Here is a really cool video about an entire coral reef spawning. Triggered by a full moon, this Okinawa coral reef put on this beautiful display.
Corals can reproduce both asexually and sexually. In reefkeeping hobby, we constantly see new polyps and branches growing on our SPS; this is a form a asexual reproduction. However, approximately 3/4 of all stony corals produce a male and/or female gametes. Many of of these stony coral species, release enormous amounts of eggs and sperm into the water, to distribute their offspring. This species are known as Broadcast Spawners.
Spawning, must be a synchronized process, whereby many species within an area release their eggs and sperms. These events, are normally timed as a response to several environmental cues.