Coral are marine invertebrates which typically live in colonies, and are often associated with the limestone reefs built by hard coral. Individual coral organisms are called coral polyps, which are soft-bodied and range in size from about the size of a pin head to a foot in diameter. These polyps have sac-like bodies and a mouth surrounded by tentacles, which they extend nocturnally to feed.
There are many species of coral, which are generally classified into two kinds: hard coral and soft coral. Hard coral create bony skeletons of limestone, or calcium carbonate. These coral live in colonies, and it is their bony skeletons that form coral reefs. Both coral polyps and their skeletons are colorless; the coral takes on its often bright coloring from zooxanthellae. Zooxanthellae are tiny algae which live within the coral polyps and create food through photosynthesis. The zooxanthellae nourish the coral, and the coral meet the rest of its nutritional needs by catching zooplankton with their stinging tentacles. Hard coral need zooxanthellae to survive, whereas soft coral do not.
Soft corals do not build stony skeletons, but rather have flexible cores and fleshy rinds which support and protect them. Soft corals resemble plants in their structure and bendability rather than rock, which hard coral becomes.
Coral colonies are formed when coral polyps reproduce and divide. These polyps connect to one another, and the colony acts as a single organism; gradually these colonies join other colonies and begin to form reefs. Coral are ancient organisms; it is estimated that the ancestors to coral reefs existed over 240 million years ago. Some extant reefs are estimated to have begun over 50 million years ago, and most are at least 5,000 years old. Because hard coral rely on the photosynthesis performed by zooxanthellae, coral reefs grow in water which has exposure to sunlight. Coral reefs grow best in water which is warm, shallow and clear although coral reefs can be found as deep as 300 feet below the surface.
Coral reefs cover only a small percentage of the ocean floor (less than one percent) and yet support nearly a quarter of all marine animals. Coral is endangered, and coral reefs are extremely sensitive to climate change and pollution.