Coconut crabBirgus latroWhere and when to see them

The coconut crab is the largest living terrestrial arthropod, a powerful island scavenger related to hermit crabs. Adults have a hardened abdomen and no longer carry borrowed shells. Despite its placement in many marine wildlife lists, adult life is overwhelmingly terrestrial; only the early larval phase develops at sea. Across Polynesia its distribution is patchy, and harvesting, habitat loss and introduced predators have reduced populations on many inhabited islands.

Size

Adult body length up to about 40 cm

Weight

1-4 kg

Lifespan

Not reliably established

Status

Vulnerable

Coconut crabs usually emerge at night or in humid conditions, using a strong sense of smell to locate food. They shelter in burrows, rock cavities or root tangles and can climb trees. Their large claws tear plant material and manipulate difficult food items. Adults moult in protected retreats and are vulnerable while the new covering hardens. Females move towards the coast to release larvae, but adults cannot live submerged. They possess an acute sense of smell, rivaling that of rats, which helps them detect food sources from a distance. During molting, they remain hidden for several weeks until their new exoskeleton fully hardens.

• Young juveniles use borrowed shells before developing the adult hardened abdomen.
• Adults breathe air using specialised branchiostegal lungs.
• Their antennae detect airborne odours rather than functioning only in water.
• Females release larvae into the sea, linking terrestrial adults to a marine early life.

There is no single dependable season across Polynesia's widely separated islands. Warm, humid evenings can encourage activity wherever protected populations persist. Local rainfall, harvest rules and access restrictions should guide visits; a regional tourism calendar cannot establish crab abundance or reproductive timing. Peak larval release often correlates with the lunar cycle, typically around the full moon. Local guides can provide the most accurate information on current activity patterns.

Suitable habitat combines humid coastal woodland, rocky shelter or burrowable soil, and access to the sea for reproduction. Adults may range inland where conditions allow. Intact island forest and low harvesting pressure are more important than the simple presence of coconut palms. Habitat destruction due to coastal development is a primary threat across their range. They often prefer areas with a deep leaf litter layer and sufficient moisture to prevent desiccation.

Fallen fruit, seeds, nuts and other plant material form much of the diet, supplemented by carrion and opportunistically captured animals. Coconuts are only one resource. Strong claws help open tough foods, although handling a whole coconut requires considerable effort. They play an important role as scavengers and detritivores in their island ecosystems. Their diet can also include small crustaceans and even other crabs, especially if carrion is scarce.

Common name

Coconut crab

Scientific name

Birgus latro

Animal group

Marine life

Size

Adult body length up to about 40 cm

Weight

1-4 kg

Lifespan

Not reliably established

Status

Vulnerable

Identification

Its massive claws, long walking legs and exposed hardened abdomen distinguish an adult coconut crab from smaller land hermit crabs carrying shells. Juveniles are often mistaken for terrestrial hermit crabs as they use discarded shells for protection. The color of adult coconut crabs can vary, ranging from purplish-blue to orange-red or light brown, influenced by diet and environment.