Coral is a marine organism in the class Anthozoa. There are over 2,000 species in this class, many of which form distinctive colonies of genetically identical organisms. Many people refer to these colonies as “coral,” and to the individuals inside the colonies as “polyps.” The organism plays a critical role in the marine environment, with collections of colonies known as reefs hosting around 25% of the known marine organisms, despite the fact that it is present on less than 1% of the ocean floor.
Individual polyps have soft bodies with a hard foot known as a calicle. They settle on the ocean floor, on a sandy or rocky substrate, depending on the species, and then they begin to build hard skeletons from the minerals present in seawater. Some polyps live as solitary individuals, but others reproduce through asexual budding, cloning numerous identical polyps to expand into a “coral head,” a large structure that can host hundreds or thousands of individual polyps, depending on how old it is.
Coral heads can take a variety of forms, depending on the species. Some are very hard, while others are soft, and a number of fantastic shapes from huge platters to sprouting antlers can develop. The landscape of a reef is often quite striking as a result.
In addition to reproducing asexually through budding, corals can also reproduce sexually by spawning. When they spawn, they release clouds of eggs or sperm into the water, with gametes forming in these clouds when they drift together and eggs and sperm fuse. The resulting gametes free-float and develop into new polyps, which take root and repeat the process all over again.
Many of these organisms have a symbiotic relationship with zooxanthellic algae, algae that live inside the coral and photosynthesize, generating energy for the polyps. These algae are responsible for the distinctive colors of coral, with non-zooxanthellic ones being generally white to cream in color. In addition to gathering food and energy from the algae, they are also capable of trapping prey with sweeper tentacles covered in minute stinging cells.
Coral polyps look very similar to sea anemones, which is perhaps not too surprising, since the two are close relatives. These small organisms are very vulnerable to damage and changes in their environment, such as elevated temperatures, increased solar radiation, nutrient runoff, and human interference in the form of harvesting, trawling for fish, or accidental damage inflicted by divers and boats.