Coconut crab
Coconut crab | |
---|---|
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Arthropoda |
Class: | Malacostraca |
Order: | Decapoda |
Suborder: | Pleocyemata |
Infraorder: | Anomura |
Family: | Coenobitidae |
Genus: | Birgus Leach, 1816 |
Species: | B. latro
|
Binomial name | |
Birgus latro | |
Coconut crabs live on most coasts in the blue area; red points are primary and yellow points secondary places of settlement | |
Synonyms[3] | |
The coconut crab (Birgus latro) is a
The coconut crab is the
Adult coconut crabs feed primarily on fleshy fruits, nuts, seeds, and the pith of fallen trees, but they will eat carrion and other organic matter opportunistically. Anything left unattended on the ground is a potential source of food, which they will investigate and may carry away – thereby getting the alternative name of "robber crab". The species is popularly associated with the coconut palm, yet coconuts are not a significant part of its diet. Although it lives in a burrow, the crab has been filmed climbing coconut and pandanus trees. No film shows a crab selectively picking coconut fruit, though they might dislodge ripe fruit that otherwise would fall naturally. When a crab is not near its burrow, climbing is an immediate escape route from predators. Sea birds eat young crabs, and both humans and larger, older crabs eat crabs of all ages.
Mating occurs on dry land, but the females return to the edge of the sea to release their fertilized eggs, and then retreat up the beach. The larvae that hatch are planktonic for 3–4 weeks, before settling to the sea floor, entering a gastropod shell and returning to dry land. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years. In the 3–4 weeks that the larvae remain at sea, their chances of reaching another suitable location is enhanced if a floating life support system avails itself to them. Examples of the systems that provide such opportunities include floating logs and rafts of marine or terrestrial vegetation. Similarly, floating coconuts can be a very significant part of the crab's dispersal options.[4] Fossils of this crab date back to the Miocene.[5]
Taxonomy
The coconut crab has been known to western scientists since the voyages of Francis Drake around 1580[6] and William Dampier around 1688.[7] Based on an account by Georg Eberhard Rumphius (1705), who had called the animal "Cancer crumenatus", Carl Linnaeus (1767) named the species Cancer latro,[8] 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 terrestrial hermit crabs.[3][9]
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).[10] In Japan (where the species lives on some of the country's southerly island chains), the species is typically referred to as yashigani (ヤシガニ), meaning 'palm crab'.[11]
Description
B. latro is both the largest living terrestrial arthropod and the largest living terrestrial invertebrate.[12][13] Reports of its size vary, but most sources give a body length up to 40 cm (16 in),[14] a weight up to 4.1 kg (9 lb), and a leg span more than 0.91 m (3 ft),[15] with males generally being larger than females.[16] The carapace may reach a length of 78 mm (3+1⁄16 in), and a width up to 200 mm (8 in).[13]
The body of the coconut crab is, like those of all
Although B. latro is a
The hardened abdomen protects the coconut crab and reduces water loss on land, but must be periodically moulted. Adults moult annually, digging a burrow up to 1 m (3 ft 3 in) long in which to hide while their soft shell hardens.[18] Depending on the size of the individual 1–3 weeks are needed for the exoskeleton to harden. The animals remain in this burrow for 3–16 weeks, again depending on size.[18][21]
Respiration
Except as
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 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
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.[23]
Sense of smell
The coconut crab has a well-developed sense of smell, which it uses to locate its food.
