marine animal vs fauna

marine animal and fauna both are nouns.

Word	Noun	Adjective	Verb	Adverb
marine animal	Yes	No	No	No
fauna	Yes	No	No	No

As nouns, fauna is a hypernym of marine animal; that is, fauna is a word with a broader meaning than marine animal:

marine animal: any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals
fauna: a living organism characterized by voluntary movement

Other hypernyms of marine animal include animal, animate being, beast, brute, creature.

marine animal (noun)	fauna (noun)
any of numerous animals inhabiting the sea including e.g. fishes and molluscs and many mammals	a living organism characterized by voluntary movement
	all the animal life in a particular region or period