The coconut crab is the only species of the genus Birgus, and is related to the terrestrial hermit crabs of the genus Coenobita. It shows a number of adaptations to life on land. Like other hermit crabs, juvenile coconut crabs use empty gastropod shells for protection, but the adults develop a tough exoskeleton on their abdomen and stop carrying a shell. Coconut crabs have organs known as branchiostegal lungs, which are used instead of the vestigial gills for breathing and they will drown if immersed in water for long. They have an acute sense of smell, which has developed convergently with that of insects, and which they use to find potential food sources.
Adult coconut crabs feed primarily on fruits, nuts, seeds, and the pith of fallen trees, but they will eat carrion and other organic matter opportunistically. Anything left unattended on the ground is a potential source of food which they will investigate and may carry away - thereby getting the alternative name of "Robber crab." The species is popularly associated with the coconut palm, yet coconuts are not a significant part of its diet. Although it lives in a burrow the crab has been filmed climbing coconut and pandanus trees. There is no film showing a crab selectively picking coconut fruit, though they might dislodge ripe fruit that otherwise would fall naturally. Climbing is an immediate escape route (if too far from the burrow) to avoid predation (when young) by large sea birds, or cannibalism (at any age) by bigger, older crabs.
Mating occurs on dry land, but the females return to the edge of the sea to release their fertilised eggs and then retreat back up the beach. The larvae that hatch are planktonic for 3–4 weeks, before settling to the sea floor, entering a gastropod shell and returning to dry land. Sexual maturity is reached after about 5 years, and the total lifespan may be over 60 years. In the 3–4 weeks that the larvae remain in the sea their chance of reaching another suitable location will be enhanced if they find a floating life support system. It is commonly agreed that floating logs or rafts of storm-struck vegetation would be suitable, although rather chancy and definitely seasonal. In contrast, floating coconuts can be a very significant part of the crab's dispersal options.
Birgus latro is the largest terrestrial arthropod, and indeed terrestrial invertebrate, in the world; reports about the size of Birgus latro vary, but most sources give a body length of up to 40 cm (16 in), a weight of up to 4.1 kg (9.0 lb), and a leg span of more than 0.91 m (3.0 ft), with males generally being larger than females. The carapace may reach a length of 78 mm (3.1 in), and a width of up to 200 mm (7.9 in).
The body of the coconut crab is, like that of all decapods, divided into a front section (cephalothorax), which has 10 legs, and an abdomen. The front-most pair of legs has large chelae (claws), with the left being larger than the right. The next two pairs, as with other hermit crabs, are large, powerful walking legs with pointed tips, which allow coconut crabs to climb vertical or overhanging surfaces. The fourth pair of legs is smaller with tweezer-like chelae at the end, allowing young coconut crabs to grip the inside of a shell or coconut husk to carry for protection; adults use this pair for walking and climbing. The last pair of legs is very small and is used by females to tend their eggs, and by the males in mating. This last pair of legs is usually held inside the carapace, in the cavity containing the breathing organs. There is some difference in color between the animals found on different islands, ranging from orange-red to purplish blue; in most regions, blue is the predominant color, but in some places, including the Seychelles, most individuals are red.