What is the marine world's equivalent of the rainforest in terms of biodiversity or species richness?

The world's coral reefs are the equivalent of the world's rainforest in terms of biodiversity or species richness.

Coral reefs are also called the tropical rainforest of the sea, it is the number one most productive ecosystems on earth. Coral reefs are in danger by human influences. Reefs provide home for millions of species.