Real Dictionary

crustacean
countable noun: A crustacean is an animal with a hard shell and several pairs of legs, which usually lives in water. Crabs, lobsters, and shrimps are crustaceans.

crustacean in American English
any of a subphylum (Crustacea) of arthropods, including shrimps, crabs, barnacles, and lobsters, that usually live in the water and breathe through gills: they have a hard outer shell and jointed appendages
of crustaceans
adjective: belonging or pertaining to the crustaceans
noun: any chiefly aquatic arthropod of the class Crustacea, typically having the body covered with a hard shell or crust, including the lobsters, shrimps, crabs, barnacles, and wood lice

crustacean in British English
adjective: of, relating to, or belonging to the Crustacea
noun: any arthropod of the mainly aquatic class Crustacea, typically having a carapace hardened with lime and including the lobsters, crabs, shrimps, woodlice, barnacles, copepods, and water fleas