The coconut crab looks like something from science fiction – or a fisherman’s fever dream. Also known as the robber crab, it is the world’s largest land-dwelling crustacean and the largest land-dwelling arthropod to boot, with a leg span of up to one metre and a body weight of more than 4kg.
It can climb trees, crack coconuts, steal unattended objects and deliver a pinch that no bare finger should volunteer to test.
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Where does the coconut crab live?
The coconut crab is found on tropical islands across the Indian and Pacific Oceans. It favours coastal forest with opportunities for hiding among rocks and digging burrows, which are often found under tree roots, sometimes lined with coconut husk fibres.
Hotspots include Aldabra Atoll in the Seychelles, Chumbe Island in Tanzania, the Chagos Archipelago in the Indian Ocean, and Palmyra Atoll in the Central Pacific Ocean. The Australian territory of Christmas Island is a well-known stronghold, with up to one million coconut crabs.
As an adult, it is a land animal. It breathes air using special respiratory organs called branchiostegal lungs, which act as an ‘evolutionary bridge’ between gills and lungs, and may drown if submerged for too long. It only returns to the sea at one crucial stage of its life-cycle: when the female releases its eggs into the water.
How big are coconut crabs?
Their leg-span can be up to 1m and they can weigh more than 4kg
How does the coconut crab grow so large?
The coconut crab starts life on a very different scale. After hatching at sea, it passes through a floating larval stage that lasts four to six weeks, before settling on the seafloor as a juvenile crab (called a glaucothoe) and finding an empty shell for protection, much like its hermit crab relatives. As it grows, its abdomen hardens and the shell is abandoned, allowing the adult to become a heavily armoured land animal.
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A coconut crab does not become a forest-floor heavyweight overnight. The species can live up to 60 years and large adults are the product of many decades of moulting, feeding and avoiding the many hazards of island life. Moulting in itself can be hazardous as the animal is vulnerable to drying out and predation while without its protective exoskeleton.
Can the coconut crab really open a coconut?
Yes, though opening a coconut is less a single mighty crack than a piece of determined demolition work. It uses its powerful claws to strip away the fibrous husk, then probe and break into the harder shell beneath. Some accounts describe it carrying a coconut up a tree and dropping it to help split it open.
Researchers measuring the pinching force of wild coconut crabs recorded forces of just over 1,765 newtons, and estimated that a very large 4kg individual might exert around 3,300 newtons – which is similar to the reported bite force of a gorilla.
What do coconut crabs eat?
Despite its name, the coconut crab does not survive on coconuts alone. It is an omnivorous scavenger and opportunist, feeding on fallen fruit, seeds, nuts and carrion. It is also capable of hunting and has been recorded attacking animals including rats, other crabs and ground-nesting seabirds, such as boobies.
Why is it called the robber crab?
The name comes from anecdotal reports of it ‘stealing’ objects from people, including pots, cutlery and other (often shiny) unattended items. They have the strength to lift items up to around 23kg.
This behaviour is less about a criminal masterplan than curiosity and an excellent sense of smell, but more research is needed. On Christmas Island, campers and swimmers avoid leaving bags on the ground because a coconut crab may investigate any belongings and carry off whatever catches its interest.
How dangerous is the coconut crab?
A coconut crab could hurt a human, but it is not looking for a fight. The danger is in the claws. A large individual can deliver a crushing pinch that may cause serious pain and injury, especially to fingers and toes. Picking one up from the front would be a regrettable decision.
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But actually, the more serious danger usually runs the other way. Across much of its range, the coconut crab has been hunted by people for food, and its large size makes it an obvious prize. For an animal that can inspire such caution, it is ironic that its downfall partly lies in its vulnerability – it’s too easy to catch, and too tempting to eat.
Does it face any threats?
The coconut crab is listed as Vulnerable by the IUCN. Its problems are mostly human-made: overharvesting for food, loss and fragmentation of coastal habitat, introduced predators and deaths on roads. The significant length of time it takes for an individual to reach maturity means that it is difficult for populations to recover quickly. Healthy populations exist on islands with fewer humans or increased protection.
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