Coconut crab  (Birgus latro)

The coconut crab is classified as Data Deficient (DD) on the IUCN Red List (1).

The coconut crab (Birgus latro) is a type of land hermit crab with a spectacular appearance and intriguing biology. Able to grow to relatively gigantic proportions, the coconut crab is probably the largest terrestrial arthropod in the world (2). Indeed, Charles Darwin described the coconut crab as “monstruous” when he encountered it on the Keeling Islands during the voyage of the Beagle (3). Unlike most other hermit crabs, only the very small coconut crab juveniles find and use gastropod shells to protect their soft-skinned abdomen as they develop. Larger juveniles abandon the shell-carrying habit and instead their abdomen develops a hard skin, the exoskeleton, as over the rest of the body (4). This protects the crab, reduces water loss and does not restrict its growth, allowing it to reach up to a metre in size toe-to-toe (2).

This huge crustacean is well adapted to life on land with long strong legs. It also has large muscular claws which are used for husking coconuts and opening the shell to eat the flesh (4). This is a unique behaviour amongst crabs and explains why this species is called the coconut crab. The claws are in fact so powerful they can lift objects such as vegetation or rocks weighing up to 28 kilograms (5). Its stalked eyes are red and this crab’s body colour varies between islands from purplish-blue to orange-red (6). Studies show that male coconut crabs are considerably larger than females (2).

The coconut crab is found on oceanic islands and small offshore islets adjacent to large continental islands across a broad geographical range in the tropical Indo-Pacific region, with reports stretching from the Aldabras Islands in the Indian ocean to the Pitcairn group and Easter Island in the Pacific ocean (2) (4) (7).

The coconut crab inhabits rock crevices and sand burrows along the coastline, though preferences vary between islands, depending on the habitat available. For example, on Olango Island in the Philippines, the coconut crab lives in burrows in coral rock and thick undergrowth, while on Guam Island, in Oceania, it establishes burrows within the porous limestone substrate (2).

The coconut crab 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 crustacean species, 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).

The coconut crab is threatened by intensive hunting for food as it is considered, on many islands, as a delicacy and an aphrodisiac (10). It is uncommon throughout much of its present range, though on less populated islands it may be abundant. In more recent years, development along the coastline of islands has modified or destroyed much of this crab’s habitat. With increased tourism these crabs are also caught and sold as curios. Predation by introduced pigs, rats, monitor lizards and monkeys are also a threat to juvenile crabs (2).

On some islands these crabs have limited protection. For example, in Papua New Guinea villagers are asked not to collect coconut crabs for food, and on Saipan Island it is prohibited to collect coconut crabs with a thoracic length of less than 3.5 centimetres, or between 1st June and 30th September when breeding occurs (2) (4). However, it is important to conduct thorough surveys to determine the full distribution of this species, and ascertain the extent to which populations are threatened in order to help develop conservation measures. There have been proposals for a reserve in the Togian Islands, Sulawesi for this species’ protection, and also the establishment of captive breeding programs (2). Though the coconut crab is not severely threatened, increasing populations, tourism and development on Pacific and Indian Ocean islands will soon threaten this crab as it has done so many species worldwide. It is therefore important to be pro-active and protect this unique species for the future (2).

View information on this species at the UNEP World Conservation Monitoring Centre.

More information on the coconut crab:

• Cook Islands Natural Heritage Trust:
  http://cookislands.bishopmuseum.org