polyplacophore vs shellfish

polyplacophore and shellfish both are nouns.

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

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

polyplacophore: primitive elongated bilaterally symmetrical marine mollusk having a mantle covered with eight calcareous plates
shellfish: invertebrate having a soft unsegmented body usually enclosed in a shell

Other hypernyms of polyplacophore include mollusc, mollusk.

polyplacophore (noun)	shellfish (noun)
primitive elongated bilaterally symmetrical marine mollusk having a mantle covered with eight calcareous plates	invertebrate having a soft unsegmented body usually enclosed in a shell
	meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)