shellfish vs mollusk

shellfish and mollusk both are nouns.

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

As nouns, shellfish and mollusk are synonyms defined as:

shellfish and mollusk: invertebrate having a soft unsegmented body usually enclosed in a shell

Other synonyms of shellfish include mollusc.

shellfish (noun)	mollusk (noun)
invertebrate having a soft unsegmented body usually enclosed in a shell	invertebrate having a soft unsegmented body usually enclosed in a shell
meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)