The Coconut Crab
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 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.
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, the 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 moulted at periodic intervals. After moulting, 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.
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 a 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 the Diogenidae, they are reduced in size and have comparatively less surface area. While these gills were probably used to breathe under water 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 sensilia. 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, catch their attention especially.
Coconut crabs mate frequently and quickly on dry land in the period from May to September, especially between early June and late August Male crabs have spermatophores and deposit a mass of spermatophores on the abdomen of the female; the abdomen opens at the base of the third pereiopods, and fertilization is thought to occur on the external surface of the abdomen as the eggs pass through the spermatophore mass. The extrusion of eggs occurs on land in crevices or burrows near the shore. Shortly thereafter, the female lays her eggs and glues them to the underside of her abdomen, carrying the fertilized eggs underneath her body for a few months. At the time of hatching, usually October or November, the female coconut crab releases the eggs into the ocean at high tide. The larvae are of the zoea type, as is usual for decapod crustaceans.
The crab larvae float in the pelagic zone of the ocean with other plankton for about a month, during which a large number of them are eaten by predators. Upon reaching the glaucothoe stage of development, they settle to the bottom, find and wear a suitably-sized gastropod shell, and migrate to the shoreline with other terrestrial hermit crabs. At that time, they sometimes visit dry land. As with all hermit crabs, they change their shells as they grow. After these 28 days, they leave the ocean permanently and lose the ability to breathe in water. Young coconut crabs that cannot find a seashell of the right size also often use broken coconut pieces. When they outgrow even coconut shells, they develop a hardened abdomen. About four to eight years after hatching, the coconut crab matures and can reproduce. This is an unusually long development period for a crustacean.
The diet of coconut crabs consists primarily of fleshy fruits (particularly Ochrosia ackeringae, Arenga listeri , Pandanus elatus, P. christmatensis), nuts (coconuts Cocos nucifera, Aleurites moluccana), and seeds (Annona reticulata). However, as they are omnivorous creatures, they will consume certain other organic materials such as tortoise hatchlings and dead animals. They have also been observed to prey upon other sympatric crabs like Gecarcoidea natalis and Discoplax hirtipes, as well as scavenge on the carcasses of dead conspecifics. During a tagging experiment, one coconut crab was observed catching and eating a Polynesian Rat (Rattus exulans). Recent discoveries suggest that the coconut crab may be responsible for the disappearance of Amelia Earhart, consuming her remains and hording her bones in its crab burrow. Coconut crabs often try to steal food from each other and will pull their food into their burrows to be safe while eating.
The coconut crab climbs trees to eat coconuts or fruit, to escape the heat or to escape predators. It is a common perception that the coconut crab cuts the coconuts from the tree to eat them on the ground (hence the German name Palmendieb, which literally means "palm thief," and the Dutch Klapperdief). Prehistoric looking, the crab can take a coconut from the ground and cut it to a husk nut, take it with its claw, climb up a tree ten metres high and drop the husk nut, to access the content inside. Coconut crabs cut holes into coconuts with their strong claws and eat the contents; this behavior is unique in the animal kingdom.
Thomas Hale Streets discussed the behavior in 1877—while doubting that the crab would climb trees to get at the nuts. In the 1980s, Holger Rumpf was able to confirm Streets's report, observing and studying how the crabs open coconuts in the wild. The animal has developed a special technique to do so: if the coconut is still covered with husk, it will use its claws to rip off strips, always starting from the side with the three germination pores, the group of three small circles found on the outside of the coconut. Once the pores are visible, the crab will bang its pincers on one of them until they break. Afterwards, it will turn around and use the smaller pincers on its other legs to pull out the white flesh of the coconut. Using their strong claws, larger individuals can even break the hard coconut into smaller pieces for easier consumption.
Habitat and distribution
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 protect itself from predators and reduce water loss from heat. The crabs' burrows contain very fine yet strong fibers 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 also come out during the day, perhaps to gain an advantage in the search for food. Coconut crabs will also sometimes come out during the day if it is moist or raining, since these conditions allow them to breathe more easily. They live almost exclusively on land, and some have been found up to 6 kilometres (3.7 mi) from the ocean.
