Coconut crab

• 1. Giant of the Islands

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  The coconut crab, Birgus latro, can weigh as much as 4.1 kilograms, which makes it the largest arthropod living on land. Stretch its legs out and the distance from one tip to another can reach a full metre. Biologically it is a hermit crab, a group of animals better known for squatting in borrowed snail shells. Its nicknames accuse it of theft, and its habits give those accusations some weight. How did a hermit crab grow so large? Why can an animal that lives on land drown in the sea? What does it eat, given its name? And why is it missing from so many places where people have settled?

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• 2. A Thief by Name

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  Western science first heard of this animal through the voyages of Francis Drake around 1580 and William Dampier around 1688. A formal name took longer. In 1705 Georg Eberhard Rumphius described it as Cancer crumenatus. Building on his account, Carl Linnaeus renamed it Cancer latro in 1767, taking latro from the Latin word for robber.

  In 1816 William Elford Leach erected the genus Birgus to hold that one species, so Linnaeus's animal became Birgus latro. It belongs to the family Coenobitidae, beside a single other genus, Coenobita, made up of terrestrial hermit crabs.

  The English names coconut crab, robber crab and palm thief have a close echo in German, where the animal is called Palmendieb. In Japan, where it lives on some of the country's southerly island chains, the name means palm crab. Its relatives in Coenobita stay much smaller, and the reason lies in what the adult crab leaves behind.

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• 3. Life Without a Shell

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  Juvenile coconut crabs push their soft abdomens into empty snail shells, just as other hermit crabs do. Adolescents sometimes switch to broken pieces of coconut shell. Adults give up borrowing altogether and harden the back of the body with deposits of chitin and calcium carbonate. With no borrowed shell to limit its size, the species grows far larger than its relatives. Even so, like a true crab, it folds its tail beneath its body for protection.

  Most sources give a body length of up to 40 cm, with males generally larger than females. The carapace can reach 78 mm in length and 200 mm in width.

  Ten legs are attached to the front section of the body, and each pair has its own job. The front pair carries large claws, with the left bigger than the right. The next two pairs are powerful walking legs with pointed tips that grip vertical and even overhanging surfaces. The fourth pair is smaller and ends in tweezer-like claws. Juveniles use them to hold the inside of a shell or a coconut husk, while adults use them for walking and climbing. The last pair is tiny and kept inside the cavity that holds the breathing organs. Females use it to tend their eggs, and males use it in mating.

  Colour changes from island to island, ranging from orange-red to purplish blue. Blue dominates in most regions, but in the Seychelles most individuals are red.

  The hardened abdomen reduces water loss on land, but it must be shed. Adults moult once a year, digging a burrow up to 1 m long to hide in while the new shell sets. Hardening takes 1-3 weeks depending on the crab's size, and the animal stays underground for 3-16 weeks. Down there, it depends on an organ that sits somewhere between a gill and a lung.

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• 4. Breath and Scent

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  An adult coconut crab left in water for more than an hour will drown. Its answer to breathing air is the branchiostegal lung, which can be read as a halfway stage between gills and lungs. It contains tissue like that of a gill, but suited to taking oxygen from air instead of water. The organ is expanded sideways and folded outward to add surface area. Because it sits in the front section of the body, the distance for gas exchange stays short, and so does the return trip for oxygenated blood.

  A rudimentary set of gills remains. Their number is comparable to that of aquatic crabs in the families Paguridae and Diogenidae, but they are smaller and offer less surface area.

  The smallest, hindmost pair of legs works as a maintenance crew for the lungs. It cleans the breathing organs and keeps them damp, stroking wet leg tips over the spongy tissue nearby, because the organs need water to function. The crab can also drink from small puddles by passing water from its claws to its mouthparts.

  Smelling works very differently in water than in air. Crabs that live in water carry organs called aesthetascs on their antennae to judge both the strength and the direction of a scent. On the coconut crab these organs are shorter and blunter, and they resemble those of insects. Insects and coconut crabs come from different clades, yet the shared need to track smells in the air produced similar organs through convergent evolution. Like an insect, the crab flicks its antennae to improve reception.

  It can pick up interesting odours over large distances. Rotting meat, bananas and coconuts are the smells that catch its attention most. In its brain, the olfactory system is well developed compared with other regions. Those scents lead it to a menu stranger than its name suggests.

