerly island chains), the species is typically referred to as Yashigani (ヤシガニ),[59] meaning 'palm crab'.
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