Coral reefs are ecosystems that are considered the rainforest of the sea due to the incredible diversity of their inhabitants. While coral reefs cover less than 1 percent of the area of the ocean, they are home to fully 25 percent of marine species. Coral reefs are large geological structures made from the calcium carbonate (CaCO3 ) skeleton of marine cnidarian animals called corals.
Individual coral animals are small anemonelike polyps, but they form colonies of hundreds or thousands of individuals. Reef-building corals contain symbiotic algal partners called zooxanthellae. Zooxanthellae are photosynthetic and provide corals with energy in the form of glucose. In return, corals provide zooxanthellae with a protected environment and the compounds needed to photosynthesize. Coral reefs are distributed in tropical oceans because they require shallow, warm, clear, nutrient-poor water to grow.
