Warmer water temperatures of about 25-26 degrees Celcius seem to be the initial cue for spawning. When the male encounters a female, he dances, swims in front of her, turning on his side in a shimmering nature. Possibly this stimulates the female in some way, but she pays no attention until dusk. Then towards dusk she rushes towards the surface, off goes the male, and spawning begins. They are mostly very small.
Plankton occurs mainly near the surface of seas where the plants get suitable illumination.
One of the most important aspects of reef ecology is that the majority of reef inhabitants develop through, and are dispersed by planktonic larval stages. Most animals and plants reproduce by shedding thousands of eggs and sperm into the currents. Nearly all of the resulting embryos are eaten in infancy. The chances of survival for most are in the order of a million to one against. Without this planktonic dispersal of larvae, corals on a reef devastated by say a cyclone, would be very slow to recover, since it would have to depend on the few surviving coral colonies to provide progeny.