Why do corals all spawn on the same night?

Corals spawn together because a coral can only reproduce if its eggs and sperm meet, and the way to make sure they do is for a whole reef to release them at once. About 75 percent of reef-building corals are broadcast spawners: they release eggs and sperm into the water, where they meet and fertilise. That works only if the timing is shared, so corals synchronise it. On a reef, often all the corals spawn on the same night, even when many species are present.

The timing is precise. Spawning usually happens in the evening or at night, can occur as rarely as once a year, and can fit inside a window of 10 to 30 minutes. Corals use several cues, among them temperature change, the lunar cycle and day length. Mass spawning often happens on nights after a full moon. Corals contain light-sensitive proteins called cryptochromes, which let them detect changes in sunlight and moonlight.

Strangely, moonlight itself may suppress spawning. The cue that seems to matter most is the dark part of the night between sunset and moonrise. After the full moon, moonrise comes later each night, leaving a dark window after sunset, and that is when corals let go.

The sight can be dramatic: the usually clear water clouds with gametes. The fertilised eggs become microscopic pink larvae called planulae. Most do not survive, and a colony may need to release several thousand larvae a year to start one new colony.

On the Great Barrier Reef, reefs in the inner part spawn in the week after the full moon in October, and the outer reefs in November and December. It is a calendar written in light and darkness, and studies suggest light pollution can scramble it.