A good way to imagine a coral reef is to think of it as a bustling city or community, with the buildings made of coral, and thousands of inhabitants coming and going, carrying out their business. In this sense, a coral reef is like a metropolis of the sea. Although coral is often mistaken for a rock or a plant, it is composed of tiny, fragile animals called coral polyps . There are two types of coral, hard coral and soft coral . Hard corals are the architects of coral reefs and include such species as brain coral and elkhorn coral.
Hard corals are hermatypes or reef-building corals and often require zooxanthellae for survival.
Soft corals such as sea fingers and sea whips, are soft and bendable and often resemble plants or trees. They are referred to as ahermatypes or non-reef building corals and they do not always possess zooxanthellae. Soft corals are found in both tropical seas and in cool, dark regions. A coral polyp is the actual tiny, coral animal. Coral polyps can be the size of a pinhead while others are larger, sometimes a foot in diameter.