Crabs are the common name for any of a group of crustaceans characterized by a reduced abdomen and an enlarged and broadened anterior portion of the body. Mainly marine, some crabs live in fresh water or on land. Crabs are divided into two groups: true crabs (about 4500 species) and hermit crabs and their allies (about 1400 species). They are related to lobsters and shrimps, but their evolutionary development has enabled them to walk or run sideways and to burrow as well as swim. Mostly active animals with complex behavior patterns, crabs have compound eyes and can see well. The senses of smell and taste, also well developed, allow them to identify both food and prospective mates.