That is amazing!
Anyone know how to make corals spawn? Is it stress factors? Or chemicals? Or simply large coral maturity?
How come in our home tanks this does not happen? We skim it out? Eaten by microfauna?
Some reefs (such as the Great Barrier Reef) have mass spawnings that occur roughly the same time every year, and we can predict them with a surprising amount of accuracy. As mentioned by lattehiatus, the timing is determined by lunar cycles (they occur roughly every 12 full moons) and temperature. But since the lunar year doesn't match up with the solar year, every 3-4 years there are two mass spawnings. For example, this year, the main mass spawning will occur around October 23rd and November 21st. Also, different species spawn a different number of days after the full moon. So most Acropora species will spawn one day, the favids the next day.
Unfortunately, our understanding of spawning cues is still incomplete. Obviously all reefs share the same lunar cycle, but reefs around the world spawn in different months, or might not have a mass spawning at all. Temperature seems to be important, near shore , warmer reefs often spawn a month sooner than off shore reefs, but there are other reefs with almost no temperature swing that still have a mass spawning.
As for our aquariums, even if you got them to spawn, I wouldn't expect any larvae to successfully settle, since they require 3-5 days of floating around as plankton before they can settle. During this time they'll either end up as delicious fish snacks, or get taken out by the filters.
