gastropod vs shellfish

gastropod and shellfish both are nouns.

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

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

gastropod: a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes
shellfish: invertebrate having a soft unsegmented body usually enclosed in a shell

Other hypernyms of gastropod include mollusc, mollusk.

gastropod (noun)	shellfish (noun)
a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes	invertebrate having a soft unsegmented body usually enclosed in a shell
	meat of edible aquatic invertebrate with a shell (especially a mollusk or crustacean)