What is a coral reef?

Coral reefs, and their associated systems of mangroves and seagrasses, are the world's most biologically diverse marine ecosystems. Important assets to local and national economies, they provide fisheries for food, materials for new medicines, and income from tourism and recreation, as well as protect coastal communities from storms.

Coral reefs are some of the oldest ecosystems on earth. Built by a variety of coral and other species that secrete skeletons of calcium carbonate that accumulate over time, coral reefs can grow to be massive structures extending over wide areas of shallow tropical and subtropical seas. Reef building corals contain tiny cells of symbiotic algae that convert sunlight and nutrients into fuel for coral growth and production. Because they depend on light, coral reefs require clear water for growth and can be severely damaged by sediment or other factors that reduce water clarity or quality. Other types of corals that do not require warm water or sunlight are found in deep water, providing important habitats for commercial, recreational and other species.

	Boulder and massive corals, like this boulder star coral, are the "builders" of the reef. A coral head is a colony of small animals called polyps. These corals provide the reef foundations that is home to millions of reef species. Over 4,000 species of reef fish have been described so far.
	Elkhorn coral is a branching coral. Branching corals grow in the shallow areas of the reef crest and serve to break up the wave action as it comes onto the reef. The branches of elkhorn coral resemble an elk's rack of antlers, thus its name.
	Pillar coral forms numerous heavy cylindrical spires that grow upward from an encrusting base. Unlike other hard corals, which feed at night, pillar coral extends its polyps to feed during the day. Fallen pillers may give rise to new upward growing spires.
	In addition to the hard corals, there are a variety of soft corals like this common sea fan. The calcium carbonate skeleton of soft corals is located within their bodies, allowing them to move with the wave action. Over 800 species of corals have been described to date.
	Sponges like this orange elephant ear sponge are water filters for the reef. They filter up to 30,000 times their body volume every day. Researchers are discovering unique chemical compounds in sponges and other reef species that may have important medicinal properties and other uses.
	Corals are large colonies of small animals called polyps. These polyps reside within a cup-like calcium carbonate skeleton. They have a central opening surrounded by tentacles which can be extended to feed on phytoplankton in the water column.