The coconut crab, Birgus latro, is the largest land-living arthropod in the world and is probably at the upper limit of how big terrestrial animals with exoskeletons can become in today’s atmosphere. The species inhabits the coastal forest regions of many Indo-Pacific islands, although localized extinction has occurred where the crab is sympatric with man. 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 that 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 crabs”; 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.

Physical description

Reports about the size of Birgus latro vary, but most references give a body length of up to 40 cm (16 in), a weight of up to 4.1 kg (9.0 lb), and a leg span of more than 0.91 m (3.0 ft), with males generally being larger than females. There have been reports in the literature of specimens measuring 6 feet (1.8 m) across the thorax and weighing 30 pounds (14 kg) They can live more than 30 years .

The body of the coconut crab is, like that of all decapods, divided into a front section (cephalothorax), which has 10 legs, and an abdomen. The front-most pair of legs has large claws used to open coconuts, and these claws (chelae) can lift objects up to 29 kg (64 lb). The next two pairs, as with other hermit crabs, are large, powerful walking legs which allow coconut crabs to climb vertically up trees (often coconut palms). The fourth pair of legs is smaller with tweezer-like chelae at the end, allowing young 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 serves only to clean the breathing organs. These legs are usually held inside the carapace, in the cavity containing the breathing organs. There is some difference in color between the animals found on different islands, ranging from light violet through deep purple to brown.

Although Birgus latro is a derived type of hermit crab, only the juveniles use salvaged snail shells to protect their soft abdomens, and adolescents sometimes use broken coconut shells to protect their abdomens. Unlike other hermit crabs, adult coconut crabs do not carry shells but instead, harden their abdominal terga by depositing chitin and chalk. Not being constrained by the physical confines of living in a shell allows this species to grow much larger than other crabs in the family Coenobitidae. Like most true crabs, B. latro bends its tail underneath its body for protection. The hardened abdomen protects the coconut crab and reduces water loss on land, but has to be molted at periodic intervals. After molting, it takes about 30 days for the exoskeleton to harden, during which time the animal’s body is soft and vulnerable, and it stays hidden for protection.

Respiration

Except as larvae, coconut crabs cannot swim, and even small specimens will drown in water. They use a special organ called a branchiostegal lung to breathe. This organ can be interpreted as a developmental stage between gills and lungs and is one of the most significant adaptations of the coconut crab to its habitat. The branchiostegal lung contains tissue similar to that found in gills but suited to the absorption of oxygen from air, rather than water. This organ is expanded laterally and is evaginated to increase the surface area; located in the cephalothorax, it is optimally placed to reduce both the blood/gas diffusion distance and the return distance of oxygenated blood to the pericardium. Coconut crabs use their hindmost, smallest pair of legs to clean these breathing organs and to moisten them with water. The organs require water to properly function, and the crab provides this by stroking its wet legs over the spongy tissues nearby. Coconut crabs may also drink water from small puddles by transferring it from their chelipeds to their maxillipeds.

In addition to the branchiostegal lung, the coconut crab has an additional rudimentary set of gills. Although these gills are comparable in number to aquatic species from the families Paguridae and Diogenidae, they are reduced in size and have comparatively less surface area. While these gills were probably used to breathe underwater in the evolutionary history of the species, they no longer provide sufficient oxygen, reflecting a decreased dependence on the gills for gas exchange and the development of other respiratory surfaces.

Sense of smell

Another distinctive organ of the coconut crab is its “nose”. The process of smelling works very differently depending on whether the smelled molecules are hydrophilic molecules in water or hydrophobic molecules in air. As most crabs live in the water, they have specialized organs called aesthetascs on their antennae to determine both the concentration and the direction of a smell. However, as coconut crabs live on the land, the aesthetascs on their antennae differ significantly from those of other crabs and look more like the smelling organs of insects, called sensilla. While insects and the coconut crab originate from different evolutionary paths, the same need to detect smells in the air led to the development of remarkably similar organs, making it an example of convergent evolution. Coconut crabs also flick their antennae as insects do to enhance their reception. They have an excellent sense of smell and can detect interesting odors over large distances. The smell of rotting meat, bananas, and coconuts, all potential food sources, catches their attention especially.

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