Since the discovery of massed coral spawning twenty five years ago, the event has been filmed to death in every coral reef film ever broadcast. The sight is indeed spectacular, but we have seen it so many times and I have it on good authority, we will be seeing it once again in an upcoming series of programmes. What is seldom shown in any detail is the very complex nature of the packaging that accompanies each release and the subsequent orgy of sexual activity and predation that follows hot on the heels of the main event.

Early research showed that, in species that liberate their gametes (not all do), the eggs are released by the coral polyps at the appropriate combination of water temperature and lunar phase (see above). In time it was realised that eggs were in fact clusters of many eggs - maybe ten or so squeezed , in most species, into one spheroidal mass. Now it is realised that the hermaphrodite species of coral glue the eggs together with a matrix of viscous, but as yet still inactive and unprimed, sperm.

Coral egg cluster

When the "eggs" are released in many species of branching and meandering coral, the whole mass floats to the surface of the sea and there absorbs water. The individual eggs soften, swell and round off. The glutinous sperm thins as water dilutes the mass and steadily billions of live, wriggling spermatozoa swim free and cause the glistening sheen sometimes noticed when calm conditions accompany the event. Amongst this soup of sex cells, fertilization is expected to take place, but sophisticated chemical systems seem to prevent self-fertilization. Sperm from another package must ideally find their way to an unfertilised egg, if the union is to be successful. In Bermuda we have seen pilchard spawning coincide with coral spawning, so producing a scum that extended the length of the fringing reef and smelt strongly of cod liver oil.

Coral and fish egg aggregation

On the Australian Great Barrier Reef it has been recently recognised that vast vertical migrations of deep sea Myctophid fish feast upon the spawn and trigger a food web concentration, now referred to as the "aggregation", that Japanese fisherman began to exploit nearly ten years ago. The western Australian reefs, in contrast, have for many years been seen as the focus of a similar, but different, event, the climax of which is the arrival of some hundreds of plankton-feeding whale sharks.

This article was written by Peter Parks

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