Coconut Crabs — the World’s Largest Terrestrial Arthropods and Invertebrates

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Coconut crabs (Scientific name: Birgus latro) are the world’s largest terrestrial arthropods and the world’s largest terrestrial invertebrates. They are also so adapted to life on land if they spend more than an hour in the sea they drown. At the back of the main carapace they have an opening to an air chamber lined with moist skin through which the crab absorbs oxygen. It returns to the sea to lay its eggs but otherwise lives out its life on land.

Also known as robber crabs or or palm thieves, coconut crabs are land-living crustaceans that are so large they can hug the trunk of palm tree between their legs and touch their feet on the other side.. Males can weigh up to 4.1 kilograms (9 pounds) and grow to up to one meter (3 feet 3 inches) from the the tip of one leg to the tip of another. They reportedly climb up palm trees and use their gigantic pincers to cut down the young coconuts. There is little evidence to support this claim. Even if they could removing the coconut husk and breaking open the nut is beyond the capability.

Coconut crabs are a kind of land hermit crabs that doesn’t wear shell. They are primarily found on land, living holes in the ground and feeding on food they can forage or scavenge. They are good climbers but are slow on land. Coconut crabs and other land crabs such as mangrove crabs do well on islands, at least party because there are no vertebrae predators to threaten them. They are listed as Vulnerable on the International Union for Conservation of Nature (IUCN) Red List.

Fossils of coconut crab date back to the Miocene.Period (23 million to 5.3 million years ago). The crabs have been known to western scientists since the voyages of Francis Drake around 1580 and William Dampier around 1688. Based on an account by Georg Eberhard Rumphius (1705), who had called the animal "Cancer crumenatus", Carl Linnaeus (1767) named the species Cancer latro, from the Latin latro, meaning "robber". It took a while for the coconut crab's range to become known. Darwin believed it was only found on "a single coral island north of the Society group".

The coconut crab is the only species of the genus Birgus, which is the family Coenobitidae, alongside one the genus, Coenobita, which contains terrestrial hermit crabs. In Japan, where the crabs live on some of the country's southerly island chains, the species is referred to as Yashigani, meaning 'palm crab'. [Source: Wikipedia]

Websites and Resources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Fishbase ; Encyclopedia of Life ; Smithsonian Oceans Portal

Coconut Crab Habitat and Where They Live

Coconut crabs once lived throughout the western Pacific and eastern Indian Ocean. There numbers have sharply declined and now they are only found in a few places. The fact they are easy to catch and good to eat is the primary reason their numbers have shrunk. On some islands they are regarded as delicacies and are becoming increasingly popular with tourists. In some places they are kept as pets.

Coconut crabs are found on islands across the Indian Ocean, and parts of the Pacific Ocean as far east as the Gambier Islands, Pitcairn Islands and Caroline Island. Their distribution is similar to that of the coconut palm. They also live off the coast of Africa near Zanzibar but are now largely gone from places with significant human population such as mainland Australia and Madagascar. [Source: Wikipedia]

Christmas Island in the Indian Ocean has the largest and densest population of coconut crabs in the world, but they are outnumbered there by more than 50 times by the Christmas Island red crab. Some are found on the Seychelles islands of Aldabra and Cosmoledo but are extinct on the central islands. Coconut crabs occur on several of the Andaman and Nicobar Islands in the Bay of Bengal. In the Pacific, large populations exist on the Cook Islands. Close to the eastern limit of its range, in the Line Islands of Kiribati, the crabs are abundant on Teraina (Washington Island), which has a large coconut palm forest. The Gambier Islands mark the species' eastern limit.

Coconut Crab Characteristics

The coconut crabs have a number of adaptations to life on land. Juveniles use empty gastropod shells for protection like other hermit crabs, but adults develop a tough exoskeleton on their abdomens and stop carrying a shell. Coconut crabs also have organs known as branchiostegal lungs, which they use for breathing instead of their vestigial gills. If immersed in water for too long adults will drown. The crabs have an acute sense of smell which they use to find potential food sources, and which has developed convergently with that of insects. [Source: Wikipedia]

Not only are coconut crabs the largest terrestrial arthropods, they are the largest terrestrial invertebrates. Most sources give a body length up to 40 centimeters (16 inches), with males generally being larger than females. The carapace may reach a length of 78 millimeters (3.1 inches), and a width up to 200 millimeters (8 inches).

As is true with all decapods, the body of the coconut crab 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. 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. 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 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 inside the carapace, in the cavity containing the breathing organs. Some difference in color occurs between the animals found on different islands, ranging from orange-red to purplish blue; in most regions, blue is the predominant color, but in some places, including the Seychelles, most individuals are red.

