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Moonlight Triggers Mass Coral "Romance"

Dave Hansford
for National Geographic News
October 22, 2007
 
Australian and Israeli scientists have discovered the trigger for the planet's biggest group sex spectacle: the mass spawning of hard corals along Australia's Great Barrier Reef.

One week each year in spring, after a full moon, millions of corals release eggs and sperm in what Bill Leggat, a co-author of the new study, called "a slow symphony."

But until now how the primitive animals—which lack brains or eyes—synchronized the mass spawning was a mystery.

In today's issue of the journal Science, researchers reveal that they have isolated an ancient gene in the corals' DNA that can detect moonlight.

By exposing corals to different colors and intensities of light, the team found that the gene—known as Cry2—was most active in Acropora corals during a full moon.

Leggat, a lecturer at James Cook University in Cairns, Australia, said Cry2 encodes a type of protein known as a cryptochrome, which appears to trigger the corals' reproductive cycle.

"This particular gene allows the coral to sense blue light and to actually work out what phase the moon is in," he added.

The research also suggests that the basic ability to sense changes in light and adapt a 24-hour cycle appeared early in the evolution of animals.

Sophisticated Spawning

Cry2 prompts a series of biochemical reactions that is surprisingly sophisticated, Leggat said.

Some 400 or 500 species of corals all spawn simultaneously during the week, creating vast slicks across the ocean, he pointed out.

"It's just magical," Leggat said. "To just sit in front of an individual coral and watch the pink sperm bundles get slowly pushed out of the corals' mouth and float away—it's incredible to watch."

"It's one of the greatest sights in nature, but the amazing thing is that, after going on for millions of years, it wasn't witnessed until the 1980s," he added.

How the right sperm ends up with the right egg is a complicated process that may rely on refined chemical pathways, he said. But scientists are still working on unraveling the exact details.

"To me, the really exciting thing is this huge, well-orchestrated symphony [is] going on, yet we still don't know how it works," Leggat said.

"We're only really just starting to understand corals and reefs in general, and something that's both exciting and worrying is that these reefs are threatened, that they may not be around in 50 years."

(Related: "Coral Reefs Vanishing Faster Than Rain Forests" [August 7, 2007].)

Night and Day

The new research also offers insights into the development of vision and the evolution of daily rhythms in animals.

Ove Hoegh-Guldberg, director of marine science at the University of Queensland, said cryptochromes are closely linked to primitive proteins known as photolyases—which harness blue light to repair DNA damaged by ultraviolet radiation.

"[In the Precambrian era] there were very high doses of ultraviolet reaching the planet surface, so organisms probably had to retreat out of range of the UV" in addition, said Hoegh-Guldberg, who was also a co-author on the new research.

"One way to do that would be to go into the deeper layers of the ocean during the day and to rise during the night as levels dropped" by adapting the light-sensing properties of the photolyases, he added.

From there, it was "a very simple step to evolve cryptochromes to set your clock to do the right things at the right time," he said.

"The first creatures wouldn't have had eyes," Hoegh-Guldberg continued. "They would have been depending on cellular biochemistry to detect changes in light. So cryptochromes are, in a sense, the functional forerunners of eyes."

Cryptochromes are still present in humans and other mammals, as well as insects, he said. They play an important role in regulating the circadian system, a "body clock" attuned to Earth's 24-hour rhythms that regulates things like cycles of metabolism and alertness.

"[These proteins] are the Swiss timing mechanism of biology," Hoegh-Guldberg said.

Surprising Complexity

In addition, the work shows the surprising level of sophistication of even the earliest animals.

"We think of corals as being very simple, but they're not," Leggat, of James Cook University, said. "They're actually incredibly complex—they have almost the same number of genes and proteins as humans.

"Many of these genes developed in deep time, in the earliest phases of organized life on the planet," he added. "They were preserved for hundreds of millions of years before being inherited by corals when they developed about 240 million years ago, and are still found today in modern animals and humans."

They are an indicator that corals and humans are in fact distant relatives, sharing a common ancestor way back."

Hoegh-Guldberg said the team would head back to the reef this spring to delve deeper into the secrets of cryptochromes.

"We've got all the smoking guns of this mechanism," he said. "The next step is to track down the way they drive things like reproduction. We fully expect to uncover other behavior that they are controlling."

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