Overview

Brief Summary

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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Description

This species is a type of land hermit crab with a spectacular appearance and intriguing biology. It is probably the largest terrestrial arthropod in the world and is able to grow to relatively gigantic proportions (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 juveniles of this species 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 males are considerably larger than females (2).
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Distribution

Range Description

Coconut crabs live in areas throughout the Indian and western Pacific oceans.
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Range

Birgus latro 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).
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Ecology

Habitat

Habitat and Ecology

Habitat and Ecology
Coconut crabs live alone in underground burrows and rock crevices. 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. They live almost exclusively on land, and some have been found up to 6 km from the ocean.

Mating occurs near the sea. After this the female lives within 100 metres of the sea to regularly moisten herself with seawater. The young (as zoea) are spawned from the eggs into the water. The zoea takes 3-6 weeks to go through 4-5 zoea stages and form an ampbibious stage called a glaucothoe. The benthic, shrimplike glaucothoe finds a minute shell and after 3-4 weeks it migrates ashore. After about 4 weeks of living around the high tide mark, it transforms into a juvenile crab, which continues to use a gastropod shell for 1-2 years, and lives very secretively in burrows.

They are solitary and usually nocturnal, especially where human activity is frequent. They are omnivorous, commonly eating the fallen fruit of Pandanus and the Coconut Palm.

Systems
  • Terrestrial
  • Marine
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Depth range based on 1 specimen in 1 taxon.
Water temperature and chemistry ranges based on 1 sample.

Environmental ranges
  Depth range (m): 4 - 4
  Temperature range (°C): 28.954 - 28.954
  Nitrate (umol/L): 0.099 - 0.099
  Salinity (PPS): 34.131 - 34.131
  Oxygen (ml/l): 4.525 - 4.525
  Phosphate (umol/l): 0.113 - 0.113
  Silicate (umol/l): 1.579 - 1.579
 
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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, coconut crabs live in burrows in coral rock and thick undergrowth, while on Guam Island, in Oceania, they establish burrows within the porous limestone substrate (2).
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Associations

Known predators

Birgus latro (coconut crabs) is prey of:
Homo sapiens

Based on studies in:
Polynesia (Reef)

This list may not be complete but is based on published studies.
  • W. A. Niering, Terrestrial ecology of Kapingamarangi Atoll, Caroline Islands, Ecol. Monogr. 33(2):131-160, from p. 157 (1963).
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Known prey organisms

Birgus latro (coconut crabs) preys on:
coconut

Based on studies in:
Polynesia (Reef)

This list may not be complete but is based on published studies.
  • W. A. Niering, Terrestrial ecology of Kapingamarangi Atoll, Caroline Islands, Ecol. Monogr. 33(2):131-160, from p. 157 (1963).
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Conservation

Conservation Status

IUCN Red List Assessment


Red List Category
DD
Data Deficient

Red List Criteria

Version
2.3

Year Assessed
1996
  • Needs updating

Assessor/s
Eldredge, L.G.

Reviewer/s

Contributor/s

History
  • 1994
    Rare
    (Groombridge 1994)
  • 1990
    Rare
    (IUCN 1990)
  • 1988
    Rare
    (IUCN Conservation Monitoring Centre 1988)
  • 1986
    Rare
    (IUCN Conservation Monitoring Centre 1986)
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Status

According to the IUCN Red List criteria, there is not enough information on this species to make an assessment of its risk of extinction based on its distribution and/or population status. It is therefore classified as Data Deficient (DD) on the IUCN Red List (1).
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Population

Population
The populations are reportedly quite large, with one of the largest populations being on Caroline Island. It is believed that the coconut crab is quite common on some islands, but rather rare on others.
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Threats

Major Threats
The Coconut Crab is esteemed as food. Unfortunately, it is easily over-harvested, because of its complex life-cycle and slow growth rate. Coastal development on many islands also reduces the natural habitat of the crab.

