1. What is a coral?

Early biologists used to think that corals were plants. In fact they are animals and are related to sea anemones, jellyfish and hydroids. All have been classified within the phylum, Coelenterata, since they have the same basic body design -- that is, they are symmetrical about a central axis (termed radially symmetrical). Basically each has a sac-like body which has a central cavity or stomach. There is a single opening to this cavity which serves as both a mouth and anus. This is often surrounded by a circle of tentacles (often more than one) which contain stinging cells (nematocysts).

Members of the coelenterate phylum may occur in a bell-shaped form (termed a medusa) which floats or swims in the water (eg. jellyfish) or they may occur as polyps which are attached to hard surfaces in the ocean (eg. corals). Some occur only as polyps (eg. corals) whilst others occur only as medusae (eg. jellyfish). Others may occur in both forms during their life cycle (eg. hydroids).

Corals may consist of only one polyp (solitary) or they may comprise many hundreds of polyps (colonial), all joined together in the one structure (termed a colony). There are several different types of corals. Those that possess 8 tentacles (or multiples thereof) and produce an almost jelly-like body are called soft corals. Those that usually have a plant-like growth and a horny skeleton which runs along the inside of the stem (making it very strong) are called gorgonians. They are more commonly referred to as sea-whips and sea-fans. Finally there are the 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 zooxanthellae are of great benefit to corals. They live inside the tissues of most corals and produce nutrients (organic carbon) which are sources of food for the corals. As these nutrients are produced via photosynthesis 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 d u ring prolonged periods in very shallow water. These compounds are being investigated at 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 into the water column and consequently fertilisation occurs externally (ie. 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 / November). It is a spectacular event!

Whilst corals occur throughout the world they are most abundant in the tropics where water temperatures are highest. Corals feed 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.

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Last updated - 12 December 97

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