Crustaceans are a very diverse group of invertebrate animals which includes active animals such as the crabs, lobsters, shrimp, krill, copepods, amphipods and more sessile creatures like barnacles.
All crustaceans have:
Crustaceans are diverse in form and live around the world in a variety of habitats - even on land. Marine crustaceans live anywhere from shallow intertidal areas to the deep sea. (Continue reading from ThoughtCo)
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. (Continue reading from Earth Day Network)