Coconut crab
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 terrestrial arthropod in the world, with a weight of up to 4.1 kg (9 lb). It can grow to up to 1 m (3 ft 3 in) in width from the tip of one leg to the tip of another. 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, similar to the distribution of the coconut palm; it has been extirpated from most areas with a significant human population, including mainland Australia and Madagascar. Coconut crabs also live off the coast of Africa near Zanzibar.
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.
Adult coconut crabs feed primarily on fleshy fruits, nuts, seeds, and the pith of fallen trees, but they will eat carrionand 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 by large sea birds (when young) or by humans, 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 fertilized 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] Fossils of this crab date back to the Miocene.[5]
The coconut crab has been known to western scientists since the voyages of Francis Drake around 1580[6] and William Dampier around 1688.[7] Based on an account by Georg Eberhard Rumphius (1705), who had called the animal "Cancer crumenatus", Carl Linnaeus (1767) named the species Cancer latro,[8] 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][9]
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