The juvenile coconut crab is vulnerable to introduced carnivores such as rats and pigs, and ants such as the yellow crazy ant.
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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).
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Management

Conservation Actions

Conservation Actions
The coconut crab is protected in some areas, with minimum sizes for taking and a protected breeding period.
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Conservation

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).
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Wikipedia

Coconut crab

The coconut crab, Birgus latro, is a species of terrestrial hermit crab, also known as the robber crab, ganjo crab or palm thief. It is the largest land-living arthropod in the world, and is probably at the upper size limit for terrestrial animals with exoskeletons in recent Earth atmosphere, with a weight of up to 4.1 kg (9.0 lb). It can grow to up to 1 m (3 ft) in length from leg to leg. It is found on islands across the Indian Ocean and parts of the Pacific Ocean as far east as the Gambier Islands, mirroring the distribution of the coconut palm; it has been extirpated from most areas with a significant human population, including mainland Australia and Madagascar.

The coconut crab is the only species of the genus Birgus, and is related to the terrestrial hermit crabs of the genus Coenobita. It shows a number of adaptations to life on land. Like hermit crabs, a juvenile coconut crab uses empty gastropod shells for protection, but the adult develops a tough exoskeleton on its abdomen and stops carrying a shell. Coconut crabs use organs known as "branchiostegal lungs" instead of the vestigial gills for breathing. They cannot swim and will drown if immersed in water for long. They have developed an acute sense of smell, evolved convergently with that of insects, which they use to find potential food sources. Mating occurs on dry land, but the females migrate to the sea to release their fertilised eggs as they hatch. The larvae are planktonic for 3–4 weeks before settling to the sea floor and entering a gastropod shell. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years.

Adult coconut crabs feed on fruits, nuts, seeds, and the pith of fallen trees, but will eat carrion and other organic matter opportunistically. The species is popularly associated with the coconut, and has been widely reported to climb trees to pick coconuts, which it then opens to eat the flesh. While coconut crabs can climb trees, and can eventually open a coconut collectively, coconuts are not a significant part of their diet. Coconut crabs are hunted wherever they come into contact with people, and are subject to legal protection in some areas. In the absence of precise information, the IUCN lists the species as Data Deficient.

Description[edit]

B. latro is the largest land-living arthropod in the world;[4][5] reports about its size vary, but most sources give a body length of up to 40 cm (16 in),[6] a weight of up to 4.1 kg (9.0 lb), and a leg span of more than 0.91 m (3.0 ft),[7] with males generally being larger than females.[8] The carapace may reach a length of 78 mm (3.1 in), and a width of up to 200 mm (7.9 in).[5]

The body of the coconut crab, like that of all decapods, is divided into a front section (cephalothorax), which has 10 legs, and an abdomen. The front-most pair of legs has large chelae (claws), with the left being larger than the right.[9] 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.[10] 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, very small, is used by females to tend their eggs, and by the males in mating.[9] This last pair of legs is usually held inside the carapace, in the cavity containing the breathing organs. Some difference in colour is seen between the animals found on different islands, ranging from orange-red to purplish-blue;[11] in most regions, blue is the predominant colour, but in some places, including the Seychelles, most individuals are red.[9]

Although B. 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 have hardened abdominal terga from deposited chitin and chalk. Not being constrained by the physical confines of living in a shell allows this species to grow much larger than other hermit crabs in the family Coenobitidae.[12] Like most true crabs, B. latro bends its tail underneath its body for protection.[9] The hardened abdomen protects the coconut crab and reduces water loss on land, but must be moulted periodically. Adults moult annually, and dig a burrow up to 1 m (3 ft 3 in) long in which to hide while vulnerable.[10] It remains in the burrow for three to 16 weeks, depending on the size of the animal.[10] After moulting, it takes one to three weeks for the exoskeleton to harden, depending on the animal's size, during which time the animal's body is soft and vulnerable, and it stays hidden for protection.[13]

Respiration[edit]

Print of a coconut crab from the Dictionnaire d'Histoire Naturelle of 1849

Except as larvae, coconut crabs cannot swim, and they will drown if left in water for more than an hour.[9] 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.[14] 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 its surface area;[10] 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.[15] 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 function properly, and the coconut crab provides this by stroking its wet legs over the spongy tissues nearby. Coconut crabs may drink water from small puddles by transferring it from their chelipeds to their maxillipeds.[16]

