bathyal zone vs ocean bottom

bathyal zone and ocean bottom both are nouns.

Word	Noun	Adjective	Verb	Adverb
bathyal zone	Yes	No	No	No
ocean bottom	Yes	No	No	No

As nouns, ocean bottom is a hypernym of bathyal zone; that is, ocean bottom is a word with a broader meaning than bathyal zone:

bathyal zone: the steep descent of the seabed from the continental shelf to the abyssal zone
ocean bottom: the bottom of a sea or ocean

Other hypernyms of bathyal zone include Davy Jones's locker, Davy Jones, ocean floor, sea bottom, sea floor, seabed.

bathyal zone (noun)	ocean bottom (noun)
the steep descent of the seabed from the continental shelf to the abyssal zone	the bottom of a sea or ocean