nudibranch vs sea slug

nudibranch and sea slug both are nouns.

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

As nouns, nudibranch and sea slug are synonyms defined as:

nudibranch and sea slug: any of various marine gastropods of the suborder Nudibranchia having a shell-less and often beautifully colored body

nudibranch (noun)	sea slug (noun)
any of various marine gastropods of the suborder Nudibranchia having a shell-less and often beautifully colored body	any of various marine gastropods of the suborder Nudibranchia having a shell-less and often beautifully colored body