The surface of the reef is covered by the living part of the corals. Where dead coral skeletons are exposed, algae and abundant invertebrates settle.
Many fish use the structure as a temporary or permanent place to feed or hide.
The tentacled polyps of stony corals resemble anemones and belong to the same phylum, Cnidaria. The tentacles on eac h polyp have stinging cells that help to stun and catch small animals that venture too close to the coral. When they come in contact with a bit of food, the tentacles pull it down to the central mouth to be digested. Many stony corals feed this way at nig ht, sinking into the protective skeleton during the day, while some other cnidirians feed during the day. Reef-building corals have very specific requirements for growth which limit their distribution.