mussel vs marine mussel

mussel and marine mussel both are nouns.

Word	Noun	Adjective	Verb	Adverb
mussel	Yes	No	No	No
marine mussel	Yes	No	No	No

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

mussel: marine or freshwater bivalve mollusk that lives attached to rocks etc.
marine mussel: marine bivalve mollusk having a dark elongated shell; live attached to solid objects especially in intertidal zones

Other hyponyms of mussel include mytilid, freshwater clam, freshwater mussel.

mussel (noun)	marine mussel (noun)
marine or freshwater bivalve mollusk that lives attached to rocks etc.	marine bivalve mollusk having a dark elongated shell; live attached to solid objects especially in intertidal zones
black marine bivalves usually steamed in wine