Corals are well known mass spawners. Many corals manage to emit eggs and sperm into water on the same evenings of the year; some coordinate the releases to within 20 minutes of each other, even though miles apart.
Researchers have noted that on nights after a full moon, when the moon lags below the horizon until after sunset, twilight takes on an especially blue hue. Yet corals lack both eyes and a central nervous system.
Researchers first used sensors to demonstrate that the blue shift can be detected underwater. They also found that only two light-sensing pigments of the opsin type [see http://en.wikipedia.org/wiki/Opsin], one tuned to a greenish and the other to a blue wavelength, would be sufficient to detect the shift. Why opsin? Because in recent genetic analyses opsin pigments have been found in abundance in invertebrates.
There is still no certainty that this is the mechanism that explains the synchrony, but it is an interesting thesis which has biological plausibility, though certainly it is in need of further exploration.
Two reports on the thesis can be found at http://jeb.biologists.org/content/214/5/770.abstract?maxtoshow=&hits=10&RESULTFORMAT=&searchid=1&FIRSTINDEX=60&resourcetype=HWFIG and http://jbr.sagepub.com/content/26/1/82.abstract.