sea animal vs fauna

sea animal and fauna both are nouns.

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

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

sea 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 sea animal include animal, animate being, beast, brute, creature.

sea 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