Coral reefs only account for about one-tenth of one
percent of the world’s area, but harbor at least five
percent of its known species. These remarkable systems
depend on a symbiotic relationship between a tiny animal
and an even tinier alga. The coral itself is a primitive
animal related to jellyfish, consisting mostly of a stomach
topped with tentacles they use to sting and eat plankton.
In addition, each coral polyp can ingest tiny
photosynthetic algae called zooxanthellae and harbor
them in its stomach cavity without digesting them. In this
symbiotic relationship, the coral provides the algae with
carbon dioxide, the essential building block of
photosynthesis, important nutrients like nitrogen and
phosphorus, as well as a protected place to live and
reproduce. In turn, the photosynthetic action of the
zooxanthellae provides the coral with up to 90% of its
energetic requirements, as well as oxygen, a by-product of
photosynthesis. Because of this unique relationship, coral
reefs are tremendously productive even in nutrient-poor
waters. In fact, in waters that are enriched with nitrogen
and phosphorus—for instance, near river outlets—other
types of algae quickly outcompete the coral. Coral reefs
are found exclusively in warm, clear, shallow waters where
enough light reaches the bottom to enable
photosynthesis.