The Sundarbans, the world's largest coastal mangrove forest, stretches for almost 6,000 square miles across India and Bangladesh, a natural barrier against tsunamis and frequent cyclones that blow in from the Bay of Bengal.

Sunderbans delta constitutes one of the' dominant crab fauna of the intertidal mangrove areas. Four species viz. Uca .acutus (u. roseus) U. annulipes, U. dussumieri and U. triangularis are quite common. The members of the family Grapsidae, namely, the genera Sesarma and Metaplax are found in the estuaries and as well as in the areas with very low or devoid of salinity. The members of the genus Sesarma prepare a deep bole on the marshy ground. An important edible crab, viz., Scylla seata is also found to occur abundantly in the intertidal mudflat as a burrower.