Coral Reefs

Overview

Coral reefs are underwater ecosystems made from the skeletons of hard coral colonies. Each colony is composed of multiple polyps called corals—animals with tentacles around a mouth at one end and sac-like bodies at the other that attach to a surface and secrete calcium carbonate for protection. Over thousands of years, these secretions accumulate to form habitats that support about 25% of marine species, even though they cover less than 1% of the ocean floor.

Known as the "rainforest of the seas," coral reefs are found in tropical and subtropical waters of more than 100 countries, with larger reef systems concentrated in the Indo-Pacific, Caribbean, and the Red Sea. Shallow-water coral reefs rely on a symbiotic relationship with colorful photosynthetic algae called zooxanthellae, which provide coral with nutrients and oxygen in exchange for a protective environment within coral tissue. Deepwater corals instead consume plankton and dead organic matter to survive.

In the US, coral reefs provide $3.4B in annual economic value through fisheries, coastal protection against wave damage, and tourism. However, stressors such as rising temperatures, ocean acidification, and pollution have weakened reefs and caused coral bleaching. This phenomenon sees corals expel zooxanthellae, turning corals white, depriving them of their food source, and making them susceptible to disease.

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