Coconut crab

Thecoconut crab(Birgus latro) is aterrestrialspeciesof gianthermit crab, and is also known as therobber craborpalm thief. It is the largest terrestrialarthropodknown, with a weight of up to 4.1 kg (9 lb). The distance from the tip of one leg to the tip of another can be as wide as 1 m (3 ft 3 in). It is found on islands across theIndianandPacificOceans, as far east as theGambier Islands,Pitcairn IslandsandCaroline Islandand as far west asZanzibar. While its range broadly shadows the distribution of thecoconut palm, the coconut crab has beenextirpatedfrom most areas with a significant human population such as mainlandAustraliaandMadagascar.
The coconut crab is theonly speciesof thegenusBirgus, and is related to the other terrestrial hermit crabs of the genusCoenobita. It shows a number ofadaptationsto life on land. Juvenile coconut crabs use emptygastropod shellsfor protection like other hermit crabs, but the adults develop a tough exoskeleton on their abdomens and stop carrying a shell. Coconut crabs have organs known asbranchiostegal lungs, which they use for breathing instead of their vestigialgills. After the juvenile stage, they will drown if immersed in water for too long. They have an acute sense of smell which they use to find potential food sources, and which has developedconvergentlywith that ofinsects.
Adult coconut crabs feed primarily on fleshy fruits, nuts, seeds, and thepithof fallen trees, but they will eatcarrionand 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 andpandanustrees. 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 areplanktonicfor 3–4 weeks, before settling to the sea floor, entering agastropod shelland returning to dry land.Sexual maturityis 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.Fossils of this crab date back to theMiocene.
The coconut crab has been known to western scientists since the voyages ofFrancis Drakearound 1580andWilliam Dampieraround 1688.Based on an account byGeorg Eberhard Rumphius(1705), who had called the animal "Cancer crumenatus",Carl Linnaeus(1767) named the speciesCancer latro,from theLatinlatro, meaning "robber". The genusBirguswas erected in 1816 byWilliam Elford Leach, containing only Linnaeus'Cancer latro, which was thus renamedBirgus latro.
Birgusis classified in the familyCoenobitidae, alongside one other genus,Coenobita, which contains terrestrial hermit crabs.
Common namesfor the species include coconut crab, robber crab, and palm thief,which mirrors the animal's name in other European languages (e.g.German:Palmendieb).In Japan (where the species lives on some of the country's southerly island chains), the species is typically referred to asyashigani(ヤシガニ), meaning 'palm crab'.
B. latrois both the largest living terrestrialarthropodand the largest living terrestrial invertebrate.Reports of its size vary, but most sources give a body length up to 40 cm (16 in),a weight up to 4.1 kg (9 lb), and a leg span more than 0.91 m (3 ft),with males generally being larger than females.Thecarapacemay reach a length of78 mm (3+1⁄16in), and a width up to 200 mm (8 in).
The body of the coconut crab is, like those of alldecapods, divided into a front section (cephalothorax) with 10legs, and anabdomen. The front-most pair of legs has largechelae(claws) with the left being larger than the right.The next two pairs of legs, as with other hermit crabs, are large, powerful walking legs with pointed tips that allow coconut crabs to climb vertical or even overhanging surfaces.The fourth pair of legs is smaller, withtweezer-like chelae at the end allowing young coconut crabs to grip the inside of the shell or coconut husks that juveniles habitually 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 in the cavity containing the breathing organs, inside the carapace. Some difference in color occurs between individuals found on different islands, ranging from orange-red to purplish blue,In most regions, blue is the predominant color, but in some places such as theSeychellesmost individuals are red.
AlthoughB. latrois aderivedtype ofhermit crab, only juveniles use salvagedsnailshellsto protect their soft abdomens while adolescents sometimes use broken coconut shells for the same purpose. Unlike other hermit crabs, the adult coconut crabs do not carry shells but instead harden their abdominaltergaby depositingchitinandcalcium carbonate. Absent the physical constraint of living within another creature's shellB. latrogrows much larger than its relatives in the family Coenobitidae.Despite being the product ofcarcinization, like mosttrue crabsB. latrobends itstailbeneath its body for protection.
