WORLD'S LARGEST CRAB

The coconut crab (Birgus latro) is the largest terrestrial crab and also the largest land-living arthropod in the world. It is a highly apomorphic hermit crab and is known for its ability to crack coconuts with its strong pincers in order to eat the contents. It is the only species of the genus Birgus. It is also called the robber crab or palm thief, because some coconut crabs are rumored to steal shiny items such as pots and silverware from houses and tents.

Reports about the size of Birgus latro vary, but most references give a weight of up to 4 kg (8.8 lb), a body length of up to 40 cm (16 in), and a leg span of around 20 cm (0.66 ft), with males generally being larger than females. It is believed that this is near the theoretical limit for a terrestrial arthropod. They can reach an age of up to 30–60 years (references vary). In India, they are found in large numbers on the Sentinel island of the Andaman & Nicobar Islands archipelago.

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 claws used to open coconuts, and these claws (chelae) can lift objects up to 29 kg (64 lb) in weight. The next two pairs, as with other hermit crabs, are large, powerful walking legs which allow coconut crabs to climb vertically up trees (often coconut palms) up to 6 m high. The fourth pair of legs is smaller with tweezer-like chelae at the end, allowing young 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 serves only to clean the breathing organs. These legs are usually held inside the carapace, in the cavity containing the breathing organs. There is some difference in colour between the animals found on different islands, ranging from light violet through deep purple to brown.

Although Birgus latro is a derived type of hermit crab, only the juveniles use salvaged snail shells to protect their soft abdomens, and adolescents sometimes use broken coconut shells to protect their abdomens. Unlike other hermit crabs, the adult coconut crabs do not carry shells, but instead harden their abdominal armor by depositing chitin and chalk. They also bend their tails underneath their bodies for protection, as do most true crabs. The hardened abdomen protects the coconut crab and reduces water loss on land, but has to be moulted at periodic intervals. Moulting takes about 30 days, during which the animal's body is soft and vulnerable, and it stays hidden for protection.