The formation of coral begins with the polyp, a small invertebrate with a cylindrical body that is closed at one end and has a mouth and minute tentacles at the open end. Coral feeds on plankton, bacteria, dead-animal material and even small floating prey that are caught by tiny tentacles. These tentacles have a stinging mechanism which the polyps use to repel attackers or to paralyze their prey. The ecosystem of a reef can be compared with that of a rain forest insofar as growth occurs in tiers, providing numerous habitats for the myriad species inhabiting the different levels. Although a single coral polyp matures quickly, it can take decades for a large coral colony to grow.
Reproduction occurs when the parent polyp releases fertilized eggs.
These grow into larvae that, after days or weeks of swimming about, settle and begin to develop into polyp bodies. This settlement forms the new coral colony. In this way, the coral colonies grow. A live colony can be 400 years old, and since a colony is the aggregation of these divisions, in a sense the polyp lives to a very old age. Sediment, or silt, is one of coral's deadliest enemies.