Crustaceans are mandibulate arthropods that are traditionally a part of the paraphyletic subphylum Crustacea. Crustaceans are mainly aquatic, including decapods, seed shrimps, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods and mantis shrimp. It is now well accepted that hexapods are cladistically crustaceans, with the two groups now combined in the monophyletic clade Pancrustacea. The three classes Cephalocarida, Branchiopoda, and Remipedia are more closely related to the hexapods than they are to any other crustaceans.