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• 5. More Than Coconuts

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  In 1877 Thomas Hale Streets doubted that the crab would climb trees to get at coconuts. As late as the 1970s, doubts remained about whether it could open them at all. In the 1980s Holger Rumpf confirmed Streets's report by watching crabs do it in the wild.

  Rumpf found that the crab follows a fixed routine. If the coconut still has its husk, the crab rips off strips with its claws, always starting from the side with the three germination pores. These are three small circles on the outside of the nut. Once the pores show, the crab bangs its pincers on one of them until it breaks. Then it turns around and pulls out the white flesh with the smaller pincers on its other legs. Larger crabs can even break the hard nut into pieces, and getting a coconut open may take several days.

  A crab can cut a coconut down to a husk nut, carry it up a tree 10 m high and drop it to reach the flesh inside. It often comes down by falling, and it can survive a drop of at least 4.5 m unhurt.

  Yet coconuts are not a significant part of the adult diet. Fleshy fruits come first, particularly Ochrosia ackeringae, Arenga listeri, Pandanus elatus and P. christmatensis. The crab also eats the nut Aleurites moluccanus, seeds of Annona reticulata, and the pith of fallen trees. Because it is an omnivore, it also takes tortoise hatchlings, dead animals and the moulted shells of other crustaceans. It has been seen hunting the crabs Gecarcoidea natalis and Discoplax hirtipes, and scavenging on the bodies of other coconut crabs.

  During a tagging experiment, one crab was seen killing and eating a Polynesian rat. In 2016 a large crab on the Chagos Archipelago was seen climbing a tree to disable and eat a red-footed booby.

  Anything left unattended on the ground is worth investigating, and the crab may carry it away. That habit earned it the nickname robber crab. Filmed crabs have climbed coconut and pandanus trees, but none has been filmed selectively picking coconut fruit, though a crab might knock loose ripe fruit that was about to fall anyway. When it is far from its burrow, a tree is an immediate escape from predators. Sea birds eat the young crabs, and the young crabs' journey begins in the open sea.

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• 6. From Egg to Giant

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  From May to September, coconut crabs mate often and quickly on dry land, with activity peaking from early June to late August. The male deposits a mass of spermatophores on the female's abdomen. The oviducts open at the base of the third pair of pereiopods, and fertilisation is thought to happen on the outside of the abdomen as the eggs pass through that mass.

  The female lays soon after mating, in crevices or burrows near the shore. She glues the eggs to the underside of her abdomen and carries them for a few months. When they are ready to hatch, she walks to the seashore and releases the larvae into the ocean. This is a gamble, because she cannot swim. If she falls in or is swept away, her weight makes it difficult or impossible to get back to land. The release usually happens on rocky shores at dusk, especially when that coincides with high tide. Afterward the empty egg cases stay on her body, and she eats them within a few days.

  The larvae drift with other plankton for 3-4 weeks, and many are eaten by predators. They pass through three to five zoea stages before moulting into a postlarval form called the glaucothoe, a process taking 25 to 33 days. Floating logs, rafts of vegetation and floating coconuts can carry them to a suitable new shore.

  At the glaucothoe stage the young crab settles to the sea floor, finds a gastropod shell of the right size and travels to the shoreline with other terrestrial hermit crabs. It sometimes visits dry land at this point. Then it leaves the ocean for good and loses the ability to breathe in water. Young crabs unable to find a seashell of the right size often use broken coconut pieces. When they outgrow their shells, they develop the hardened abdomen.

  Sexual maturity arrives around 5 years after hatching, and the total lifespan may be over 60 years. Maximum size comes only after 40-60 years. Michelle Drew of the Max Planck Institute has posited that full size could take up to 120 years. Fossils of the coconut crab date back to the Miocene.

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• 7. Islands and Burrows

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  Zanzibar, off the coast of Tanzania, marks the western limit of the coconut crab's range. The species lives in the Indian Ocean and the central Pacific, in a pattern that closely matches the distribution of the coconut palm. The tropics of Cancer and Capricorn form its northern and southern limits, with very few populations in the subtropics, such as the Ryukyu Islands. The Gambier Islands mark its eastern limit.

  Some evidence suggests the crab once lived on mainland Australia, Madagascar, Rodrigues, Easter Island, Tokelau and the Marquesas Islands, and possibly India. It is now gone from all of them. Adults cannot swim, so the crabs must have colonised the islands as planktonic larvae.

