Ever since marine biologist Bette Willis discovered that dozens of species of corals on the Great Barrier Reef spawn to-gether at the same time each spring, she's wondered how they do it.

More than two decades after her discovery, an international group of biologists and geneticists have answered the question for her : ancient moonlight-sensitive molecules called cryptochromes.

The group knew that reef-building corals such as Acropora millepora spawn three or four nights after full moons in late spring. Since the team also knew that cryptochromes are involved in regulating the "body clocks" of many animals, from fruit flies to mice, they decided to look in A.millepora for genes known to control cryptochromes. They found two genes, cry1 and cry2, in the outer layer of the corals. Laboratory experiments showed they displayed daily rhythms under cycles of exposure to light and dark, but not to constant darkness.

According to Professor Hoegh-Guldberg, the coral clock works by combining general cues such as water temperature with detailed signals from the cryptochromes. The corals then know exactly when to launch eggs and sperm into the sea. And they do so en masse to help as many baby corals as possible to survive hungry predators.

"Cryptochromes look like the smoking gun, as the mechanism by which you tune yourself to the faint blue light of the moon" claimed Professor Hoegh- Guldberg.

He also said that as cryptochromes " entrained " the behavior and biology of corals- the simplest multi-cellular organisms on the planet - the basic cryptochrome system must have appeared hundreds of millions of years ago.

But now it has become a very sophisticated timing mechanism, essentially the Swiss clock for the animal kingdom

Unless you live in Cairns or are prepared to jump on a plane at a moments notice its going to be a TAD difficult to catch this phenomenum. Usually it occurrs late November to early December.

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