The coconut
crab, Birgus latro, is the largest land-living arthropod in the world, and is probably at the upper size limit of terrestrial animals with exoskeletons in today's atmosphere. The species in
habits the coastal forest regions of many Indo-Pacific islands, although localized extinction has occurred where the species lives in proximity to humans. Generally nocturnal, they remain hidden during the day and emerge only on some nights to forage. Their body is divided into four regions; the cephalic lobe, forepart, trunk, and opisthosoma. It is a highly apomorphic hermit
crab and is known for its ability to crack coconuts with its strong pincers to eat the contents. It is the only species of the genus Birgus.
It is also called the robber
crab or palm thief, because some coconut
crabs are rumored to steal shiny items such as pots and silverware from houses and tents. Another name is terrestrial hermit
crab, due to the use of shells by the young animals; however, there are other terrestrial hermit
crabs which do not get rid of the shell even as adults. These – typically in the closely related genus Coenobita – are the animals usually called "terrestrial hermit
crab"; given the close relationship between Coenobita and Birgus, the term would generally refer to any member of the family Coenobitidae.
The coconut
crab also has a range of local names, for example, unga or kaveu in the Cook Islands, and ayuyu in the Marianas where it is sometimes associated with taotaomo'na because of the traditional belief that ancestral spirits can return in the form of animals such as ayuyu.
Most of the early European literature on Birgus is of a popular nature and relies heavily on the accounts of early European travelers in the tropical Pacific and Indian Oceans. These papers usually discuss the coconut
crab's reputed
habits of climbing coconut palms, clipping off nuts, returning to the ground to husk and hammer open the nut, or carrying the husked nut back up the palm and dropping it repeatedly on the rocks below until breakage. Many of the early observers admitted that their descriptions were second-hand. Based on such an account by Georg Eberhard Rumphius (1705), Carl Linnaeus (1767) named the species latro, or "robber" (of nuts). These
habits continued to be described in zoological texts, despite the absence of first-hand accounts in the scientific literature of how the coconut
crab husked and opened a coconut.
The taxonomic position of Birgus as a hermit
crab has been documented through the result of studies of its larvae. Borradaile (1900) briefly described the first zoea. Harms (1932) described the shell carrying habit and growth of the glaucothoe. Orlamunder (1942), using material collected by Harms, described the embryology and first zoea. Reese and Kinzie (1968), in their description of the four to five zoeal stages, found that settling and metamorphosis to glaucothoe (a transitional developmental stage between the larval and juvenile stages) occurred primarily on the 24th to 27th days of larval life. Reese (1968) found that only those glaucothoe which entered shells survived after emigrating to land, and considered the use of small gastropod shells by the glaucothoe an example of the retention of ancestral behavior. Held (1963) found that very small Birgus dig a burrow prior to ecdysis and eat the cast-off exoskeleton.