Coconut crab

Name and taxonomy

Francis Drake's crew met the crab around 1580, and William Dampier described it around 1688. Georg Eberhard Rumphius described it in 1705 and called it Cancer crumenatus. Carl Linnaeus named it Cancer latro in 1767, from the Latin latro, a robber. William Elford Leach created the genus Birgus for it in 1816, and it has been Birgus latro since. It is the only species in its genus.

The crab belongs to the family Coenobitidae, the land hermit crabs, which holds one other genus, Coenobita. That family sits in the superfamily Paguroidea, the hermit crabs, inside the order Decapoda. Its English names are coconut crab, robber crab and palm thief, and the German Palmendieb means the same as the last. In Japan it is yashigani, the palm crab, and in the Mariana Islands it is ayuyu. In the Cook Islands it is unga or kaveu, and in the Philippines it has many names, among them tatus and umang kagang.

Evolution and relatives

The coconut crab is a hermit crab that has taken to the land more fully than any other decapod. Its closest relatives are the land hermit crabs of the genus Coenobita, which still carry shells as adults. Fossils of the coconut crab go back to the Miocene. Free of a borrowed shell, it has grown far larger than any other member of its family.

Like a true crab, it folds its tail under its body, a result of the process biologists call carcinisation. Its sense of smell evolved separately from that of insects, yet the two ended up with similar organs on the antennae. Like other hermit crabs, its ancestors probably had a soft, twisted abdomen, and the hard plates of the adult are a later addition.

Appearance

The body reaches 40 centimetres, the weight reaches 4.1 kilograms, and the leg span passes 91 cm. The carapace reaches 78 millimetres long and 200 mm wide. Males are larger than females, with an average carapace of 75 mm against 50 mm. The colour varies by island, from orange-red to purple-blue, and blue is the usual colour in most places. In the Seychelles most crabs are red.

Like all decapods it has 10 legs. The first pair carries the great claws, and the left claw is larger than the right. The next two pairs are long, strong walking legs with pointed tips that grip bark and let the crab climb a vertical trunk. The fourth pair is small and ends in tweezer-like pincers, which the young use to hold the inside of their shell. The last pair is tiny and stays tucked in the breathing chamber, where it cleans the lungs and, in females, tends the eggs.

The adult abdomen is covered in hard plates of chitin and calcium carbonate, with leathery skin and tufts of bristles between them. A female carries three large feathery pleopods under her abdomen, which hold the egg mass. The compound eyes sit on stalks.

Habitat and range

The coconut crab lives on tropical islands in the Indian Ocean and the Pacific, and its range follows that of the coconut palm. The western limit is Zanzibar, off Tanzania, and the eastern limit is the Gambier Islands. The range stops at the two tropics, north and south, with a few outposts beyond them such as the Ryukyu Islands. Christmas Island in the Indian Ocean holds the largest and densest population in the world. Other strongholds are Aldabra in the Seychelles, the Chagos Archipelago, the Cook Islands and Teraina in the Line Islands.

The crab has vanished from most places with many people, including mainland Australia, Madagascar, Easter Island and the Marquesas. It cannot swim as an adult, so it reached every island as a drifting larva, often on a floating log or coconut. GBIF holds records from 44 countries, but that count includes zoos and imports and is not the native range. iNaturalist holds 1,561 observations, with Japan and Christmas Island giving the most.

The crab digs a burrow in sand or loose soil, or uses a rock crevice, and lines it with fine coconut fibre. It closes the entrance with a claw to keep the air inside moist, because its lungs need damp air. It lives almost entirely on land and goes to the sea only to release its larvae. On Christmas Island it is common 6 kilometres from the shore.

Diet and feeding

The adult eats mostly fleshy fruit, nuts and seeds, together with the soft pith of fallen trees. Pandanus fruit is a main food on many islands. It is an omnivore and a scavenger, and it also eats carrion, dead crabs, shed crab skins and tortoise hatchlings. It kills and eats red crabs on Christmas Island, and one tagged crab killed and ate a Polynesian rat. In 2016 a large crab climbed a tree on the Chagos Archipelago and killed a red-footed booby.

Despite the name, coconuts are only a small part of the diet. To open one, the crab tears strips of husk from the end with the three germination pores. Then it hammers a pore with its claw until it breaks and pulls out the flesh with its smaller pincers. Opening a coconut can take several days, and a large crab can break the whole nut into pieces. A crab may also carry a nut up a tree 10 metres high and drop it. The crab itself often drops from a tree, and a fall of 4.5 metres does it no harm.

Naturalists doubted the coconut story for a century. Thomas Hale Streets described the behaviour in 1877, and Holger Rumpf confirmed it in the wild in the 1980s. The crab carries large food items back to its burrow to eat and store. Larvae eat other plankton, and captive crabs will eat lettuce, cabbage and live giant African snails.

Behaviour and social life

The coconut crab is solitary and mostly nocturnal. It hides by day to avoid losing water to the heat, and it comes out in rain or humid weather, when it breathes more easily. On islands with many people it is active only at night. Where crabs are dense, some feed by day to get ahead in the search for food.

