Invertebrates
Coconut Crab
Birgus latro · also called robber crab, palm thief
The coconut crab is a land animal that has taken the hermit-crab body plan to an extreme: it is the size of a house cat, it climbs palm trees, it can crack a coconut with its bare claws, and it would drown in the sea it came from.
Overview
Coconut crabs live on tropical islands across the Indian and western Pacific Oceans, sheltering by day in rock crevices and burrows they dig in loose soil and sand, and emerging at night to forage across the forest floor and shoreline. They are the largest terrestrial arthropod in the world and can only reach that size on land, where their bodies are not limited by the need to carry a heavy borrowed shell or by the buoyancy of water.
From hermit crab to giant
Coconut crabs belong to the hermit crab family, and their life story shows it:
- Larvae hatch in the ocean and spend three to four weeks drifting as plankton.
- Young juveniles crawl onto land and, like other hermit crabs, protect their soft abdomens inside empty snail shells, sometimes using broken coconut shells.
- Older juveniles outgrow the shell habit, and the abdomen hardens into a permanent tucked-under tail plate.
- Adults are fully committed to land, breathing air through a moist lung-like organ and unable to survive prolonged immersion.
The claw
An adult coconut crab’s front pincers are massively built, and the larger of the two can exert a force that, for the animal’s size, exceeds the bite of any vertebrate. The crab uses this to husk and break open hard-shelled food, to defend itself and its burrow, and to fight other crabs. The grip is slow to release, and researchers handling them describe being held for long periods until the crab decides to let go.
Diet and behaviour
Coconut crabs are omnivorous scavengers. They eat fallen fruit, nuts, seeds, pith, and leaves, gnaw on carrion and bones for calcium, catch and eat other crabs and, occasionally, small animals such as rats and nesting seabirds, and will take almost any food or shiny object left unattended — hence the names “robber crab” and “palm thief.” They have an insect-like sense of smell and can track a food source over long distances.
Reproduction and threats
Females carry their eggs under the abdomen and, when the larvae are ready, release them into the sea on a high tide — the only time an adult approaches the water, and even then it risks drowning. Growth is slow, maturity comes at around five years, and animals may live for 60 years or more.
The coconut crab is listed as Vulnerable and is disappearing from any island with a substantial human population. It is hunted for food (the meat is prized), it is slow-breeding and easy to catch by hand at night, and its larvae and juveniles are vulnerable to introduced rats, pigs, and ants. Large healthy populations now persist mainly on remote and protected islands.
Coconut Crab: common questions
How strong is a coconut crab's claw?
The largest coconut crabs can clamp with a force of around 3,300 newtons — comparable to the bite of a lion and, relative to body weight, the most powerful pinch or bite recorded in the animal kingdom. The grip is strong enough to break a coconut, snap a broom handle, and inflict serious injury on a human hand, and a crab that latches on may not let go for hours.
Do coconut crabs actually open coconuts?
Yes, though it takes a while. A crab will tear the fibrous husk off a coconut strip by strip over several days, working at the softest spot (one of the three "eyes"), then hammer and prise the shell open with its claws, sometimes after dropping it from a height in a tree. They also eat many other foods, and the "coconut" name reflects a talent, not a sole diet.
Can a coconut crab swim or breathe underwater?
Not as an adult. Coconut crabs evolved from marine ancestors but have become fully terrestrial, breathing air through a specialised organ that must be kept moist. An adult held underwater for long will drown. Only the larvae are aquatic — they hatch in the sea and drift as plankton for weeks before a juvenile crab crawls ashore, and from then on the animal cannot return to the water.
Are coconut crabs dangerous, and did one eat Amelia Earhart?
They are not aggressive and will retreat if given the chance, but a cornered crab's claw can cause real harm, so they are best not handled. The Earhart story is speculation — a hypothesis that if she died as a castaway on Nikumaroro, coconut crabs could have scattered her remains, which is plausible for their scavenging behaviour but unproven. There is no record of a coconut crab killing a person.