the.com/shellfish

means An aquatic invertebrate with a shell or shell-like exterior — including crustaceans like crabs and shrimp and mollusks like clams and oysters — grouped by cuisine, not by biology.

from From Old English scylfisc, a straightforward compound of 'shell' and 'fish' — 'fish' once being a loose catch-all word for any edible water creature, not the narrow backboned category it names today. That old looseness is exactly why crabs, which aren't fish at all, still carry the name.