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.”
Coral organisms, called polyps, can live on their own, but are primarily associated with the spectacularly diverse limestone communities, or reefs, they construct.Coral polyps are tiny, soft-bodied organisms related to sea anemones and jellyfish. At their base is a hard, protective limestone skeleton called a calicle, which forms the structure of coral reefs. Reefs begin when a polyp attaches itself to a rock on the sea floor, then divides, or buds, into thousands of clones. The polyp calicles connect to one another, creating a colony that acts as a single organism. As colonies grow over hundreds and thousands of years, they join with other colonies and become reefs. Some of the coral reefs on the planet today began growing over 50 million years ago.
Coral polyps are actually translucent animals. Reefs get their wild hues from the billions of colorful zooxanthellae (ZOH-oh-ZAN-thell-ee) algae they host. When stressed by such things as temperature change or pollution, corals will evict their boarders, causing coral bleaching that can kill the colony if the stress is not mitigated.
Corals live in tropical waters throughout the world, generally close to the surface where the sun’s rays can reach the algae. While corals get most of their nutrients from the byproducts of the algae’s photosynthesis, they also have barbed, venomous tentacles they can stick out, usually at night, to grab zooplankton and even small fish.
Coral reefs teem with life, covering less than one percent of the ocean floor, but supporting about 25 percent of all marine creatures. However, threats to their existence abound, and scientists estimate that human factors—such as pollution, global warming, and sedimentation—could kill 30 percent of the existing reefs in the next 30 years.
An ancient light-sensitive gene has been isolated by researchers from the ARC Centre of Excellence for Coral Reef Studies (CoECRS) that appears to act as a trigger for the annual mass spawning of corals across a third of a million square kilometres of Australia’s Great Barrier Reef, shortly after a full moon. The genes, known as a cryptochromes, occur in corals, insects, fish and mammals – including humans – and are primitive light-sensing pigment mechanisms which predate the evolution of eyes.
One week each year in spring, after a full moon, millions of corals release eggs and sperm. 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. 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.
Cry2 prompts a series of biochemical reactions that is surprisingly sophisticated.Some 400 or 500 species of corals all spawn simultaneously during the week, creating vast slicks across the ocean.The new research also offers insights into the development of vision and the evolution of daily rhythms in animals. 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. 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 the light-sensing properties of the photolyases. Cryptochromes set the clock to do the right things at the right time.The first creatures wouldn’t have had eyes.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.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.
Corals 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.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.
3.Some photographs of corals:
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