Coral is formed by small animals called coral polyps, which are related to sea anemones and jellyfish. Coral polyps extract calcium from seawater and then convert that calcium into limestone shelters, which eventually become the coral seen in bodies of water.
According to HowStuffWorks, a piece of coral is often made up of the former shelters of several coral polyps. When one polyp dies, another polyp builds its shelter on top of the shelter of the first polyp. Eventually, a large structure is formed due to the polyps constantly building on top of homes of other polyps.
One of the best-known structures created by coral is known as a coral reef. A coral reef is made up of several colonies of coral polyps. The majority of species that are a part of the reef-building coral population reside in waters that stay at a relatively warm temperature, which is why they can be found in waters of the South Pacific, the Indian Ocean, the Caribbean and adjacent waters.
Coral reefs are not only home to coral polyps; other creatures, including fish, turtles, sharks, eels, crabs, shrimps, urchins, sponges and algae, use coral reefs for shelter and food, forming a complex food web.