shellfish vs mollusc

shellfish and mollusc both are nouns.

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

As nouns, shellfish and mollusc are synonyms defined as:

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

Other synonyms of shellfish include mollusk.

shellfish (noun)	mollusc (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)