It is established by the scientists that the most successful free living kinds of animals judged by the number of species and standing stocks of living substance, have come from marine surroundings.
In 1964, Sir Vernon Wigglesword had suggested that, at first these animals might have lived along the shore lines and later moved in two directions. A part colonized the open sea and ocean deeps to give rise to crabs, crustacea etc and the other part moved to land areas.
Crabs are mainly marine animals – although some live in fresh water or land. They are ten-legged alert carnivores that act as scavengers. There are many species of crabs worldwide; such as European crabs, shore crabs, edible crabs, fiddler crabs, river crabs, common hermit crabs and robber crabs.
Among all these, robber crabs or commonly known as ‘coconut crabs’ are well adapted for living on land, although their larvae first live in the sea. But if an adult robber crab is left under water beyond 24 hours, it will be drowned ultimately. These are the crabs which can climb trees.
The robber crab (Birgus Latro) is an extremely large land crab of South-West Pacific and Indian Ocean. Adults of robber crabs are about one metre long (40 inches) and weigh about 15-17 kilograms. A robber crab uses two large chelae or pincers to pound or chip coconuts to eat the soft white portion of the coconut. They climb coconut trees to detach coconuts from tree tops through they are quite unable to get inside a coconut on their own. They apparently eat coconut meat already broken into the ground by other means.
The colour of a grown crab ranges from light violet to brown and deep purple with black strips on their legs. The female lays her eggs in sea. In larval state, it lives in water, feeding on small organisms. After about a month when it is grown to intermediate stage, it leaves water to live in a shell for 3-4 weeks. It then discards the shell and buries itself in moist sands, and transforms into an adult robber crab. Its meat is a local delicacy.