Crabs do not fight, but they show a size-based order, and a smaller crab gives way to a larger one. On large islands a crab is nomadic and changes burrows often, while on small islands it keeps one burrow. The home range is 40 to 250 square metres, depending on the island. It climbs coconut and pandanus trees, and climbing is its quickest escape when it is far from its burrow. It often comes down by simply dropping to the ground.

An adult moults about once a year. It digs a burrow up to 1 metre long, overeats beforehand, and hides while the new shell hardens over 1 to 3 weeks. It stays in the burrow for 3 to 16 weeks, longer for a big crab, and then eats its shed skin.

Communication and senses

Smell is the crab's main sense, and it uses it to find food. The smell organs on its antennae are short and blunt, unlike those of sea crabs, and they work like an insect's. It flicks its antennae as an insect does to pick up more scent. It can smell rotting meat, bananas and coconuts from a long way off. The smell centre of its brain is large compared with the rest.

The crab has full compound eyes on stalks, and it probably sees and feels vibration much as other decapods do. Bristles on the claws give it a sense of touch. Crabs signal to each other by sight: a larger crab moves its claws and legs up and down to make a smaller crab back off. It drinks from small puddles by passing water from its claws to its mouthparts.

Reproduction and life cycle

Mating takes place on land, quickly and often, from May to September, with a peak from early June to late August. There is little courtship. The male holds the female's claws, turns her on her back and passes a mass of sperm packets to her abdomen. The female lays soon after mating and glues the eggs under her abdomen, where she carries them for a few months. ADW gives 50,000 to 138,000 embryos per spawn, and the eggs are orange.

When the eggs are ready she walks to a rocky shore at dusk, usually at high tide near a new or full moon. She shakes the eggs into the water, and they hatch on contact. This is the most dangerous moment of her life, because she cannot swim and will drown if a wave takes her. The empty egg cases stay on her body, and she eats them within a few days. The larvae drift as plankton for 3 to 4 weeks and pass through several zoea stages, which together take 25 to 33 days.

The larva then becomes a glaucothoe, settles on the sea floor, finds a snail shell of the right size and walks ashore. Once on land it never breathes water again. The young crab changes shells as it grows, and it uses a piece of coconut shell when no snail shell fits. When it outgrows every shell it hardens its abdomen and goes without. It matures at about 5 years, reaches full size only after 40 to 60 years, and may live more than 60 years. Our fields put the lifespan at up to 60 years.

Predators and threats

An adult coconut crab has no known predator except people and larger coconut crabs. Sea birds eat the young, and larvae in the plankton are eaten in huge numbers. Monitor lizards, cane toads and feral pigs may take small crabs, but this is not confirmed. A crab that is not near its burrow escapes by climbing a tree.

The biggest threats are hunting for food and the loss of coastal forest. The crab grows slowly and is easy to catch, so a population is easy to wipe out. It is now very rare on islands with a large human population. A crab that has eaten sea mango fruit becomes poisonous, and people have been poisoned by eating such crabs.

Population and conservation

The IUCN listed the coconut crab as Vulnerable in 1981. It changed that to Data Deficient in 1996 for lack of data, and it returned the crab to Vulnerable in 2018. Japan's Ministry of the Environment lists it as Vulnerable on its 2020 red list. Taiwan protects it as a listed wild animal. Populations in several areas have shrunk or vanished through habitat loss and hunting.

Guam and Vanuatu set minimum legal sizes, and Guam and the Federated States of Micronesia ban the taking of egg-bearing females. The Northern Mariana Islands allow a licensed season in September, October and November, with a limit of five crabs a day and 15 a season. In Tuvalu the crab lives on the islets of the Funafuti Conservation Area, which covers 33 square kilometres. The Philippines declared the crab locally threatened in 2001 and banned its catch and sale, but the law is weakly enforced. Crabs are still caught openly for tourists in the Batanes Islands.

Relationship with humans

Islanders have eaten the coconut crab for centuries, and its meat is a delicacy served at weddings and feasts. On some islands people regard it as an aphrodisiac. Tourists pay to see and to eat it, so hunters find it an easy income, and some small islands now import large crabs. On other islands taboos limit the hunt: the Nicobar Islands, Flores in Indonesia, and the Tao people of Taiwan all have them.

The claws can give a painful pinch, and the crab often holds on for a long time. Streets recorded a trick from the Line Islands: tickle the soft underside with something light and the crab lets go. One measurement put the pinch at about 3,300 newtons, which no other crustacean matches. ADW records no adverse effect of the crab on people, and the species is wild.

In culture and myth

On Kamorta Island in the Nicobars, people believe that eating the crab brings bad luck and serious illness. When someone falls ill after a meal, the family carves a wooden crab and takes it to the place of capture for a ritual. In the Mariana Islands the crab, called ayuyu, is linked with the taotaomo'na, ancestral spirits that can return as animals. Charles Darwin thought the crab lived on only one coral island north of the Society Islands.

An internet legend says that coconut crabs ate the remains of Amelia Earhart on Nikumaroro after her plane came down in 1937. A partial skeleton was found there in 1940, and in one experiment crabs dragged the bones of a pig carcass into their burrows. Historians discount the story, because no trace of her plane has been found on or near the island. The crab's size and its habit of stealing camp gear probably explain why so many stories gather around it.