Most coral polyps attach themselves to a hard substrate and remain there for life.
Reef-building corals have a mutualistic relationship with zooxanthellae, microscopic algae that live with coral polyp's tissues. Both the polyp and the zooanthellae benefit. For this reason, reef-building corals are found only in areas where symbiotic zooxanthellae can take in light for photosynthesis.
Through photosynthesis, zooxanthellae convert carbon dioxide and water into oxygen and carbohydrates. The coral polyp uses carbohydrates as a nutrient. The polyp also uses oxygen for respiration and in turns, returns carbon dioxide to the zooxanthellae. Through this exchange, coral saves energy that would otherwise be used to eliminate the carbon dioxide.
Nitrogen and phosphorus are cycled between zooxanthellae and coral polyps. For example, zooxanthellae take in ammonia given off as waste by the polyp, and return amino acids.
Zooxanthellae also promote polyp calcification by removing carbon dioxide during photosynthesis. Under optimum conditions, this enhanced calcification builds the reef faster than it can be eroded by physical or physical or biological factors.
Certain toxic compounds in soft corals (Order Alcyonacea) may make the corals unappetizing and deter predators.
Corals compete for living space on the reef. Some soft corals secrete toxins to eliminate competitors. Some reef-building corals can actually digest the tissue of an invading coral.