cephalopod mollusk vs shellfish

cephalopod mollusk and shellfish both are nouns.

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

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

cephalopod mollusk: marine mollusk characterized by well-developed head and eyes and sucker-bearing tentacles
shellfish: invertebrate having a soft unsegmented body usually enclosed in a shell

Other hypernyms of cephalopod mollusk include mollusc, mollusk.

cephalopod mollusk (noun)	shellfish (noun)
marine mollusk characterized by well-developed head and eyes and sucker-bearing tentacles	invertebrate having a soft unsegmented body usually enclosed in a shell
	meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)