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)