seafood vs shellfish

seafood and shellfish both are nouns.

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

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

seafood: edible fish (broadly including freshwater fish) or shellfish or roe etc
shellfish: meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)

seafood (noun)	shellfish (noun)
edible fish (broadly including freshwater fish) or shellfish or roe etc	invertebrate having a soft unsegmented body usually enclosed in a shell
	meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)