Coral and Coral Reef

Corals are small (0.06–0.5 in; 1.5–12 mm), colonial, marine invertebrates. They belong to the class Anthozoa, phylum Cnidaria (or Coelenterata). Corals are subdivided into (1) stony corals (reef-building or hermatypic)—order Scleractinia, subclass Hexacorallia—which have six tentacles, and (2) soft corals, sea fans, and sea whips—order Gorgonacea, subclass Octocorallia—with eight tentacles.

The limestone substrate of stony coral colonies develops because each individual animal, or polyp, secretes a hard, cup-like skeleton of calcium carbonate (limestone) around itself as a protection against predators and storm waves. These limestone skeletons, or corallites, make up the majority of the reef framework. Certain coral species produce distinctively shaped colonies, while others exhibit various shapes. Some species, such as staghorn coral, are intricately branched, and are sometimes called coral stands. Brain corals are almost spherical in outline and are often called coral heads; they often display surface convolutions reminiscent of those on a human brain.

Calcareous red, or coralline, algae also contribute to the framework of reefs by secreting their own encrusting skeleton that acts as cement, stabilizing loose sediment on the reef. Coralline algae often produce as much of a reef's limestone as do the stony corals. Other calcareous organisms that contribute reef sediments include sponges, bryozoans (another colonial animal), tube worms, clams, and snails.