What system hosts more biodiversity than any other ecosystem in the oceans?

The coral reef

Explanation:

The coral reef system is widely known to be, and arguably the part of the ocean/marine ecosystem that hosts more biodiversity than all other ecosystems. The coral reef has several hundreds of individual reef systems housing up to 25 percent of the total marine life. It includes marine life such as sponges, algae, fishes, various invertebrates etc. It is home to hundreds of beautiful types of corals. The largest reef in the marine habitat is the Great Barrier Reef which is found Australia, covering about 2,400 km.