Birgus

Lore & Background

The coconut crab has been known to western scientists since the voyages of Francis Drake around 1580 and William Dampier around 1688. Based on an account by Georg Eberhard Rumphius (1705), Carl Linnaeus (1767) named the species Cancer latro. The genus Birgus was erected in 1816 by William Elford Leach, containing only Linnaeus' Cancer latro, which was thus renamed Birgus latro. Birgus is classified in the family Coenobitidae alongside the genus Coenobita.

Juvenile coconut crabs use empty gastropod shells for protection like other hermit crabs, but adults develop a tough exoskeleton on their abdomens and stop carrying a shell. They have organs known as branchiostegal lungs for breathing instead of vestigial gills, and after the juvenile stage they will drown if immersed in water for too long. Their acute sense of smell, used to find food, has developed convergently with that of insects.

Adult coconut crabs feed primarily on fleshy fruits, nuts, seeds, and the pith of fallen trees, but also eat carrion and other organic matter opportunistically. Despite its name, coconuts are not a significant part of the crab's diet. Mating occurs on dry land, but females return to the edge of the sea to release fertilized eggs. The larvae are planktonic for 3–4 weeks before settling to the sea floor, entering a gastropod shell, and returning to dry land.

Reader's Guide

Birgus latro is the sole member of the genus Birgus and represents a unique evolutionary lineage among hermit crabs. Its adaptations to terrestrial life—including branchiostegal lungs, a hardened abdomen, and a reduced dependence on shells in adulthood—allow it to reach sizes far exceeding those of its relatives in Coenobita. The species is found on islands across the Indian and Pacific Oceans, from Zanzibar to the Gambier Islands, Pitcairn Islands, and Caroline Island, though it has been extirpated from most areas with significant human populations such as mainland Australia and Madagascar.

The coconut crab's ability to climb trees, its opportunistic feeding behavior, and its long lifespan (over 60 years) contribute to its ecological role as a scavenger and seed disperser. Its sense of smell, convergent with that of insects, enables it to locate food over large distances. Fossils of this crab date back to the Miocene. The species has been known to western science since the late 16th century, and its scientific name reflects its robber-like behavior. As the largest living terrestrial arthropod, Birgus latro is a subject of biological interest for studies of carcinization, terrestrial adaptation, and island biogeography.

Did You Know?

• The coconut crab is the only species in the genus Birgus.
• Adult coconut crabs can weigh up to 4.1 kg and have a leg span of more than 0.91 m.
• Juvenile coconut crabs use gastropod shells for protection, but adults develop a hardened exoskeleton on their abdomens and stop carrying shells.