The Coconut Crab
The coconut crab (Birgus latro) is the largest land-dwelling arthropod in the world, famous for its size, strength, and unique adaptations to terrestrial life.
Found on tropical islands across the Indian Ocean and the Pacific, this remarkable crustacean fascinates scientists and travelers alike with its complex biology, striking appearance, and resourceful behaviors.
Coconut crabs inhabit coastal regions on oceanic islands from Madagascar to the southwestern Pacific, including locales like the Seychelles, Easter Island, and American Samoa.
They are most common in undisturbed areas away from heavy human populations, as hunting and habitat loss have extirpated them from many former ranges, including mainland Australia and Madagascar.
Adults primarily live on land and burrow into sandy or loose soil, especially in forests close to shore, where they can safely molt and store food. While adults are almost entirely terrestrial, the young hatch in the sea, develop as planktonic larvae for several weeks, then return to land, gradually becoming fully terrestrial as they grow.
A fully grown coconut crab can reach a leg span of up to 3 feet (about 1 meter) and weigh as much as 9 pounds (4 kilograms). Their powerful front claws are strong enough to crack open coconuts or even break through bones and hard shells.
Juvenile crabs wear borrowed gastropod shells for protection, but adults develop a tough exoskeleton on their abdomen and stop carrying shells. Their coloration ranges from blue to red or brown, depending on location and age.
Coconut crabs are omnivorous scavengers, eating almost anything they can handle. Their diet includes fruits, nuts, seeds, the pith of fallen trees, animal carcasses, smaller crabs, discarded shells for calcium, and, occasionally, live prey such as rats or seabirds.
They have developed a keen sense of smell, similar to that of insects, to seek out food sources across large distances.
A common myth is that coconuts make up the bulk of their diet. In reality, while the crabs are capable of opening coconuts using their formidable claws—and sometimes drop coconuts from trees to crack them—the coconut is just one of many foods they consume.
The crabs’ opportunistic nature earns them the nickname "robber crab," as they often steal human objects and any available food. They are primarily nocturnal, emerging under the cover of darkness to avoid predators and human disturbance, though they may also forage at dawn or dusk, especially in less populated areas.
Mating occurs on land, but the females release their fertilized eggs into the ocean, where the larvae float as plankton for several weeks before making their way back to shore. Sexual maturity is reached after about five years, and adult lives can span over 60 years.
During growth, coconut crabs molt and bury themselves for several weeks, often eating their shed exoskeleton to recoup calcium. Their only significant natural predators are humans—a major threat through hunting for food—and, to a lesser extent, some birds, lizards, and other crabs when they are juveniles.
Coconut crabs have been exterminated or severely reduced in many parts of their historical range, mainly due to overharvesting and habitat destruction.
Because they are considered a delicacy and aphrodisiac in some cultures, and because of their slow reproductive cycle and long maturation time, many populations are at risk.
Some regions have established legal protections and sanctuaries to help populations recover, but the species remains vulnerable, with careful management required for its continued survival.
The coconut crab exemplifies adaptation, strength, and vulnerability—all wrapped into one of the world’s most extraordinary terrestrial invertebrates.
Nautical Mile Magazine