Unlike other hermit crabs, the adult coconut crabs do not carry shells, but instead harden their abdominal terga by depositing chitin and chalk. Not being constrained by the physical confines of living in a shell allows these crabs to grow much larger than other hermit crabs. Like most true crabs, coconut crabs bends its tail underneath its body for protection. The hardened abdomen protects the coconut crab and reduces water loss on land, but must be moulted periodically.

The special organ — the branchiostegal lung — that the coconut crab uses to breath can be viewed as a developmental stage between gills and lungs. An extraordinary adapation to terrestrial life, it 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 to increase the surface area that can absorb more oxygen and is cleaned and moistened with water by the crab’s hindmost, smallest pair of legs. The organs require water to properly function, and the coconut crab provides this by stroking its wet legs over the spongy tissues nearby. In addition to the branchiostegal lung, coconut crab have rudimentary set of small regular gills which are not really used.

The coconut crab has a well-developed sense of smell.Crabs that live in water have specialized organs called aesthetascs on their antennae to determine both the denseness and the direction of a scent. Land-dwelling coconut crabs have shorter and blunter aesthetascs on their antennae and these are remarkably similar to those of insects and this is viewed as an example of convergent evolution. Insects and coconut crab have followed different evolutionary paths but they both have needed to pick up smells in the air. Coconut crabs flick their antennae as insects do to enhance their reception. They can detect odors of things they like to eat such a rotting flesh, bananas, and coconuts — over large distances. The olfactory part of the crab's brain is well-developed compared to other areas of the brain.

Coconut Crab Claws Pinch — Almost as Strong as a Lion’s Bite

The claws of coconut crabs have the strongest pinching force of any crustacean, according to a study published in November 2016 in the journal PLOS ONE by a team led by Shin-ichiro Oka from Okinawa Churashima Foundation, Japan. The pinching force also exceeds the bite force of most terrestrial animals. [Source: PLOS ONE, November 23, 2016]

PLOS ONE reported: Coconut crabs are remarkably strong, lifting up to 28 kilograms. The crabs use their claws to fight and defend themselves, and to eat coconuts and other foods with hard exteriors. Decapods (most crustaceans) exert the greatest pinching force relative to their mass. The researchers measured the claw pinching force of 29 wild coconut crabs from Okinawa Island, Japan.

The researchers found that pinching force increased with body mass. Based on the crabs' maximum known weight, the maximum pinching force of their claws was projected to be 3,300 newtons. This exceeds both the pinching force of other crustaceans and the bite force of all terrestrial animals except alligators. The crabs' "mighty claws" let them monopolize coconuts, which other animals are unable to access. In addition, suggest the researchers, being able to hunt other animals with hard exteriors could help these crabs maintain their large bodies.

According to New Atlas: 3,300 newtons is about four times the force generated by a human eating a a steak and about 75 percent of the force hyenas, tigers and lions generate with their jaws. [Source: Nick Lavars, New Atlas,November 24, 2016]

Coconut Crab Behavior and Burrows

Except as larvae, coconut crabs cannot swim, and they drown if left in water for more than an hour. Adults moult annually. At that time they dig a burrow up to one meter deep long in which to hide while vulnerable. They remain in the burrow for three to 16 weeks, depending on the size of the crab. One to three weeks is needed for the exoskeleton to harden after moulting. [Source: Wikipedia]

Coconut crabs are regarded as one of the most terrestrial-adapted crustaceans. Many aspects their life and anatomy are oriented towards living on land. Coconut crabs live alone in burrows and rock crevices, depending on the local terrain. In areas with large numbers of coconut crabs, 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. Sometimes they live far from the sea and return to the sea only to release their eggs. On Christmas Island, many coconut crabs can be found six kilometers (3½ mlesi) from the sea.

Coconut crabs dig their own burrows in sand or loose soil. During the day, they usually stay 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.

Although the crabs are associated with coconuts, coconuts are not a significant part of its diet. Coconut crabs have been filmed filmed climbing coconut and pandanus trees but there has never been any footage of them picking coconut fruit. Climbing is viewed more as escape route from danger than a means of collecting food. They often descend from the trees by falling, and can survive a fall of at least 4.5 meters (15 feet) unhurt.

Coconut Feeding and Predators

Adult coconut crabs feed primarily on fleshy fruits, nuts, seeds, and the pith of fallen trees, but they eat carrion and other organic matter if it is available. Anything left unattended on the ground is a potential source of food. Their habit of investigating such things and carrying them away earned them their alternative name "robber crab". Adults face few predators except maybe by large sea birds and humans but juveniles are vulnerable to attacks from such birds and cannibalism by bigger, older crabs. [Source: Wikipedia]

The diet of coconut crabs consists primarily of 1) the fleshy fruits Ochrosia ackeringae, Arenga listeri, Pandanus elatus, and P. christmatensis; 2) the nut Aleurites moluccanus; 3) the drupe Cocos nucifera; 4) the seedAnnona reticulata; and 5) the pith of fallen trees. They have also been observed eating tortoise hatchlings and dead animals and preying upon crabs and scavenging the carcasses of dead coconut crabs. During a tagging experiment, one coconut crab was observed killing and eating a Polynesian rat. In 2016, on the Chagos Archipelagom a large coconut crab was observed climbing a tree to disable and consume a red-footed booby.

