Coral reefs are an extremely species-rich habitat, rivalled only by tropical forests in terms of their diversity and productivity.

They are a community based on rock-forming coral animals and algae that exist and grow into large submerged mounds, slopes and islands.

On-reef building corals and seaweeds, soft corals and sponges are also important members of this community.

The coral animals harbour microscopic algal cells (zooxanthellae) within their own cells that allow them to use sunlight to make food and so help to build their limestone structure. The depth of coral reefs is therefore limited by light penetration with few reefs extending deeper than 40m.

1,500km of reefs
Growth of the rock-like corals varies between a few millimetres to 10cm per year, depending on the species and water conditions. In the eastern African marine ecoregion fringing coral reefs are the most common type of reef, in many places forming continual stretches of 100km or more, and constituting most of the estimated total of 1,500km of reefs along these shores.