Coral and Coral Reefs

What is a coral? Corals may consist of only one polyp solitary) or they may comprise of many hundreds of polyps (colonial) all joined together in the one structure termed as colonial. There are several different types of corals.

Soft Corals are those that possess 8 tentacles (or multiples thereof) and produce an almost jelly-like body.

Gorgonians have a plant-like growth and a horny skeleton which runs along the inside of the stem, making it very strong. they are more commonly known as Sea-whips and Sea-fans

True stony corals which are the main building blocks of reefs. Their hard calcareous skeletons are produced by the polyps themselves. Each polyp in a colony is suspended inside a tiny cup of skeleton and is connected by a thin layer of tissue which overlies the surface of the colony

small single-celled algae called zoozanthelae are a great benefit to corals. They live inside the tissue of most corals and produce nutrients (organic carbon) which are sources of food for the corals. These nutrients are produced via photsynthiesis the corals are able to build their skeletons much faster in the light than in the dark.

Corals also possess chemical compounds which prevent them from being affected by ultraviolet radiation when exposed at low tide or during prolonged periods in very shallow water. These compounds are being investigated by the Australian Institute of Marine Science because they may prove very effective as ultraviolet blocking agents in various substances (such as sunburn creams, paints and plastics).

Corals reproduce in several different ways. Some bud or split from the original polyp (a form of asexual reproduction). Sexual reproduction also occurs. In some corals the eggs are fertilised internally. These are then brooded within the body of the parent and at the appropriate time are released as planulae.

Other species shed their eggs and sperm in the water column and consequently fertilisation occurs externally (i.e. outside the parent). This strategy is very 'hit and miss' given the large volume of the oceans. To ensure that sufficient fertilisation takes place many corals on the Great Barrier Reef spawn together on one night of the year This occurs about 5 days after the full moon in late spring (October or November. It is a spectacular event!

Whilst corals occur throughout the world they are the most abundant in the tropics where water temperatures are highest. Corals fee primarily during the night by expanding their tentacles and capturing minute plankton from the water. They also occur in a variety of forms, the most common being branching sometimes called staghorn corals_ and plate-like (table corals) colonies.