Coconut Crab
Birgus latro
Coconut Crab (Birgus latro): The Largest Land Arthropod
The coconut crab (Birgus latro) is a giant land hermit crab of Indian and Pacific Ocean islands. It is the largest arthropod living on land, weighing up to about 4 kg, and its claws are among the strongest of any animal.
- Habitat
- Coastal forest and scrub on tropical islands
- Size
- Up to about 4 kg, leg span up to about 1 m
- Diet
- Fruit, nuts, coconuts, carrion, and some live prey
- Range
- Islands of the Indian and Pacific Oceans
- Conservation
- Vulnerable (IUCN)
- Authority
- Linnaeus, 1767
Habitat & range
Coconut crabs live on islands across the tropical Indian and Pacific Oceans, from islands off East Africa to the central Pacific. Christmas Island in the Indian Ocean has one of the largest remaining populations. They have disappeared from many islands where people live in large numbers (Drew et al. 2010). Adults are fully terrestrial. They breathe air with modified gill chambers called branchiostegal lungs and cannot swim; they drown if kept underwater for too long (Natural History Museum). Only the larvae live in the sea, drifting for several weeks before coming ashore.
Diet & claws
Coconut crabs are omnivores and scavengers. They eat fallen fruit, nuts, seeds, tree pith and carrion, and they also take live prey (Drew et al. 2010). On the Chagos Archipelago in 2016, a coconut crab was seen climbing to a red-footed booby at night, breaking its wing and dragging it down to eat (Laidre 2017). They can open coconuts with their claws by tearing off the husk and breaking through the shell. A study of 29 wild crabs on Okinawa measured pinching forces up to 1,765 newtons, and estimated that the largest 4 kg crabs could reach about 3,300 newtons, more than any other crustacean and more than the bite force of most land predators (Oka et al. 2016).
Body & life cycle
The coconut crab is the largest arthropod living on land, weighing up to about 4 kg with a leg span of up to about 1 m (Natural History Museum; Oka et al. 2016). Colour varies from orange-red to deep blue or purple. The front legs carry two large claws, and the left claw is usually the larger one. It is a hermit crab. Young coconut crabs carry snail shells like their relatives, but they soon outgrow them, and adults rely on a hardened body instead, with the abdomen tucked under the body (Oka et al. 2016). They grow slowly, mature at about five years and can live up to 60 years (Natural History Museum).
Conservation
The IUCN lists the coconut crab as Vulnerable. Its slow growth and late maturity make it easy to overharvest, and it is hunted for food in many places. Habitat loss and introduced predators add to the pressure (Natural History Museum; Drew et al. 2010). Many Pacific islands now set minimum harvest sizes, protect egg-carrying females or use quotas and export permits (Natural History Museum).
Fun facts about Coconut Crab
- A 2.1 kg coconut crab pinched with 1,765 newtons in field tests; the biggest crabs are estimated to reach about 3,300 newtons (Oka et al. 2016).
- Coconut crabs can lift up to about 30 kg (Natural History Museum).
- Adults cannot swim and will drown if they stay underwater too long (Natural History Museum).
- Young coconut crabs use snail shells like other hermit crabs before outgrowing them (Oka et al. 2016).
Coconut Crab vs Pistol Shrimp (claw comparison)
The coconut crab wins on raw force; the pistol shrimp wins on speed and strangeness.
This is a comparison of two crustacean claws, not a fight: one lives on land, the other in the sea. The coconut crab's claw is a crusher, measured at up to 1,765 newtons and estimated at about 3,300 newtons in the largest crabs (Oka et al. 2016). The pistol shrimp's claw is a weapon of a different kind: it snaps shut so fast that it fires a collapsing bubble that stuns small prey. Both are relatives in the class Malacostraca, but only the coconut crab could crack a coconut.
Read more about Pistol Shrimp.
Common questions about Coconut Crab
How strong is a coconut crab's pinch?
Up to about 1,765 newtons measured, and an estimated 3,300 newtons for the largest crabs. That is stronger than any other crustacean tested and greater than the bite force of most land predators (Oka et al. 2016).
Can a coconut crab really open a coconut?
Yes. Coconut crabs tear the fibrous husk away with their claws and then break into the nut. Coconuts are only part of the diet, which also includes fruit, nuts and carrion (Drew et al. 2010).
Can a coconut crab kill a cat?
There is no scientific record of one killing a cat, but they do kill animals. On the Chagos Archipelago a coconut crab was seen breaking a red-footed booby's wing and eating it (Laidre 2017). Stories of them taking kittens are anecdotal.
Are coconut crabs dangerous to humans?
They are not aggressive, but a pinch can be painful and hard to release. Researchers measuring their claws were pinched several times (Oka et al. 2016). Leave them alone and don't handle them.
Why is the coconut crab endangered?
It is listed as Vulnerable by the IUCN, mainly because of overharvesting and habitat loss. It grows slowly, matures at about five years and can live 60 years, so populations recover slowly (Natural History Museum).
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