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.[15]

Sense of smell[edit]

The coconut crab has a well-developed sense of smell, which it uses to locate its food.[17] 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 specialised 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 are shorter and blunter than those of other crabs and look more like those of insects.[17] While insects and the coconut crabs originate from different paths, the same need to detect smells in the air led to the development of remarkably similar organs. Coconut crabs flick their antennae as insects do to enhance their reception. They have an excellent sense of smell and can detect interesting odours over large distances. The smells of rotting meat, bananas, and coconuts, all potential food sources, catch their attention especially.[18] The olfactory system in the coconut crab's brain is well-developed compared to other areas of the brain.[19]

Lifecycle[edit]

Coconut crabs mate frequently and quickly on dry land from May to September, especially between early June and late August.[20] Male coconut crabs deposit a mass of spermatophores on the abdomen of the female;[21] the abdomen opens at the base of the third pereiopods, and fertilisation is thought to occur on the external surface of the abdomen as the eggs pass through the spermatophore mass.[22] The extrusion of eggs occurs on land in crevices or burrows near the shore.[23] Shortly thereafter, the female lays her eggs and glues them to the underside of her abdomen, carrying the fertilised eggs underneath her body for a few months. At the time of hatching, the female coconut crab releases the eggs into the ocean.[22] This usually takes place on rocky shores at dusk, especially when this coincides with high tide.[24] The empty egg cases remain on the female's body after the larvae have been released, and the female eats the egg cases within a few days.[24]

The larvae float in the pelagic zone of the ocean with other plankton for three to four weeks,[5] during which a large number of them are eaten by predators. The larvae pass through three to five zoea stages before moulting into the postlarval glaucothoe stage; this process takes from 25 to 33 days.[25] 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.[26] At that time, they sometimes visit dry land. Afterwards, they leave the ocean permanently and lose the ability to breathe in water. As with all hermit crabs, they change their shells as they grow. Young coconut crabs that cannot find a seashell of the right size often use broken coconut pieces. When they outgrow their shells, they develop a hardened abdomen. The coconut crab reaches sexual maturity around five years after hatching.[22] They reach their maximum size only after 40 to 60 years.[10]

Distribution[edit]

Coconut crabs live in the Indian Ocean and the central Pacific Ocean; their distribution closely matches that of the coconut palm.[27] The western limit of the range of B. latro is Zanzibar (with a stronghold on Chumbe island), off the coast of Tanzania,[28] while the Tropic of Cancer and Tropic of Capricorn mark the northern and southern limits, respectively, with very few population in the subtropics, such as the Ryukyu Islands.[5] 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.[5] As they cannot swim as adults, coconut crabs must have colonised the islands as planktonic larvae.[29]

Christmas Island in the Indian Ocean has the largest and densest population of coconut crabs in the world,[17] although it is outnumbered there by more than 50 times by the Christmas Island red crab, Gecarcoidea natalis.[30] Other Indian Ocean populations exist on the Seychelles, including Aldabra and Cosmoledo,[31] but the coconut crab is extinct on the central islands.[32] 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.[33]

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".[34] The coconut crab is far more widespread, though it is not abundant on every Pacific island it inhabits.[34] 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 (Washington Island), with its abundant coconut palm forest.[34] The Gambier Islands marks the species' eastern limit.[28]

Ecology[edit]

Diet[edit]

A coconut crab atop a coconut

The diet of a coconut crab consists primarily of fleshy fruits (particularly Ochrosia ackeringae, Arenga listeri, Pandanus elatus, P. christmatensis), nuts (coconuts Cocos nucifera, Aleurites moluccana), and seeds (Annona reticulata),[35] and on the pith of fallen trees.[36] However, as they are omnivores, they will consume other organic materials such as tortoise hatchlings and dead animals.[10][37] They have been observed to prey upon crabs, such as Gecarcoidea natalis and Discoplax hirtipes, and scavenge on the carcasses of other coconut crabs.[35] During a tagging experiment, one coconut crab was observed killing and eating a Polynesian rat (Rattus exulans).[38] Coconut crabs may be responsible for the disappearance of Amelia Earhart's remains, consuming them after her death and hoarding her skeletal remnants in their burrows.[39]

