How Do Coral Reefs Form?

Question: How Do Coral Reefs Form?

Answer: Stony corals are a remarkable group of animals that inhabit tropical marine waters around the world and form vast colonies known as coral reefs. Coral reefs often resemble rock formations or even plants but such resemblances are only superficial. Coral reefs are in fact made up of many tiny animals known as coral polyps. Each coral polyp is an individual animal and the individual polyps coexist as part of a larger colony of polyps.

Coral reefs provide habitat for an immense diversity of other organisms including fishes, marine mammals, marine turtles, and invertebrates.

Stony corals inhabit marine habitats where the water is quite clear, the temperature remains within the range of 18°C-29°C, and the salinity of the water is in the range of 32ppt-42ppt.

A stony coral colony begins as a single free-swimming founder coral polyp that attaches itself to a hard substrate such as submerged rocks. The founder polyp replicates itself repeatedly through asexual reproduction, producing a colony. The colony consists of a base which is attached to the reef substrate, a growing edge zone (where new coral polyps are produced), and an upper surface that is exposed to light that filters down through the water.

Stony corals are so named because of the stone-like skeleton that forms within their bodies. This hard skeleton is made up of calcium carbonate (CaCO₃) and is the substance that gives a coral reef much of its structure. Over time, stony coral polyps lift up from their base and new calcium carbonate is deposited on top of the old surface. As a result, the coral grows upward on a mound of hard rock-like deposits.