Climate change is knocking coral mating dance out of sync, Israeli study finds

In groundbreaking research, Tel Aviv University academics discover changes to mass spawning events that may lead to extinction of some species of reef builders

Sue Surkes is The Times of Israel's environment reporter

A branching coral spawns pinkish bundles of eggs and sperm that are carried away by the currents. (Tom Shlesinger/Courtesy)
A branching coral spawns pinkish bundles of eggs and sperm that are carried away by the currents. (Tom Shlesinger/Courtesy)

Species of reef-building corals off the coast of Eilat and Aqaba may be facing extinction due to changes in the environment where they reproduce, according to two Tel Aviv University researchers.

According to the scientists, Prof. Yossi Loya and PhD candidate Tom Shlesinger of Tel Aviv University’s School of Zoology, ocean warming and pollution may be changing the extraordinary way in which corals synchronize the release of eggs and sperm, leading to declining growth rates.

According to the scientists, whose groundbreaking study appeared in Science Thursday, fears are that the potentially catastrophic phenomenon could spread.

The study focused on coral spawning events, which take place annually between June and September. Most of the spawning corals are hermaphrodite — they have both male and female reproductive organs. At a specific hour, thousands of corals along hundreds of kilometers of a coral reef will spout out millions of bundles, each containing eggs and sperm. The bundles float to the ocean surface, where they are carried on the waves and break down to release the male and female cells. Fertilization takes place in the water column, creating coral larvae which develop into juvenile corals.

Timing is everything. Because the window of opportunity for fertilization is just a few hours, the corals spawn together in a precisely synchronized dance that increases the chances of male and female cells from different parents to meet one another. The synchronization relies on environmental cues such as sea temperature, solar irradiance, wind, the phase of the moon and the time of sunset.

Critical mass is also essential. The more cells that are released together, the higher the probability that the cells will find “mates.”

In 2015, Loya and Shlesinger initiated a long-term monitoring of coral spawning in the Gulf of Eilat and Aqaba in neighboring Jordan. Over four years, they performed 225 night field surveys, lasting three to six hours each, during the annual coral reproductive season, recording the number of spawning individuals of each coral species.

They found that spawning synchronization had become erratic among some common species.

There were differences both within and between coral colonies, Shlesinger told the Times of Israel. Where there were problems, corals would spawn in dribs and drabs lasting anything up to two months, rather than in a single burst.

The researchers then investigated whether this breakdown in spawning synchronization translated into reproductive failure. They mapped thousands of corals within permanent reef plots, then revisited these plots every year to examine and track changes in the coral community looking, for example, at how many corals of a given species had died compared with new juveniles recruited to the reef.

What they found was that while the overall state of the reefs was good, with many new corals joining, the specific species where the spawning synchronization had gone awry were seeing more deaths than reproduction.

Sunset on the reefs of Eilat, Red Sea. (Tom Shlesinger)

“The reef itself is not threatened,” Shlesinger said. “But some species that currently appear to be abundant may be lost in a few decades because of their failure to reproduce. And the fear is that erratic synchronization will start to occur in other species as well.”

The affected species, the researchers report, fall into the three genera: brain corals, staghorns (also known as branching corals) and Galaxea (for which there is no common English term).

A spawning brain coral. (Tom Shlesinger)

“Several possible mechanisms may be driving the breakdown in spawning synchrony that we found,” Loya concluded. “For example, temperature has a strong influence on coral reproductive cycles. In our study region, temperatures are rising fast, at a rate of 0.31 degrees Celsius per decade, and we suggest that the breakdown in spawning synchrony reported here may reflect a potential sub-lethal effect of ocean warming. Another plausible mechanism may be related to endocrine [hormonal] disrupting pollutants, which are accumulating in marine environments as a result of ongoing human activities that involve pollution.”

Shlesinger said the next step was to pinpoint the causes of the breakdown in synchronization.

“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.”

Corals are invertebrate animals related to jellyfish and sea anemones. Each individual coral animal, called a polyp, has a stomach with a single mouth opening surrounded by stinging tentacles. Coral polyps live in groups of hundreds to thousands of genetically identical individuals that form a colony.

A species of Galaxea spawning. (Tom Shlesinger)

Around 1,000 reef-building corals have been recorded worldwide, with around 150 of them living off the coast of Eilat and Aqaba.

In a remarkable example of symbiosis that has been going on for millions of years, most corals (which, as animals, cannot produce their own food) team up with algae (which can, through photosynthesis). The algae receive protective board and lodging within the coral’s tissue and use the coral’s waste products for photosynthesis. The coral benefits from the sugars, oxygen and waste removal services supplied by the algae.

When the seawater warms too much, however, corals will expel their algae lodgers and turn white in an event called bleaching. Bleached corals can continue to live but will eventually starve without their algae partners. In 2016, bleaching on Australia’s Great Barrier Reef killed between 29 and 50 percent of the reef’s coral. UNESCO has said that “climate change remains the most significant overall threat to the future” of the iconic 2,300-kilometer (1,400-mile) reef.

Bleached coral (YouTube/National Geographic screenshot)

So far, bleaching has not affected the reefs off Eilat and Aqaba. For evolutionary reasons, local corals demonstrate resilience to water temperatures that are several degrees higher than those in places where bleaching is taking place.

Among the most diverse and productive ecosystems on the planet, coral reefs provide a framework and home for hundreds of thousands, if not millions, of other species. They are the largest living structure on the planet, and the only one that can be seen from space.

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