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.
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. Crabs can breed throughout the year, but for mud crab peak seasons in Bangladesh are May to August, and December to February. Crab fishery is now an important economic activity in Bangladesh. Worldwide crustacean aquaculture mainly represents culture of shrimp and lobsters. But in recent decades crab culture particularly the culture of mangrove mud crab, Scylla serrata, has received the most attention because of its market demand and decreasing natural production.