Mass coral spawning

node · earth · Global tropical reefs; timing local

The synchronised release of gametes by many coral colonies and many species over a few nights of the year, cued by a combination of seasonal water temperature, the lunar cycle and the timing of sunset. It was described from the Great Barrier Reef in 1984, where the main event falls in late spring and early summer a few nights after a full moon, and comparable synchrony has since been documented on reefs elsewhere with different timings. Most reef-building corals are hermaphroditic broadcast spawners releasing buoyant egg-sperm bundles that rise to the surface, where fertilisation happens in a slick that can be kilometres across. The synchrony is a predator-swamping strategy and a fertilisation strategy at once: gametes are only viable for hours, so an unsynchronised colony in a broadcast-spawning species is a colony that does not reproduce. For this Journey, spawning is the recovery mechanism — a reef recovers by recruitment, and recruitment happens on this calendar, once a year.