Crustaceans are the largest group of sea-dwelling arthropods, occupying a vast variety of environments and with more than 40,000 living species, including some in freshwater and on land. They include crabs, lobsters, shrimps, prawns and the shrimp-like krill in the seas, as well as pillbugs and woodlice on land, water-fleas in ponds, and barnacles along seashores.