Shrimp and crabs (Subphylum: Crustacea) are arthropods (Phylum: Arthropoda) which have a well developed, calcified shell, with certain modifications specific to each group. As well as the shrimp and crabs, this group contains lobsters, langoustines, prawns, scampi, barnacles, blue crabs, woodlice, water fleas, crayfish, hermit crabs and krill. Their bodies are segmented and generally divided into three regions: head, thorax and abdomen. They have two pairs of antennae and numerous appendages (or extremities), some of which are specialized for feeding or swimming. Their size varies considerably, from less than 1mm to more than 4 meters in length, such as the spider crab of Japan. Those of largest size belong to the group of lobsters and crabs.
Their diversity of forms is so wide that no other group of animals or plants can equal it (Martin and Davis, 2001). The majority are aquatic, although some terrestrial species exist. They live at all depths of the marine and continental aquatic environment. They have adapted to every conceivable habitat within this medium, including hypersaline lakes and polar waters.