The Cry2 gene, stimulated by the faint blue light of the full moon, appears to play a central role in triggering the mass synchronised coral spawning, said the scientists in a paper published in the international journal Science on Friday.

"This is the key to one of the central mysteries of coral reefs," said Professor Ove Hoegh-Guldberg, who lead the University of Queensland laboratory which discovered the genes.

"We have always wondered how corals without eyes can detect moonlight and get the precise hour of the right couple of days each year to spawn," Hoegh-Guldberg said in a statement.

The annual mass spawning of corals occurs across a third of a million square kilometres of Australia's Great Barrier Reef, shortly after a full moon.

Exposing corals to different colours and intensities of light and sampling live corals on reefs around the time of the full moon, Israeli researcher Oren Levy found the Cry2 gene at its most active in Acropora corals during full moon nights.

The genes developed in primitive life forms in the Precambrian, more than 500 million years ago, as a way of sensing light to synchronise their body clocks and breeding cycles, said the researchers.

"They are, in a sense, the functional forerunners of eyes," said Hoegh-Guldberg.

Cryptochromes still tune humans to the rhythms of the planet, he said, but had lost their light-sensing function.