The coconut crab, Birgus latro, is the world’s largest terrestrial arthropod. Also called the robber crab or palm thief, this crustacean is an iconic resident of tropical island ecosystems. Its immense size and adaptations allow it to live almost entirely on land in the Indo-Pacific region.
Defining the Coconut Crab
The coconut crab is a giant hermit crab species that can weigh up to 9 pounds (4.1 kilograms) and possess a leg span exceeding 3 feet (1 meter). Unlike smaller relatives, the adult develops a tough, calcified exoskeleton on its abdomen and stops carrying a borrowed shell. This adaptation is supported by specialized breathing organs called branchiostegal lungs, which allow the crab to extract oxygen from the air. Adults cannot survive prolonged submersion and will drown if kept underwater, a complete reversal from their aquatic ancestry.
Its diet is omnivorous and opportunistic, consisting of fruits, nuts, seeds, and carrion. Coconuts are a notable food source it can crack open with its powerful claws. The life cycle begins in the ocean, where females release fertilized eggs and larvae spend several weeks drifting. Larvae then transition into a terrestrial stage, initially using gastropod shells before developing hardened abdominal plates and moving permanently onto land. Coconut crabs are long-lived, with some individuals surviving up to 60 years in the wild.
Current Conservation Status
The global conservation status for the coconut crab is currently classified as Vulnerable on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. This means the species faces a high risk of extinction in the wild. This status was elevated from the previous “Data Deficient” in 2018/2020 following a thorough reassessment of population trends and threats.
The Vulnerable status is designated under the criteria A2cd+4cd, which details the reasons for the risk. The “A2” indicates an inferred population reduction of at least 30% over the last three generations (approximately 60 to 90 years). The letters “c” and “d” specify the causes: reduction in habitat quality and area, and actual or potential exploitation. The “+4cd” indicates a projected continued decline of at least 30% over the next three generations due to the same factors.
Geographic Range and Island Ecology
The coconut crab is found across a vast, highly fragmented area of the Indo-Pacific. Its range extends from islands off East Africa, such as Zanzibar, eastward to the Gambier Islands in the Pacific. The species has been extirpated from many areas with dense human populations, including mainland Australia and Madagascar, leaving remaining populations primarily on smaller, more remote islands.
The crab’s habitat is typically coastal forest, where it digs burrows or utilizes rock crevices up to 6 kilometers inland. Within these isolated island ecosystems, the coconut crab plays a significant ecological role, often filling niches vacant due to the absence of large terrestrial mammals. As a large-bodied omnivore and scavenger, it aids in nutrient cycling by consuming organic material and carrion. By moving and discarding partially consumed fruits, the crab also acts as a seed disperser for various island flora.
Factors Driving Population Decline
Overharvesting is the primary driver of population decline, as the coconut crab is considered a delicacy and, in some areas, an aphrodisiac, leading to intense collection pressure. This commercial exploitation often targets the largest individuals, known as “selective hunting.” This disproportionately removes the biggest and most reproductively successful males, disrupting the population’s sex ratio and overall reproductive capacity.
Habitat destruction is another major threat, driven by coastal development, urbanization, and agricultural expansion on islands. The crabs require undisturbed coastal forest for feeding and burrowing, and habitat fragmentation severely restricts their movement and survival. Their life history characteristics make recovery difficult: they are slow-growing, take up to five years to reach sexual maturity, and have a long lifespan. This slow reproductive rate means populations cannot quickly rebound from sustained harvesting or habitat loss.