Since scientists discovered that corals reproduce by synchronous spawning in 1981, they have been searching for its catalyst. In October 2007, Australian, Israeli, and American scientists discovered the trigger for the mysterious procreation habits of coral.
Corals reproduce both sexually and asexually, and each individual coral, called a polyp, may reproduce both ways within its lifetime. Coral live in colonies that may consist of one or both sexes.
At least a third of corals in the Great Barrier Reef reproduce by synchronous spawning, a process in which the eggs and sperm are released into the water at the same time. Eventually the sperm and eggs merge together and create embryonic corals that sink to the ocean floor, and, if conditions are right, form new colonies. Synchronous spawning is dependent on the time of year, water temperature, and tidal and lunar cycles. Spawning happens in the spring during the third through six nights following a full moon, layering the sea so completely with eggs that they are visible to the human eye.
Until 1981, corals were thought to be primitive creatures without a brain or eyes and knew nothing of their environment. Graduate students at James Cook University changed that thinking when they discovered a mass spawning in the Great Barrier Reef. Over the last 25 years, the spawning rituals have been observed by scuba divers and scientists, and documented on PBS by photographer Al Giddings.
Corals have primitive photoreceptors, idea discovery first introduced by Israeli scientist, Dr. Oren Levy. In October 2007, scientists discovered that these photoreceptors have photosensitive chemicals that respond to moonlight like human lovers to each other. The photoreceptor response to the Moon triggers the largest spawning event on Earth. The Moon functions like a clock for corals, alerting them when to release sperm and eggs. The discovery is a big step forward for coral researchers and also sheds light on evolutionary questions. Corals emerged over 500 million years ago, which means we now know light receptors evolved much earlier in the development of animals than was previously thought.