Coral reefs are built by coral polyps as they secrete layers of calcium carbonate beneath their bodies. The corals that build reefs are known as “hard” or “reef-building” corals. Soft corals, such as sea fans and sea whips, do not produce reefs; they are flexible organisms that sometimes resemble plants or trees. Soft corals do not have stony skeletons and do not always have zooxanthellae. They can be found in both tropical seas and in cooler, darker parts of the ocean.
The coral polyps that build the reef survive by forming a symbiotic relationship with microscopic algae called zooxanthellae. The polyps offer the algae shelter while the zooxanthellae create energy—through photosynthesis—that the corals use as food. In a sense, the coral polyps are “farming” the algae. The waste products of the polyps also serve as food for the zooxanthellae. Corals are also predators; they extend their tentacles at night and capture tiny organisms (zooplankton, small fish, or other potential food item) that happen to be floating by with stinging cells called nematocysts. The captured prey is then moved into the polyps’ mouths and digested in their stomachs.
Other types of animals and plants also contribute to the structure of coral reefs. Many types of algae, seaweed, sponges, sediment, and even mollusks like giant clams and oysters add to the architecture of coral reefs. When these organisms die, they also serve as foundations for new corals.