  Christmas Island in the Indian Ocean holds the largest and densest population in the world. Even there, the Christmas Island red crab, Gecarcoidea natalis, outnumbers it by more than 50 times. In the Seychelles, populations survive on Aldabra and Cosmoledo, but the crab is extinct on the central islands. It occurs on several of the Andaman and Nicobar Islands in the Bay of Bengal, and on most islands and the northern atolls of the Chagos Archipelago.

  In the Pacific, the range became known gradually. Charles Darwin assumed the crab was found only on "a single coral island north of the Society group". It is far more widespread, though not abundant on every island. Large populations live in the Cook Islands, especially Pukapuka, Suwarrow, Mangaia, Takutea, Mauke, Atiu and Palmerston Island. In the Line Islands of Kiribati, the crab is especially frequent on Teraina, also called Washington Island, which has an abundant coconut palm forest.

  Coconut crabs live alone, in burrows they dig in sand or loose soil or in rock crevices, depending on the terrain. By day they stay hidden to avoid losing water to the heat. Fine, strong fibres of coconut husk line the burrow as bedding. While resting, the crab seals the entrance with one claw, which creates the moist microclimate its breathing organs need. Where populations are large, some crabs come out by day, perhaps to get ahead in the search for food. Others emerge when the weather is humid or rainy, since breathing is easier then. They return to the sea only to release eggs, and on Christmas Island the species is abundant 6 km from the shore.

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• 8. Hunted and Protected

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  Adult coconut crabs have no known predators apart from other coconut crabs and humans. Their size and the quality of their meat make them a heavy target for hunters. On various islands the crab is eaten as a delicacy and regarded as an aphrodisiac, and intensive hunting has threatened the species in some areas.

  Elsewhere, taboos protect it. Such taboos have been recorded in the Nicobar Islands of India, on Flores Island in Indonesia, and among the Tao people of Taiwan. On Kamorta Island in the Nicobars, eating the crab is believed to bring bad luck and severe, sometimes fatal, illness. When locals fall ill after eating it, their families make a wooden image of the creature. They carry it to the place where the crab was caught and perform specific rituals there.

  The crab is not innately poisonous, but its diet can make it so, and cases of poisoning have occurred. If it eats the sea mango, Cerbera manghas, it may become toxic because of cardiac cardenolides in the plant.

  Its pincers can cause noticeable pain, and the crab often holds on for a long time. Thomas Hale Streets reported a trick used by Micronesians of the Line Islands. A gentle tickling of the crab's soft underside with any light material makes it loosen its grip.

  In the Cook Islands the crab is known as unga or kaveu. In the Mariana Islands it is called ayuyu, and it is sometimes linked with taotaomo'na because of a traditional belief that ancestral spirits can return in the form of animals such as the coconut crab. A popular internet legend claims that Amelia Earhart crash-landed on Nikumaroro and that coconut crabs quickly devoured her remains. No evidence of her plane has been found on or near the island, and historians generally discredit the theory.

  Populations in several areas have declined or disappeared through habitat loss and human predation. The IUCN Red List first listed the species as vulnerable in 1981. A lack of biological data led to a change to data deficient in 1996, and in 2018 the assessment returned to vulnerable.

  Some regions have set rules. Guam and Vanuatu impose minimum legal sizes. Guam and the Federated States of Micronesia ban the capture of females carrying eggs. In the Northern Mariana Islands, adults without eggs and above a carapace length of 76 mm may be hunted only in September, October and November, and only under license. The bag limit is five crabs a day and 15 for the whole season. In Tuvalu, crabs live on the motu, or islets, within the Funafuti Conservation Area, a marine reserve of 33 km2 on the western side of Funafuti atoll.

  In the Philippines, the crab goes by names such as tatus, umang kagang, kasaso, kuray and manla. The 2001 Fisheries Administrative Order No. 208 declared it locally threatened. Catching, selling, buying, transporting or possessing coconut crabs is illegal, with a fine of ₱120,000 and up to six years in prison. Even so, open capture and eating for the tourist trade continues, particularly in the Batanes Islands. Lax enforcement and conflicts with laws that take precedence, such as the Indigenous Peoples' Rights Act of 1997, are the stated reasons. Populations across the islands keep falling rapidly from overharvesting and habitat destruction.

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