The crustaceans (Crustacea) are a large group of arthropods, comprising almost 52,000 described species ,and are usually treated as a subphylum .They include various familiar animals, such as crabs, lobsters, crayfish, shrimp and barnacles.

The majority of them are aquatic, living in either marine or fresh water environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs and woodlice. Crustaceans are among the most successful animals, and are as abundant in the oceans as much as insects are on land. Over half of animals in the world are marine copepod crustaceans.