mollusk vs shellfish

mollusk and shellfish both are nouns.

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

As nouns, mollusk and shellfish are synonyms defined as:

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

Other synonyms of mollusk include mollusc.

mollusk (noun)	shellfish (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)