Coral form a compact colony. The hard calcium carbonate that builds reefs comprises the exoskeleton for hundreds of thousands of tiny polyps. Each polyp is only a few millimeters in diameter.
Although some coral catch small fare using stinging cells, most live symbiotically with zoox, a photosynthetic unicellular algal protist. Zoox are endosymbiotic, living within coral polyps. Requiring sunlight so the zoox can provide nutrition, coral typically live shallower than 60 meters.
Coral polyps on a reef reproduce by simultaneous release of eggs and sperm. This happens on just 1 night, or a very few consecutive nights of the year, shortly after sunset, just after a full moon, for 20–30 minutes, beginning at 9:20 pm local time.
The synchronicity is astonishing, especially considering that coral have no eyes, and so supposedly cannot see. But polyps have photoreceptors that detect hue shifts in the twilight sky.
Prior to a full Moon, the Moon hits the sky before sunset. Reflecting the ruddy light of a setting sun, the mixture with moonlight sets the sky slightly redder. Just after a full moon, sunset precedes moonrise. With no Moon reflecting pinkish twilight, there is a slightly bluer glow.
As scuba divers know, underwater, red is the first color to go. That’s because red is the color of long wavelength, and relatively easily disrupted by water movement.
Blue and green are at short wavelengths and carry farther underwater. While blue is the shortest visible wavelength for humans, green is the last color to disappear underwater. At extremity of color detection, the mind-brain cannot compensate for the relatively few blue cone receptors in the human eye, leaving green the most visible color in an aquatic murk.
In the lunar cycle blue light is rarer, appearing only after a full Moon. As with all things biological, the coherent solution is derived via evolution: polyps detect the higher concentration of blue light.
How coral polyps manage their calendar for release only once a year is not yet understood. The smart money is on temperature variation averaging: coral calendar calculus.