It has been described by scientists as “the greatest orgy in the world”; an annual gamete-fest, where entire colonies of coral reefs release their sperm and eggs simultaneously in a slick on the ocean surface that has been seen from space. But now scientists fear the climate crisis may be disrupting the ability of corals to synchronise this marine phenomenon, threatening them with extinction.

A Tel Aviv university study, published in Science, has found the release of eggs and sperm in certain reef-building corals in the Gulf of Eilat in the Red Sea have changed over time and have lost their synchronicity. For a coral, reliant on a chance encounter, timing is everything. But researchers have found some are spawning “out of tune” with normal patterns, with the result that fewer baby corals are forming.

“Coral spawning … is one of the greatest examples of synchronised phenomena in nature,” said Professor Yossi Loya, of the university’s School of Zoology. “Once a year, thousands of corals along hundreds of kilometres of a coral reef release their eggs and sperm simultaneously into the open water, where fertilisation 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.”

Although visually, the coral reefs seemed healthy, the corals suffering a breakdown in spawning synchrony produced fewer baby corals, creating circumstances for extinction. While it is not yet fully understood exactly how spawning synchrony works, scientists say it relies on environmental cues, including sea temperature, solar irradiance, wind, the phase of the moon and the time of sunset.

The researchers began monitoring corals in the Gulf of Eilat in 2015 and, over four years, recorded the number of spawning individuals during the annual coral reproductive season, between June and September. They compared spawning timings of five coral species between 2015 and 2018 to results from two other studies conducted on the same species in the 1980s.

“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 Tom Shlesinger, a PhD student and co-author of the study.

To find out whether this failure in spawning synchrony translated into reproductive failure, they mapped thousands of corals within permanent reef plots, revisiting the plots every year to see how many corals of a given species had died, compared to new juveniles.

“Although it appeared that the overall state of the coral reefs at Eilat was quite good 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,” said Shlesinger.

It is not yet known what was causing the loss of synchrony. However, the researchers found temperature had a “strong influence” on coral reproductive cycles. In the study region, temperatures are rising “fast”, at rate of 0.31C a decade. Another plausible cause could be endocrine or hormonal disrupting pollutants, accumulating in marine environments, they said. The study was based on five corals; however, the researchers believe their findings could be a warning to other species.

“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.”