shellfish vs seafood

shellfish and seafood both are nouns.

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

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

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

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