what are crustaceans

Crustaceans are a diverse group of invertebrate animals that belong to the subphylum Crustacea. They have a hard exoskeleton, jointed, paired appendages, and three body regions (head, thorax, and abdomen) . Crustaceans include a large number of species, such as crabs, lobsters, shrimp, krill, copepods, amphipods, barnacles, water fleas, and pill bugs. Most crustaceans are aquatic, living in either marine or freshwater environments, but some are terrestrial. They have evolved a variety of specialized body forms and behaviors to cope with both aquatic and terrestrial environments. 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. Small crustaceans eat substantial amounts of algae, which keeps the plants in check, making for clearer waters which, in turn, give seagrass beds access to light and oxygen.