Birgus latro, (Linnaeus, 1767)
The coconut crab (Birgus latro) is a terrestrial species of giant hermit crab, and is also known as the robber crab or palm thief. It is the largest living terrestrial arthropod, with a weight up to 4.1 kg (9.0 lb). The distance from the tip of one leg to the tip of another can be as wide as 90 cm (35 in). It is found on islands across the Indian and Pacific Oceans, as far east as the Gambier Islands, Pitcairn Islands, and Caroline Island, and as far west as Zanzibar. While its range broadly shadows the distribution of the coconut palm, the coconut crab has been extirpated from most areas with a significant human population such as mainland Australia and Madagascar.
The coconut crab is the only species of the genus Birgus, and is related to the other terrestrial hermit crabs of the genus Coenobita. It shows a number of adaptations to life on land. Juvenile coconut crabs use empty gastropod shells for protection like other hermit crabs, but the adults develop a tough exoskeleton on their abdomens and stop carrying a shell. Coconut crabs have organs known as branchiostegal lungs, which they use for breathing instead of their vestigial gills. After the juvenile stage, they will drown if immersed in water for too long. They have an acute sense of smell, which they use to find potential food sources, and which has developed convergently with that of insects.
B. latro is both the largest living terrestrial arthropod and the largest living terrestrial invertebrate. Reports of its size vary, but most sources give a body length up to 40 cm (16 in), a weight up to 4.1 kg (9 lb), and a leg span more than 0.91 m (3 ft), with males generally being larger than females. The carapace may reach a length of 78 mm (3+1⁄16 in), and a width up to 200 mm (8 in).
Coconut crabs live in the Indian and the central Pacific Ocean, with a distribution that closely matches that of the coconut palm. The western limit of the range of B. latro is Zanzibar, off the coast of Tanzania, while the tropics of Cancer and Capricorn mark the northern and southern limits, respectively, with very few populations in the subtropics, such as the Ryukyu Islands. Some evidence indicates the coconut crab once lived on the mainland of Australia, Madagascar, Rodrigues, Easter Island, Tokelau, the Marquesas Islands, and possibly India, but is now extirpated in those areas. As they cannot swim as adults, coconut crabs must have colonised the islands as planktonic larvae.
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); and the pith of fallen trees. As they are omnivores, though, they consume other organic materials, such as tortoise hatchlings and dead animals, including other crustaceans, as well as the molted exoskeletons of other crustaceans. 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). In 2016, a large coconut crab was observed climbing a tree to disable and consume a red-footed booby on the Chagos Archipelago.
Coconut crabs mate frequently and quickly on dry land in the period from May to September, especially between early June and late August. Males have spermatophores and deposit a mass of spermatophores on the abdomens of females; the oviducts 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.
Adult coconut crabs have no known predators apart from other coconut crabs and humans. Their large size and the quality of their meat means that they are extensively hunted and are very rare on islands with a human population. The coconut crab is eaten as a delicacy – and regarded as an aphrodisiac – on various islands, and intensive hunting has threatened the species' survival in some areas. In other regions, taboos associated with the crab prohibit or limit hunting and consumption of B. latro. Such taboos have been recorded in the Nicobar Islands in India, on Flores Island in Indonesia, and among the Tao people of Taiwan. On the Nicobarian Kamorta Island, eating the crab is believed to lead to bad luck and to cause severe, sometimes fatal, illnesses. In cases where locals fall ill after consuming the crab, their families create a wooden image of the creature.
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