Coconut crab

Appearance

"B. latro" is the largest terrestrial arthropod, and indeed terrestrial invertebrate, in the world; reports about its size vary, but most sources give a body length up to 40 cm, a weight up to 4.1 kg, and a leg span more than 0.91 m, with males generally being larger than females. The carapace may reach a length of 78 mm, and a width up to 200 mm.

The body of the coconut crab is, like that of all decapods, divided into a front section, which has 10 legs, and an abdomen. The front-most pair of legs has large chelae, with the left being larger than the right. The next two pairs, as with other hermit crabs, are large, powerful walking legs with pointed tips, which allow coconut crabs to climb vertical or overhanging surfaces.

The fourth pair of legs is smaller with tweezer-like chelae at the end, allowing young coconut crabs to grip the inside of a shell or coconut husk to carry for protection; adults use this pair for walking and climbing. The last pair of legs is very small and is used by females to tend their eggs, and by the males in mating. This last pair of legs is usually held inside the carapace, in the cavity containing the breathing organs. Some difference in color occurs between the animals found on different islands, ranging from orange-red to purplish blue; in most regions, blue is the predominant color, but in some places, including the Seychelles, most individuals are red.

Naming

Common names for the species include coconut crab, robber crab, and palm thief, which mirrors the animal's name in other European languages. In Japan, the species is typically referred to as "Yashigani", meaning 'palm crab'.

Distribution

Coconut crabs live in the Indian Ocean 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 population in the subtropics, such as the Ryukyu Islands. Some evidence indicates the coconut crab once lived on the mainlands of Australia and Madagascar and on the island of Mauritius, but it no longer occurs in any of these places. As they cannot swim as adults, coconut crabs must have colonised the islands as planktonic larvae.

Christmas Island in the Indian Ocean has the largest and densest population of coconut crabs in the world, although it is outnumbered there by more than 50 times by the Christmas Island red crab, "Gecarcoidea natalis". Other Indian Ocean populations exist on the Seychelles, including Aldabra and Cosmoledo, but the coconut crab is extinct on the central islands. Coconut crabs occur on several of the Andaman and Nicobar Islands in the Bay of Bengal. They occur on most of the islands, and the northern atolls, of the Chagos Archipelago.

In the Pacific, the coconut crab's range became known gradually. Charles Darwin believed it was only found on "a single coral island north of the Society group". The coconut crab is far more widespread, though it is not abundant on every Pacific island it inhabits. Large populations exist on the Cook Islands, especially Pukapuka, Suwarrow, Mangaia, Takutea, Mauke, Atiu, and Palmerston Island. These are close to the eastern limit of its range, as are the Line Islands of Kiribati, where the coconut crab is especially frequent on Teraina, with its abundant coconut palm forest. The Gambier Islands mark the species' eastern limit.

Status

Coconut crab populations in several areas have declined or become locally extinct due to both habitat loss and human predation. In 1981, it was listed on the IUCN Red List as a vulnerable species, but a lack of biological data caused its assessment to be amended to "data deficient" in 1996.

Habitat

Coconut crabs are considered one of the most terrestrial-adapted of the decapods, with most aspects of its life oriented to, and centered around such an existence; they will actually drown in sea water in less than a day. Coconut crabs live alone in underground burrows and rock crevices, depending on the local terrain. They dig their own burrows in sand or loose soil. During the day, the animal stays hidden to reduce water loss from heat.

The coconut crabs' burrows contain very fine yet strong fibres of the coconut husk which the animal uses as bedding. While resting in its burrow, the coconut crab closes the entrances with one of its claws to create the moist microclimate within the burrow necessary for its breathing organs. In areas with a large coconut crab population, some may come out during the day, perhaps to gain an advantage in the search for food. Other times, they emerge if it is moist or raining, since these conditions allow them to breathe more easily. They live almost exclusively on land, returning to the sea only to release their eggs; on Christmas Island, for instance, "B. latro" is abundant 6 km from the sea.

Food

The diet of coconut crabs consists primarily of fleshy fruits; nuts, drupes and seeds; and the pith of fallen trees. However, as they are omnivores, they will consume other organic materials such as tortoise hatchlings and dead animals. They have been observed to prey upon crabs such as "Gecarcoidea natalis" and "Discoplax hirtipes", as well as scavenge on the carcasses of other coconut crabs.

The coconut crab can take a coconut from the ground and cut it to a husk nut, take it with its claw, climb up a tree 10 m high and drop the husk nut, to access the coconut flesh inside. They often descend from the trees by falling, and can survive a fall of at least 4.5 m unhurt. Coconut crabs cut holes into coconuts with their strong claws and eat the contents, although it can take several days before the coconut is opened.