Coconut crabs live in areas from the Indian to the central Pacific Ocean. Christmas Island in the Indian Ocean has the largest and best-preserved population in the world. Other Indian Ocean populations exist on the Seychelles, especially Aldabra, the Glorioso Islands, Astove Island, Assumption Island and Cosmoledo, but the coconut crab is extinct on the central islands. They are also known on several of the Andaman and Nicobar Islands in the Bay of Bengal. Large numbers roam freely in the British-owned Chagos Archipelago, also known as the British Indian Ocean Territories (BIOT). They are protected on these islands from being hunted and/or eaten, with fines of up to 1,500 British pounds (roughly $3,000 USD) per crab consumed. On Mauritius and Rodrigues, they are extinct.
In the Pacific, its range only gradually became known. Charles Darwin believed it was only found on "a single coral island north of the Society group." The crab is actually far more widespread, though certainly not abundant on each and 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 crab is especially frequent on Teraina (Washington Island), with its abundant coconut palm forest, and on Caroline Island.
Coconut crabs are considered one of the most terrestrial decapods, with most aspects of its life linked to a terrestrial existence. The crab drowns in sea water in less than a day. As they cannot swim as adults, coconut crabs over time must have colonized the islands as larvae, which can swim. However, due to the large distances between the islands, some researchers believe a larva stadium of 28 days is not enough to travel the distance, and they assume juvenile coconut crabs reached other islands on driftwood and other flotsam.
The distribution shows some gaps, as for example around Borneo, Indonesia or New Guinea. These islands were within easy reach of the crab and also have a suitable habitat, yet have no coconut crab population. This is due to the coconut crabs being eaten to extinction by people. However, coconut crabs are known to live on the islands of the Wakatobi Marine National Park in Sulawesi, Indonesia.
Relationship with humans
This hermit crab, with its intimidating size and strength, has a special position in the culture of many human societies which share its range. The coconut crab is admired for its strength, and it is said that villagers use this animal to guard their coconut plantations. The coconut crab, especially if it is not yet fully grown, is also sold as a pet, for example, in Tokyo. The cage must be strong enough that the animal cannot use its powerful claws to escape. Should a coconut crab pinch a person, it will not only cause pain, but is unlikely to release its grip. Thomas Hale Streets reports the following trick, used by Micronesians of the Line Islands, to get a coconut crab to loosen its grip:
It may be interesting to know that in such a dilemma a gentle titillation of the under soft parts of the body with any light material will cause the crab to lose its hold.
The coconut crab is eaten by Southeast Asians and Pacific Islanders and is considered a delicacy and an aphrodisiac, with a taste similar to lobster and crab meat. The most prized parts are the eggs inside the female coconut crab and the fat in the abdomen. Coconut crabs can be cooked like other large crustaceans, by boiling or steaming. Different islands also have a variety of recipes, as for example, coconut crab cooked in coconut milk. While the coconut crab itself is not innately poisonous, it may become so depending on its diet, and cases of coconut crab poisoning have occurred. It is believed that the poison comes from plant toxins, which would explain why some animals are poisonous and others not. Reputedly, this poison is considered an aphrodisiac, similar to the highly poisonous pufferfish eaten in Japan. However, coconut crabs are not a commercially significant species and are usually not sold.
Status and conservation
Coconut crab populations in several areas have declined or become extinct due to both habitat loss and human predation. It is a worldwide protected species according to the IUCN Invertebrate Red Data Book. However, according to the IUCN Red List criteria, there is not enough data at present to decide if the coconut crab is an endangered species, and therefore, it is provisionally listed as DD (data deficient), cautioning that this assessment is in need of update. There have been conservation management strategies effected in some regions, such as minimum legal size limit restrictions in Guam and Vanuatu, and the capture of ovigerous females has been banned in Guam and the Federated States of Micronesia.
The juvenile coconut crab is vulnerable to introduced carnivores, such as rats and pigs, and ants, such as the yellow crazy ant (Anoplolepis gracilipes). Adult coconut crabs have few natural predators, and significant numbers are eaten only by people. The adults have poor eyesight and detect enemies based on ground vibration.
Overall, it seems that large human populations have a negative effect on the coconut crab population, and in some areas, populations are reported to be declining due to over-harvesting. The coconut crab is protected in some areas, with minimum sizes for taking and a protected breeding period.