The corals only reproduce once a year and it all depends on the water temperature, the moon and their own timing.
The best chance to experience this is with a night dive or snorkeling in September or October, seven nights after the full moon. That is exactly what biologists from STINAPA did last week, and they captured it all on film.
The timing on the magical event on October 1st was perfect. When the biologists from STINAPA got in the water, 9.30 pm, they could see all the small egg bundles ready in each polyp of the star corals.
All corals release their egg bundles at the same time, containing both eggs and sperm, so that they can reproduce with other corals further away on the reef. Once one coral let the egg bundles go and the small packages start rising to the surface, other corals perceive that. They can actually smell each other.
Not all species of coral reproduce during the same night. On October 1st when the STINAPA crew went diving it was the star corals that spawned. If you want to know more about when and where to go, there are coral spawning calendars estimating when the different species spawn based on last year’s event. Link to a calendar: http://www.researchstationcarmabi.org/predictions-coral-spawning-2018/