The hardened abdomen protects the coconut crab and reduces water loss on land, but must be periodicallymoulted. Adults moult annually, digging a burrow up to 1 m (3 ft 3 in) long in which to hide while their soft shell hardens.Depending on the size of the individual 1–3 weeks are needed for theexoskeletonto harden. The animals remain in this burrow for 3–16 weeks, again depending on size.
Except aslarvae, coconut crabs cannot swim, and they drown if left in water for more than an hour.They use a special organ called abranchiostegal lungto breathe. This organ can be interpreted as a developmental stage betweengillsandlungs, and is one of the most significant adaptations of the coconut crab to itshabitat.The branchiostegal lung contains atissuesimilar to that found in gills, but suited to the absorption ofoxygenfrom air, rather than water. This organ is expanded laterally and isevaginatedto 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 thepericardium.
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 theirchelipedsto theirmaxillipeds.
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 familiesPaguridaeandDiogenidae, they are reduced in size and have comparatively less surface area.
The coconut crab has a well-developed sense of smell, which it uses to locate its food.The process of smelling works very differently depending on whether the smelled molecules arehydrophilicmolecules in water orhydrophobicmolecules in air. Crabs that live in water have specialized organs calledaesthetascson theirantennaeto determine both the intensity and the direction of a scent. Coconut crabs live on the land, so the aesthetascs on their antennae are shorter and blunter than those of other crabs and are more similar to those ofinsects.
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.Theolfactory systemin the coconut crab's brain is well-developed compared to other areas of the brain.
Coconut crabs mate frequently and quickly on dry land in the period from May to September, especially between early June and late August.Males havespermatophoresand deposit a mass of spermatophores on the abdomens of females;theoviductsopens at the base of the thirdpereiopods, andfertilisationis thought to occur on the external surface of the abdomen, as the eggs pass through the spermatophore mass.
The extrusion ofeggsoccurs on land in crevices or burrows near the shore.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 thelarvaeinto the ocean.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.The egg laying usually takes place on rocky shores at dusk, especially when this coincides with hightide.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.
The larvae float in thepelagic zoneof the ocean with otherplanktonfor 3–4 weeks,during which a large number of them are eaten by predators. Thelarvaepass through three to fivezoeastages before moulting into the postlarvalglaucothoestage; this process takes from 25 to 33 days.
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. 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 reachessexual maturityaround 5 years after hatching.They reach their maximum size only after 40–60 years.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.
Coconut crabs live in theIndian Oceanand the centralPacific Ocean, with a distribution that closely matches that of thecoconut palm.The western limit of the range ofB. latroisZanzibar, off the coast ofTanzania,while the tropicsof CancerandCapricornmark the northern and southern limits, respectively, with very few populations in thesubtropics, such as theRyukyu Islands.Some evidence indicates the coconut crab once lived on the mainland of Australia, Madagascar,Rodrigues,Easter Island,Tokelau, theMarquesasislands, and possiblyIndia, but is nowextirpatedin those areas.As they cannot swim as adults, coconut crabs must have colonised the islands as planktonic larvae.
Christmas Islandin 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 theChristmas Island red crab(Gecarcoidea natalis).Other Indian Ocean populations exist on theSeychelles, includingAldabraandCosmoledo,but the coconut crab is extinct on the central islands.Coconut crabs occur on several of theAndamanandNicobar Islandsin theBay of Bengal. They occur on most of the islands, and the northernatolls, of theChagos Archipelago.
In the Pacific, the coconut crab's range became known gradually.Charles Darwinbelieved it was only found on "a single coral island north of theSociety group".The coconut crab is far more widespread, though it is not abundant on every Pacific island it inhabits.Large populations exist on theCook Islands, especiallyPukapuka,Suwarrow,Mangaia,Takutea,Mauke,Atiu, andPalmerston Island. These are close to the eastern limit of its range, as are theLine IslandsofKiribati, where the coconut crab is especially frequent onTeraina(Washington Island), with its abundant coconut palm forest.TheGambier Islandsmark the species' eastern limit.
