Corals are invertebrates: animals without backbones. They belong to the group of creatures called Cnidaria, which also includes sea anemones and jellyfish. Like all cnidarians, corals have no head. Their bodies are saclike with a mouth surrounded by a crown of tentacles in groups of six or eight. Coral polyps are some of the smaller members of this group, and the polyps range in size from less than an inch to several inches in diameter. Corals feed using their tentacles to capture their prey, which includes tiny animals called zooplankton as well as small fish.
Coral reefs are colonies of small animals, known as polyps, which release calcium carbonate to form a hard skeleton on which they live. Coral reefs are found in shallow, tropical waters around the world. Living coral reefs provide homes for large numbers of marine animals. In fact, coral reefs make up only 1 percent of the ocean’s habitat but include over 25 percent of its species. Barrier reefs are the largest living structures in the world and are formed by coral running roughly parallel to the shore, which is separated from it by a wide, deep lagoon.