shellfish

noun, plural (especially collectively) shell·fish, (especially referring to two or more kinds or species) shell·fish·es.

1. an aquatic animal having a shell, as the oyster and other mollusks and the lobster and other crustaceans.

noun plural -fish or -fishes

1. any aquatic invertebrate having a shell or shell-like carapace, esp such an animal used as human food. Examples are crustaceans such as crabs and lobsters and molluscs such as oysters

n.also shell-fish, Old English scylfiscas (plural); see shell (n.) + fish (n.).