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Sunset upon the reefs of Eilat, Red Sea.(Photo by: TOM SHLESINGER)
Breakdown in spawning synchrony places corals at ‘risk of extinction’
By EYTAN HALON
09/05/2019
Researchers at TAU found that highly synchronized spawning events of certain reef-building corals in the Red Sea’s Gulf of Eilat have completely changed and lost their vital synchrony.
Reef-building corals, which reproduce by broadcast spawning – simultaneously releasing eggs and sperm into open water in a mass synchronized event – could be at risk of extinction due to climate change and water pollution.

Researchers at Tel Aviv University found that highly synchronized spawning events of certain reef-building corals in the Red Sea’s Gulf of Eilat have completely changed and lost their vital synchrony, “dramatically reducing” their chances of successful fertilization.

Led by Prof. Yossi Loya and PhD candidate Tom Shlesinger of Tel Aviv University’s School of Zoology, the study discovered that the breakdown in coral spawning synchrony has led to a worrying lack of new coral recruitment and an increase in stagnant aging populations, which researchers say create circumstances for extinction.

The findings will also be published Friday on the cover of Science, a world-leading academic journal published by the American Association for the Advancement of Science.

“Coral spawning, often described as ‘the greatest orgy in the world,’ is one of the greatest examples of synchronized phenomena in nature,” said Loya. “Once a year, thousands of corals along hundreds of kilometers of a coral reef release their eggs and sperm simultaneously into the open water, where fertilization will later take place. Since both the eggs and the sperm of corals can persist only a few hours in the water, the timing of this event is critical.”

Successful fertilization of new corals, which can only occur within a narrow window of time, has required the evolution of a precise spawning synchrony. The synchronized timing relies on environmental cues, including sea temperature, solar irradiance, wind, the phase of the moon and the time of sunset.

Since 2015, researchers have monitored coral spawning in the gulf, recording the number of spawning individuals of each coral species. They performed 225 nighttime field surveys over four years, each lasting three to six hours during the annual coral reproductive season from June to September.

“We found that in some of the most abundant coral species, the spawning synchrony had become erratic, contrasting both the widely accepted paradigm of highly synchronous coral spawning and studies performed on the exact same reefs decades ago,” said Shlesinger.

To investigate whether the breakdown in spawning synchrony translated into reproductive failure, the researchers mapped thousands of corals within permanent reefs, and revisited the locations every year to track changes and compare coral deaths to recruits.

“Although it appeared that the overall state of the coral reefs at Eilat was quite good,” Shlesinger explained, “and every year we found many new corals recruiting to the reefs, for those species that are suffering from the breakdown in spawning synchrony there was a clear lack of recruitment of new juvenile generations, meaning that some species that currently appear to be abundant may actually be nearing extinction through reproductive failure.”

According to Loya, several possible mechanisms could be driving the breakdown in spawning synchrony. These include rising sea temperatures and disrupting pollutants resulting from human activities.

Temperatures have risen at a rate of 0.31°C per decade in the gulf region, Loya said, suggesting that the breakdown “may reflect a potential sub-lethal effect of ocean warming.” The increase in endocrine disrupting pollutants in marine environments, resulting from human activities, could be another plausible cause of the breakdown.

“Regardless of the exact cause leading to these declines in spawning synchrony, our findings serve as a timely wake-up call to start considering these subtler challenges to coral survival, which are very likely also impacting additional species in other regions,” said Shlesinger. “On a positive note, identifying early-warning signs of such reproductive mismatches will contribute to directing our future research and conservation efforts toward the very species that are at potential risk of decline, long before they even display any visible signs of stress or mortality.”
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