Most reef-building corals on the Great Barrier Reef are broadcast spawners: colonies release buoyant bundles of eggs and sperm that rise and break apart at the surface, where fertilisation occurs. Because the gametes are diluted quickly, synchrony is what makes fertilisation likely at all.
Three cues are stacked. Rising sea temperature through spring sets the month. The lunar cycle sets the night, with spawning typically following a full moon. A response to the fading light after sunset sets the hour, so release across a reef falls into a window of an hour or two. The result, first documented on the Great Barrier Reef in the early 1980s, is mass spawning in which many species release together, usually in late spring after the October or November full moon on the southern reefs.
The simplification worth correcting is that the whole reef spawns on one night. Timing shifts along the 2,300-kilometre system, split spawning across two months occurs in some years, and different species reliably offset by nights or hours, which limits hybridisation between them.