sea slug vs gastropod

sea slug and gastropod both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea slug	Yes	No	No	No
gastropod	Yes	No	No	No

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

sea slug: any of various marine gastropods of the suborder Nudibranchia having a shell-less and often beautifully colored body
gastropod: a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes

Other hypernyms of sea slug include univalve.

sea slug (noun)	gastropod (noun)
any of various marine gastropods of the suborder Nudibranchia having a shell-less and often beautifully colored body	a class of mollusks typically having a one-piece coiled shell and flattened muscular foot with a head bearing stalked eyes