shellfish vs gastropod

shellfish and gastropod both are nouns.

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

As nouns, gastropod is a hyponym of shellfish; that is, gastropod is a word with a more specific, narrower meaning than shellfish:

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

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