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 (33 ft) high and drop the husk nut, to access the content inside.[40] They often descend from the trees by falling, and can survive a fall of at least 4.5 m (15 ft) unhurt.[41] 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.[36]

Thomas Hale Streets discussed their behaviour in 1877, doubting that the animal would climb trees to get at the nuts.[34] In the 1980s, Holger Rumpff was able to confirm Streets's report, observing and studying how they open coconuts in the wild.[36] 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 coconut 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.[42]

Habitat[edit]

Coconut crabs vary in size and colouring.

Coconut crabs are considered one of the most terrestrial decapods,[43] with most aspects of its life linked to a terrestrial existence; they will drown in sea water in less than a day.[16] 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 crab's burrows contain very fine yet strong fibres of the coconut husk which the animal uses as bedding.[34] While resting in its burrow, it 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 will 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 (3.7 mi) from the sea.[44]

Relationship with human beings[edit]

Adult coconut crabs have no known predators apart from other coconut crabs and people. Its large size and the quality of its meat mean the coconut crab is extensively hunted and is rare on islands with a human population.[45] The coconut crab is eaten by Southeast Asians and Pacific Islanders and is considered a delicacy and an aphrodisiac, and intensive hunting has threatened the species' survival in some areas.[11] 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.[45][46] For instance, consumption of the sea mango Cerbera manghas by the coconut crab may make the coconut crab toxic due to the presence of cardiac cardenolides.[47]

The pincers of the coconut crab are powerful enough to cause noticeable pain to a human; furthermore, the coconut crab will often keep its hold for extended periods of time. Thomas Hale Streets reported a 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 loosen its hold."[34]

In the Cook Islands, the coconut crab is known as unga or kaveu, and in the Mariana Islands it is called ayuyu, and is sometimes associated with taotaomo'na because of the traditional belief that ancestral spirits can return in the form of animals such as the coconut crab.[48]

Conservation[edit]

Coconut crab populations in several areas have declined or become locally extinct due to both habitat loss and human predation.[49][50] 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.[1][5]

Conservation management strategies have been put in place in some regions, such as minimum legal size limit restrictions in Guam and Vanuatu, and a ban on the capture of egg-bearing females in Guam and the Federated States of Micronesia.[51] In the Northern Mariana Islands, hunting of adults not bearing eggs above a carapace length of 30 mm (1.2 in) may take place in September, October, and November, and only under licence. A bag limit of five coconut crabs on any given day, and 15 across the whole season has been set.[52]

In Tuvalu, coconut crabs live on the motu (islets) in the Funafuti Conservation Area, a marine conservation area covering 33 km2 (12.74 mi2) of reef, lagoon and motu on the western side of Funafuti atoll.[53]

Names[edit]

The coconut crab has been known to western scientists since the voyages of William Dampier around 1688.[54] Based on an account by Georg Eberhard Rumphius (1705), who had called the animal "Cancer cruentatus", Carl Linnaeus (1767) named the species Cancer latro,[55] from the Latin latro, meaning "robber". The genus Birgus was erected in 1816 by William Elford Leach, containing only Linnaeus' Cancer latro, which was thus renamed Birgus latro.[3] Birgus is classified in the family Coenobitidae, alongside one other genus, Coenobita, which contains the terrestrial hermit crabs.[3][28]

Common names for the species include coconut crab, robber crab and palm thief,[1] which mirrors the animal's name in other European languages (e.g. German: Palmendieb).[56] In the Pohnpeian language of the Eastern Caroline Islands, the crab is called emp.[57]

References[edit]

  1. ^ a b c L. G. Eldredge (1996). "Birgus latro". IUCN Red List of Threatened Species. Version 2.3. International Union for Conservation of Nature. Retrieved July 25, 2011. 
  2. ^ Patsy McLaughlin (2010). "Birgus latro (Linnaeus, 1767)". In P. McLaughlin. World Paguroidea database. World Register of Marine Species. Retrieved March 3, 2011. 
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