The diet of coconut crabs consists primarily of fleshyfruits(particularlyOchrosia ackeringae,Arenga listeri,Pandanus elatus,P. christmatensis); nuts (Aleurites moluccanus), drupes (Cocos nucifera) and seeds (Annona reticulata);and thepithof fallen trees.However, as they areomnivores, they will consume other organic materials such astortoisehatchlings and dead animals, including other crustaceans, as well as the molted exoskeletons of other crustaceans.They have been observed to prey upon crabs such asGecarcoidea natalisandDiscoplax hirtipes, as well as scavenge on the carcasses of other coconut crabs.During a tagging experiment, one coconut crab was observed killing and eating aPolynesian rat(Rattus exulans).In 2016, a large coconut crab was observed climbing a tree to disable and consume ared-footed boobyon theChagos Archipelago.
The coconut crab can take a coconut from the ground and cut it to ahusknut, 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.They often descend from the trees by falling, and can survive a fall of at least 4.5 m (15 ft) 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.
Thomas Hale Streetsdiscussed the behaviour in 1877, doubting that the animal would climb trees to get at the coconuts.As late as the 1970s there were doubts about the crab's ability to open coconuts.In the 1980s, Holger Rumpf was able to confirm Streets' report, observing and studying how they 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 threegerminationpores, 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.
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 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, 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; onChristmas Island, for instance,B. latrois abundant6 km (3+1⁄2mi) from the sea.
Adult coconut crabs have no knownpredatorsapart from other coconut crabs and humans. Its large size and the quality of its meat means that the coconut crab is extensively hunted and is very rare on islands with a human population.The coconut crab is eaten as adelicacy– and regarded as anaphrodisiac– on various islands, and intensive hunting has threatened the species' survival in some areas.In other regions, there are taboos associated with the crab that prohibit or limit hunting and consumption ofBirgus latro. Such taboos have been recorded in theNicobar Islandsin India, onFlores Islandin Indonesia, and among theTao peopleof Taiwan.On the NicobarianKamorta Island, it is believed that eating the crab leads to bad luck and can cause severe, sometimes fatal, illnesses. In cases where a local falls ill after consuming the crab, their family creates a wooden replica of the creature. Thiseffigyis then taken to the crab's capture site, where specific rituals are performed.
While the coconut crab itself is not innatelypoisonous, it may become so depending on its diet, and cases of coconut crab poisoning have occurred.For instance, consumption of the sea mango (Cerbera manghas) by the coconut crab may make the coconut crab toxic due to the presence of cardiaccardenolides.
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 byMicronesiansof theLine Islandsto 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."
In theCook Islands, the coconut crab is known asungaorkaveu, and in theMariana Islandsit is calledayuyu, and is sometimes associated withtaotaomo'nabecause of the traditional belief that ancestral spirits can return in the form of animals such as the coconut crab.
A popularinternet memesuggests thatAmelia Earhartcrash-landed onNikumaroroand her remains were rapidly consumed by coconut crabs on the island.However, as no evidence of Earhart's plane has been found on or near Nikumaroro, this theory is generally discredited by historians.
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 theIUCN Red Listas avulnerable species, but a lack of biological data caused its assessment to be amended to "data deficient" in 1996.In 2018, IUCN updated its assessment to "vulnerable".
Conservation management strategies have been put in place in some regions, such as minimum legal size limit restrictions inGuamandVanuatu, and a ban on the capture of egg-bearing females in Guam and theFederated States of Micronesia.In theNorthern 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.
InTuvalu, coconut crabs live on themotu(islets) in theFunafuti Conservation Area, a marine conservation area covering 33 km(12.74 mi) of reef, lagoon andmotuon the western side ofFunafutiatoll.