shellfish vs mussel

shellfish and mussel both are nouns.

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

As nouns, mussel is a hyponym of shellfish; that is, mussel is a word with a more specific, narrower meaning than shellfish:

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

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