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.

belonging or pertaining to the crustaceans.

noun

shellfishnotes:one major difference between insect , spider , and crustacean is the antennae; one pair on most insects , none on spiders , and two pairs for crustaceans , barnacle , crab , lobster , louse , prawn , shrimp , squilla , water flea