Final answer:
Crustaceans are a large group of aquatic arthropods that include shrimp, lobsters, crabs, and crayfish. They are highly successful and dominant in the aquatic ecosystem.
Step-by-step explanation:
Crustaceans are a large group of arthropods, consisting of almost 52,000 species. They are among the most successful animals and can be considered the dominant aquatic animals. The majority of crustaceans are aquatic, living in the ocean or freshwater. Examples of crustaceans include shrimp, lobsters, crabs, and crayfish. However, there are also a few terrestrial crustacean species like land crabs and woodlice.