What are corals?
Corals are a symbiotic relationship between an animal (a Cnidarian) and a photosynthetic algae (Zooxanthellae). The growth form of the animal is known as a polyp. Zooxanthellae live within the polyp. . The algae benefit because they are glued into place in the photic zone (therefore they don’t have to worry about sinking so deep there is no light), they are protected from most grazers (except for parrotfish),and the waste products of the polyps provides them with a high concentration of nutrients (e.g., CO2 and NH3). The coral benefits because it receives energy from the zooxanthellae. The zooxanthellae produce about 100 times more energy than they need and about 90% of the excess is picked up by the polyps. In addition, the polyps can benefit from the oxygen that is produced as a byproduct of photosynthesis. Finally, the polyps benefit because they are able to have higher rates of calcification.
Coral polyps live together in large colonies. Most corals extend their polyps to feed at night (bottom photo) and do not extend their polyps at night (top photo).
Corals gain energy both by feeding on detritus and small living organisms and from sugar produced by the Zooxanthellae.
Coral Reefs
Coral polyps excrete an exoskeleton made up of Calcium carbonate (CaCO3). Different types of corals produce different types of skeletons.
Over time the colony's exoskeleton accumulates and forms a variety of growth forms including (clockwise starting upper left- staghorn corals, flower coral, brain coral, lettuce coral).
Over thousands of years the accumulation of coral exoskeletons combined with the hard bodies of other dead marine creatures can cause the formation of a coral reef.
https://www.youtube.com/watch?v=8NzyQwTnRUs
