Crustaceans are a diverse group of invertebrate animals that belong to the subphylum Crustacea. They have a hard exoskeleton, a segmented body that is bilaterally symmetrical, more than four pairs of jointed appendages, and an open circulatory system. Crustaceans include a large variety of arthropods such as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods, and mantis shrimp. They can be treated as a subphylum under the clade Mandibulata. Most crustaceans are aquatic, living in either marine or freshwater environments, but some are terrestrial, such as terrestrial crabs, terrestrial hermit crabs, and woodlice. Crustaceans have an important role in the ecosystem as they serve as vital food sources for both marine animals and humans. Small crustaceans can recycle nutrients as filter feeders, and larger crustaceans can act as a food source for large aquatic mammals. Terrestrial crustaceans also have ecological importance as decomposers of dead organisms.