While insects and the coconut crab originate from different clades, the same need to track smells in the air led to convergent evolution of similar organs. Coconut crabs flick their antennae as insects do to enhance their reception. Their sense of smell can detect interesting odors over large distances. The smells of rotting meat, bananas, and coconuts, all potential food sources, catch their attention especially.[26] The olfactory system in the coconut crab's brain is well-developed compared to other areas of the brain.[27]
Life cycle
Coconut crabs mate frequently and quickly on dry land in the period from May to September, especially between early June and late August.[28] Males have spermatophores and deposit a mass of spermatophores on the abdomens of females;[29] the oviducts 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.[30]
The extrusion of eggs occurs on land in crevices or burrows near the shore.[31] The female lays her eggs shortly after mating 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 migrates to the seashore and releases the larvae into the ocean.[30] The coconut crab takes a large risk while laying the eggs, because coconut crabs cannot swim: If a coconut crab falls into the water or is swept away, its weight makes it difficult, or impossible, for it to swim back to dry land.[32] The egg laying usually takes place on rocky shores at dusk, especially when this coincides with high tide.[33] 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.[33]
The larvae float in the
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.[35] 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 5 years after hatching.[30] They reach their maximum size only after 40–60 years.[18] They grow remarkably slowly, and may take up to 120 years to reach full size, as posited by ecologist Michelle Drew of the Max Planck Institute.[36]
Distribution
Coconut crabs live in the
Christmas Island in the Indian Ocean has the largest and densest population of coconut crabs in the world,[25] although it is outnumbered there by more than 50 times by the Christmas Island red crab (Gecarcoidea natalis).[39] Other Indian Ocean populations exist on the Seychelles, including Aldabra and Cosmoledo,[40] but the coconut crab is extinct on the central islands.[41] 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.[42]
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".[43] The coconut crab is far more widespread, though it is not abundant on every Pacific island it inhabits.[43] 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.[43] The Gambier Islands mark the species' eastern limit.[9]
Ecology
Diet
The diet of coconut crabs consists primarily of fleshy
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 coconut flesh inside.[50] They often descend from the trees by falling, and can survive a fall of at least 4.5 m (15 ft) unhurt.[51] 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.[45]
Thomas Hale Streets discussed the behaviour in 1877, doubting that the animal would climb trees to get at the coconuts.[43] As late as the 1970s there were doubts about the crab's ability to open coconuts.[52] In the 1980s, Holger Rumpf was able to confirm Streets' report, observing and studying how they open coconuts in the wild.[45] 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 bangs its pincers on one of them until it breaks. Afterwards, it turns around and uses 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.[53]
Habitat
Coconut crabs are considered one of the most terrestrial-adapted of the decapods,[54] 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.[24] Coconut crabs live alone in 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.[43] While resting in its burrow, the coconut crab closes the entrances with one of its claws to create the moist microclimate within the burrow, which is necessary for the functioning of 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 (3+1⁄2 mi) from the sea.[55]
Relationship with humans
Adult coconut crabs have no known
While the coconut crab itself is not innately
The pincers of the coconut crab are powerful enough to cause noticeable pain to a human; furthermore, the coconut crab often keeps its hold for extended periods of time. Thomas Hale Streets reports 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."[43]
In the
A popular internet meme suggests that Amelia Earhart crash-landed on Nikumaroro and her remains were rapidly consumed by coconut crabs on the island.[63][64][65] However, as no evidence of Earhart's plane has been found on or near Nikumaroro, this theory is generally discredited by historians.[66][67]
Conservation
Coconut crab populations in several areas have declined or become locally extinct due to both habitat loss and human predation.[68][69] 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.[13] In 2018, IUCN updated its assessment to "vulnerable".[1]
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.[70] In the Northern Mariana Islands, hunting of non-egg-bearing adults above a carapace length of 76 mm (3 in) may take place in September, October, and November, and only under license. The bag limit is five coconut crabs on any given day, and 15 across the whole season.[71]
In
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- Brown, I.W.; Fielder, D.R., eds. (1991). The Coconut Crab: Aspects of the Biology and Ecology of Birgus latro in the Republic of Vanuatu. ACIAR Monograph. Vol. 8.
- Drew, M.M.; Harzsch, S.; Stensmyr, M.; Erland, S.; Hansson, B.S. (2010). "A review of the biology and ecology of the Robber Crab, Birgus latro (Linnaeus, 1767) (Anomura: Coenobitidae)". .
- Fletcher, Warwick J. (1993). "Coconut crabs". In Wright, Andrew; Hill, Lance (eds.). Nearshore Marine Resources of the South Pacific: Information for fisheries development and management. ISBN 978-982-02-0082-1.
- Hartnoll, Richard (1988). "Evolution, systematics, and geographical distribution". In Burggren, Warren W.; McMahon, Brian Robert (eds.). Biology of the Land Crabs. ISBN 978-0-521-30690-4.
- Wolcott, Thomas G. (1988). "Ecology". In Burggren, Warren W.; McMahon, Brian Robert (eds.). Biology of the Land Crabs. ISBN 978-0-521-30690-4.
External links
- Coconut crab attacks bird (video) – via YouTube. – Footage of a coconut crab preying upon a red-footed booby, at Chagos Archipelago, British Indian Ocean Territory