Sexy Corals Keep ‘Eye’ on Moon, Scientists Say
New York Times
October 19, 2007

Birds do it. Bees do it. Even lowly corals do it — but infrequently, forgoing sex for as long as a year.

Then, at night, just after the full moon, under warm tropic breezes, the corals dissolve in an orgy of reproduction, sowing waters with trillions of eggs and sperm that swirl and dance and merge to form new life. The frenzy can leave pink flotsam.

Scientists discovered the mysterious rite of procreation in 1981 and ever since have puzzled over its details. The moon clearly rules the synchronized mass spawning, which happens during different months in different parts of the globe, but usually in the summer. But how do corals monitor the moon’s phases and know when to start mingling?

Today, seven scientists from Australia, Israel and the United States report in the journal Science that corals have primitive photoreceptors, if not true eyes. In experiments, they found that the photosensitive chemicals respond to moonlight as admirably as, well, human lovers.

“This looks to be the smoking gun,” Ove Hoegh-Guldberg, a team member at the University of Queensland, said in an interview. “It triggers the largest spawning event on the planet.”

Margaret W. Miller, a coral specialist at the National Oceanic and Atmospheric Administration, called the finding by the group of scientists “a big step forward. It’s always been a mystery as to how these animals manage to synchronize themselves.”

In recent years, the undersea love-fests have become tourist attractions for divers in the Caribbean, in Australia on the Great Barrier Reef, and other coral havens. Al Giddings, a famous ocean photographer, made a PBS documentary that showed reefs around the Pacific islands of Palau exploding in blizzards of rising sperm and eggs.

Though the scientists involved say more work is needed to determine how the photoreceptor works, the finding is significant because it addresses the spawning’s main riddle, marine biologists say. “When I talk about thousands of reefs in the Caribbean releasing their spawn within minutes of each other during a specific phase of the moon, people marvel and ask, ‘How do they do it?’” said Alina M. Szmant, a coral expert at the University of North Carolina, Wilmington. “My answer is always, ‘It’s a mystery.’”

Now, she said, the discovery provides clues to the puzzle and opens up “a new direction to explore.” Biologists say the finding sheds light on hidden aspects not only of coral reproduction but of evolution, suggesting that light receptors arose surprisingly early in the development of animals. Corals emerged more than 500 million years ago, near the dawn of complex life. “Our discovery,” the scientists write in Science, suggests that the basic mechanisms for responding to light “were in place at the origins of multicellularity in animals.”

People have known about the moon’s romantic possibilities for a long time. Shakespeare in “A Midsummer Night’s Dream” relies on moonlight to set the mood. The 1987 movie “Moonstruck” features a love story centered on “La Bella Luna.”

Corals are actually colonies of individual organisms called polyps, which create the skeletal structure that binds them together. For centuries, scientists held that corals were primitive creatures with no brain or eyes that knew nothing of moonlight or other environmental nuances and reproduced mainly by brooding offspring and bringing forth live young.

In 1981, that dogma began to collapse when graduate students at James Cook University in Australia followed a trail of clues to a nighttime mass spawn on the Great Barrier Reef. Their discovery made the cover of Science in 1984, and a chapter of the 1998 book “The Enchanted Braid” (John Wiley), which called the startling find “a coup for a group of graduate students.”

Investigation showed that the swirling eggs and sperm would merge and float away, forming embryonic corals that would sink to the ocean floor and, if conditions were right, found new colonies. Scientists speculated that the moon’s phase was important in the ritual because it controlled the tides. But in some places the tides were low and in others high, and scientists now say the moon may simply act as a clock to choreograph sex among more than 100 species of corals.

The photoreceptor discovery is the brainchild of Oren Levy, a young Israeli scientist who traveled to Australia in 2004 to study in Dr. Hoegh-Guldberg’s laboratory at the University of Queensland. Dr. Levy was fascinated by a class of photoreceptors known as cryptochromes. Originally found in plants, they had been tracked to insects and mammals, too. Dr. Levy wondered if corals might possess them.

In an interview, he said one clue was that cryptochromes responded to blue light. That frequency can easily penetrate seawater, so much so that reef areas are sometimes known as “blue deserts.” Dr. Levy and his colleagues studied Acropora millepora, a coral that can grow a foot across or wider and contain thousands of the individual polyps. They found two kinds of cryptochrome arrayed on the coral’s outer edges, one of which responded to the light and dark phases of the moon.

“This is the start of the story, not the end,” Dr. Levy said in an interview. “It will take five more years at least” to uncover the light sensor’s full story. The paper’s other authors work at Stanford University, Tel Aviv University, the Australian National University and James Cook University.

Peter D. Vize, a coral specialist at the University of Calgary, called the team’s work “big in implications.” The discovery, he added, “is opening up a whole pile of questions.”