What is a coral?

The coral polyp is surrounded by a limestone skeleton Diagram by Geoff Kelley from Veron JEN. (2000). Corals of the World. AIMS, Townsville

While corals are not solely responsible for coral reef formation, they contribute a significant amount of calcium carbonate (limestone) building material, and provide a lot of the complex structure which supports such a diverse range of live forms.

Corals belong to the group of animals (Cnidarians) that also include anemones, jellyfish, bluebottles and hydroids. This group of animals is quite simple, with a body plan that includes a central cavity.

The basic structure shared by all corals is the coral polyp. This is the basic building block of a coral colony. The polyp is a small tube-like structure, with a central space opening via a mouth. The mouth of the polyp is surrounded by six (or a multiple of six) tentacles. A coral polyp resembles a small anemone. Most soft corals lack hard external skeleton, and their polyps have eight tentacles.

In hard corals, the polyp sits in a small limestone cup (corallite) that it secretes and that protects the soft polyp tissue. When the polyp grows upwards and divides into two, each new polyp secretes a new corallites. Thus as the coral grows upward and outwards, the coral tissue lifts itself up and lays down new skeleton. In most coral species, the new polyps remain joined together as they grow, and food can be moved between the polyps through the coral colony.

Some colonies stay small, and some corals will stay as a single large polyp for their entire life. Some corals can continue to growth for centuries, reaching more than 10m in height. Scientists are examining cores from these 'massive' corals to study the environmental conditions that were present throughout the corals lifetime.