IUCN threat status:

Data Deficient (DD)

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Biology

Birgus latro is almost entirely terrestrial and has adapted so well to living on land that it actually drowns in water (2). That said, it does still breathe through modified gills. The gills are surrounded by spongy tissues which need to be kept moist. The coconut crab does this by dipping its legs into water and passing them over the gills. The crab does require some contact with the sea as it often drinks the water to maintain its salt balance, and females need to return to sea to release eggs (2). By day the coconut crab inhabits burrows where it is protected from desiccation and intruders, and by night it goes in search of food. As its name suggests this crab feeds on coconuts, and is actually able to climb coconut palms, where it is thought to pinch off coconuts with its powerful claws when coconuts are not already available on the ground. If the coconut does not break open on its fall, the crab husks the coconuts by pulling back the husk from the end that was formerly attached to the palm, and evidence indicates that they then pierce the "soft eye" with a pointed walking leg, before gradually enlarging the hole by breaking off sections of the shell until they can reach in to scoop out the flesh (4) (5). This crab feeds on more than just coconuts however, and will scavenge for anything organic from fruit to leaves (6). It also feeds on the moulted exoskeletons of other crustaceans, which are thought to provide calcium for its own carapace growth (2). Courtship in most hermit crabs is a prolonged experience, but between coconut crabs it is quick, simple and infrequent. Mating occurs on land and the female carries the fertilised eggs beneath her abdomen, held in place by three specialised appendages. When the eggs are ready for hatching, the female walks down to the edge of the sea during high tide and releases the larvae (2). The larvae are pelagic and remain floating in the sea for up to 28 days while they develop. This is followed by an amphibious stage of 21 to 28 days when the young crabs occupy gastropod shells and are able to migrate on to the land (2). This shell-living habit serves to protect the juveniles from desiccation and predation during this early and vulnerable life stage (8). When they are two to three years old, and still less than two centimetres long, they abandon the shell, harden their skin, and transform into a miniature of the adult coconut crab, with a thoracic length of just five to ten millimetres (4) (9). Their exoskeleton is moulted regularly to allow the crab continuous growth (2). Moulting occurs in the safety of a burrow and takes around 30 days, after which the crab eats the cast-off exoskeleton. These crabs are slow growing, (8), and there is good evidence that they live to be more than 40 years, after which they don't increase in size, although they might live for many more years (4).

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Source: ARKive

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