mussel vs shellfish

mussel and shellfish both are nouns.

Word	Noun	Adjective	Verb	Adverb
mussel	Yes	No	No	No
shellfish	Yes	No	No	No

As nouns, shellfish is a hypernym of mussel; that is, shellfish is a word with a broader meaning than mussel:

mussel: black marine bivalves usually steamed in wine
shellfish: meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)

mussel (noun)	shellfish (noun)
marine or freshwater bivalve mollusk that lives attached to rocks etc.	invertebrate having a soft unsegmented body usually enclosed in a shell
black marine bivalves usually steamed in wine	meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)