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Cabbage CoralCorals are animals. Some species have separate sexes; others are both sexes in one. All produce eggs and sperm. During the annual spawning, corals release thousands of eggs and sperm, which rise to the surface forming slicks of pink goop. This occurs between sunset and 11pm, 4-6 days after the full moon in October, November and or December.

The floating slick is basically a soup of sperm and eggs, which hope to meet and successfully fertilise. Sperm will live several hours and eggs only 3-6 hours. The morning after, fertilised eggs have developed into embryos. The floating embryo slick will only last 12 hours before disintegrating. One day after fertilisation, the embryo develops into a little worm-shaped juvenile, a planula. Although ready to swim down to the reef in search of a home only four days later the planula can probably survive up to 20 days floating at sea.

Gorgonia The planula searches for a place to settle on the reef and probably chooses the site using a combination of small chemical cues, light intensity, water pressure and surface texture. The planula glues its bottom down to secure its new home and within hours of moving in transforms itself into the adult form, a polyp.

Some coral mums equip their eggs with zooxanthellae, or plant cells. Species without this "packed lunch" must pick up plant cells from the water around them. These are usually obtained within 14 days of settlement.

24 hours after settlement, the polyp, with the help of its plant cells, begins to lay down it's hard, calcium carbonate skeleton. After several weeks or months, this skeleton is a small 2-5mm cup shape visible to a searching human eye. The coral polyp will divide into two, genetically identical, polyps after just 14 days. The polyps continue to divide and at six months, the little colony could consist of at least half a dozen individual polyps and be 5-10mm in diameter.

Hard Coral A one year old colony can be anywhere from 4-25mm in diameter and contain between 1-50 polyps. Over the years, the polyps continue to divide, the colony continues in size and within 2-3 years begins to take on a shape that is representative of its species.

To identify the various coral species you only need to look at their shape. In most cases the common name predicts what shape they are. Corals, both soft and hard, come under a common family name, and there are many types with in that family group.

Soft Coral You will often hear the term Bommie used when discussing dive and snorkelling sites. The word is derived from the French bombax and is sometimes spelled bombie, but more commonly bommie. The term is not normally used in scientific papers but is usually understood to describe anything from a single coral boulder to a large reef structure. Technically it is a small patch reef in column form in, usually, a sheltered lagoon. It is attractive to divers because of the diversity and abundance of marine life that it attracts.

Coral Spawning - Sex On The Reef

Described as "sex on the reef", this simultaneous mass spawning of corals was first scientifically observed in 1981. This event usually only happens at night and requires several contributing factors for the coral to spawn which can make it very difficult to predict specific dates.

Predictably Unpredictable

Annual coral orgies, the simultaneous mass spawning of corals on the Great Barrier Reef was first scientifically observed in 1981 and is a great reason to night dive from a liveaboard boat during November and December. Coral spawning is now the focus of international research however nature cannot be totally predicted.

The process begins 6 months before when eggs and sperm begin to form inside the coral polyps. For spawning to take place, water temperatures must be 27 degrees or more. But corals need a specific cue so they can release eggs and sperm into the water at exactly the same time. That cue is November's full moon and on the 2nd to 6th night following the full moon the majority of corals spawn. The 2004 coral spawning is expected to occur around December 1 (give or take a few days).

The first sign of spawning is the sight of coloured bundles of eggs inside female polyps; these are held under the mouths of the polyps and are visible through the transparent tissues. The bundles are squeezed out through the mouths of the polyps and released. All the polyps in a colony can do this in minutes. Meanwhile male corals produce clouds of sperm. Floating up to the surface the eggs and sperm form a slick on the seas surface for days.

Spawning is timed to coincide with periods when there are minimum tidal movements, which allows the sex's time to find representatives from the same species and mix and match before being swept away. Some years there is 'split' spawn with corals in shallow warmer inshore reefs performing in November while those in colder waters on the outer reefs, spawn in December.

Corals make such an effort to spawn at the same time in order to increase opportunities for fertilisation. Mass spawning also overwhelms the appetite of predators. Developing larvae (planula) are swept off to begin new reefs. A planula attaches itself to a vacant patch of reef and starts to grow as the founder polyp for a new coral colony. Coral spawning is a once in a lifetime experience and highlight of night diving during November and December.

Photos courtesy of Richard Fitzpatrick (Digital Dimensions)

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Diving Cairns - The ultimate guide to a diving holiday on North Queensland's Great Barrier Reef