The coconut crab can take a coconut on the ground, take off the husk with its claws and take the a husk nut with its claws and climb up a tree 10 meters (33 feet) high and drop the husk nut, to break it open and access the coconut flesh inside. Coconut crabs cut holes into coconuts with their strong claws and eat the contents, but this can take several days.

In 1877 Thomas Hale Streets raised doubts about the crab’s ability to climb trees to get at coconuts. In the 1980s, Holger Rumpf confirm Streets' suspicions are meticulously described how coconut crabs open coconuts in the wild. When the coconut is still covered with husk, the crab 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 claws 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 Crab Reproduction

Coconut crabs spend their youth in the sea and move onto land when they mature. Mating occurs on dry land, but the females return to the edge of the sea to release their fertilized eggs, and then retreat back up the beach. For a long time there was some debate as to whether the creatures brooded their eggs on land or at sea. The dispute was settled in 2009 when a Japanese photographer took a picture of a female guarding her red-colored eggs in a cave in Okinawa.

Coconut crabs mate frequently and quickly on dry land from May to September, especially between early June and late August. Males have spermatophores (male reproductive structures that package sperm cells) and use them deposit a mass of sperm cells on the abdomens of the females. The females’ 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. The extrusion of eggs occurs 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 the larvae into the ocean.[Source: Wikipedia]

Female coconut crab takes a large risk while laying the eggs because coconut crabs can't swim. If a coconut crab falls into the water or gets swept away by a wave, 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 high tide. 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.

Coconut Crab Development

The coconut crab larvae that hatch are planktonic for three to four weeks, before settling to the sea floor. They float in the pelagic zone of the ocean with other plankton. Large numbers of them are eaten by predators. The larvae pass through three to five zoea stages before moulting into the postlarval glaucothoe stage.

Upon reaching the glaucothoe stage of development, they settle to the bottom of sea and 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. [Source: Wikipedia]

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.

Coconut crab reaches sexual maturity around 5 years after hatching. They reach their maximum size only after 40–60 years. They grows remarkably slowly, taking perhaps 120 years to reach full size, as posited by ecologist Michelle Drew of the Max Planck Institute. [Source: Wikipedia]

Coconut Crabs and Humans

Large adult coconut crabs have no known predators apart 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 a delicacy — and regarded as an aphrodisiac — on various islands, and intensive hunting has threatened the species' survival in some areas. [Source: Wikipedia]

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. 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.

Not only can coconut crabs deliver a powerful pinch with their claws, they can also grasp on to things, refsuing to let go for long periods of time. Thomas Hale Streets reported that Micronesians of the Line Islands had a trick to get coconut crabs to let go — "a gentle titillation of the under soft parts of the body with any light material will cause the crab to loosen its hold." In the Cook Islands, coconut crabs are known as unga or kaveu. In the Mariana Islands they are called ayuyu, and is sometimes associated with the traditional religion taotaomo'na because of the belief that ancestral spirits can return in the form of animals such as the coconut crab.

Coconut crab populations in several areas have declined or become locally extinct due to both habitat loss and human predation. In 1981, the International Union for Conservation of Nature (IUCN) Red List listed them as vulnerable, an assessment affirmed in 2018. Conservation efforts have included size restrictions in Guam and Vanuatu, and a ban on the capture of egg-bearing females in Guam and Micronesia. In the Northern Mariana Islands, hunting of non-egg-bearing adults above a carapace length of 76 millimeters (3 inches) may take place in September, October, and November, and only under licence. The bag limit is five coconut crabs on any given day, and 15 across the whole season. In Tuvalu, coconut crabs live on the motu (islets) in the Funafuti Conservation Area, a marine conservation area covering 33 square kilometers (12.74 square miles) of reef, lagoon and motu on the western side of Funafuti atoll.

Image Sources: Wikimedia Commons, NOAA

Text Sources: Animal Diversity Web (ADW); National Oceanic and Atmospheric Administration (NOAA); Wikipedia, National Geographic, Live Science, BBC, Smithsonian, New York Times, Washington Post, Los Angeles Times, The New Yorker, Reuters, Associated Press, Lonely Planet Guides and various books and other publications.

Last Updated April 2023

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