Crab any of the members of decapod crustaceans belonging to the suborder Brachyura with a broad, rather round, upper carapace and a small abdomen tucked beneath the body living in marine, brackish, or freshwaters. Though they differ from species to species in size, shape, colour and structure, they closely resemble each other in general morphology and biology. Sixteen species of crabs have been so far reported from Bangladesh waters, of which the common ones are Scylla serrata, Portunas pelagicus, P. sanguinolentus, Charybdis feriata, Charybdis rostrata, Matuta lunaris, M. planipes, Clappa lophos, C. pustulosa, Varuna litterata, Sartorina spinigera, Ocypoda cratopthalma, and Gelasimus annulipes. The six important genera used as food crabs are Scylla, Portunus, Charybdis, Matuta, Varuna and Sartorina.

Most of these species are economically important but the serrated mud crab, also known as mangrove crab, or Scylla serrata is now the most commercially important species and is widely distributed in the Indo-Pacific region, including the bay of bengal. Bangladesh earns about US $ 6 million per year by exporting 1,500 m tons of live mud crab to Singapore, Hong Kong, China, Taiwan and Japan.

The segmentally organised body of crab is divided into a well-developed cephalothorax, and a highly reduced abdomen with six cephalic, eight thoracic and six abdominal segments. Each segment except the first bears a pair of uniramous or biramous appendages.

There are five pairs of legs used for running, swimming, jumping, climbing and burrowing. Crabs construct burrows of different shape and size mainly for concealment but mud crabs use those also at the time of molting and breeding. Crabs can breed throughout the year, but for mud crab peak seasons in Bangladesh are May to August, and December to February.

The commercially important species Scylla serrata usually spawns in the sea and its larvae are carried to the coast by tide. They attain sexual maturity six months after hatching and maturation of gonads and sexual activity is controlled by the secretion of hormones. An adult usually weighs 300-600 g though the largest one may grow up to 1kg weight (15 cm across the carapace). The ovigerous or berried females carry the fertilized egg mass in its broad abdomen.