The coconut crab is a giant hermit crab that drowns in water
The biggest arthropod living on land starts life drifting in the ocean, yet as an adult it will drown if held underwater for more than an hour. And despite its name, the coconut crab eats few coconuts; its other nickname, robber crab, comes from hauling off anything left lying around.
Birgus latro can weigh up to 4.1 kg with legs spanning around 90 centimetres. It lives on islands across the Indian and Pacific Oceans, from Zanzibar in the west to the Pitcairn and Gambier Islands in the east, broadly tracking the coconut palm, but it has vanished from most places with many people, including mainland Australia and Madagascar. Colours vary by island: blue dominates in most regions, while in the Seychelles most are red.
It is technically a hermit crab. Juveniles shelter in snail shells like their relatives, sometimes swapping in broken coconut shell, but adults abandon borrowed homes and harden their own abdomens with chitin and calcium carbonate, which frees them to grow far larger. Once a year they moult, hiding in a burrow up to a metre long for weeks while the new armour sets. In place of working gills they breathe with a branchiostegal lung, a sort of halfway organ between gill and lung that must be kept moist, so the crab strokes it with its wet hindmost legs.
Its nose is remarkable. Water-dwelling crabs smell with antenna hairs tuned to dissolved molecules, but the coconut crab's are short and blunt like an insect's, and it flicks its antennae as insects do, a case of convergent evolution. The smell of rotting meat, bananas and coconuts can draw it from far off. It eats mostly fruit, nuts, seeds and fallen tree pith, climbs trees to escape danger, and has never been filmed deliberately picking coconuts.
Breeding is risky for an animal that cannot swim. Females carry eggs for months, then release larvae at the shoreline, often at dusk on a high tide. The larvae drift three to four weeks, perhaps hitching rides on floating logs or coconuts, before returning to land for good. Maturity takes about five years, and a lifespan beyond 60 years is possible; one Max Planck researcher suggested full size may take up to 120.
Source: Coconut crab
