Red (Sea) alert for coral

Breakdown of synchronised spawning an unrecognised threat, Israeli researchers say.

In-situ spawning of some coral species. See details at foot of story.

Tel Aviv University

The spawning patterns of some reef-building corals in the Red Sea have completely changed over time, dramatically reducing chances of successful fertilisation, new research shows.

Writing in the journal Science, Yossi Loya and Tom Shlesinger from Israel’s University of Tel Aviv say that this desynchronisation – a previously unnoticed threat – has led to a dearth of new recruits and stagnant aging populations, creating circumstances for extinction.

And they warn that while other coral populations around the globe might appear healthy, they could also be suffering silently from these reproductive struggles.

Previous studies have suggested that the synchronicity of coral reproduction relies on various environmental cues working in unison to determine the exact moment of spawning. It’s certainly a precise process.

"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,” says Loya. “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."

Over four years from 2015, the two scientists completed 225 field surveys in the Gulf of Eilat/Aqaba area during the coral reproductive season from June to September. They spent three to six hours at sea each night, recording the number of spawning individuals of each coral species.

"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," says Shlesinger.

To investigate whether this breakdown 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; that is, to see how many corals of a given species had died compared with new juveniles recruited to the reef.

"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," says Shlesinger.

Loya notes that there are “several possible mechanisms” behind the breakdown in spawning synchrony, but points in particular to rapidly rising temperatures in the study regions and the likely impact of endocrine (hormonal) disrupting pollutants, which are accumulating in marine environments.

"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," adds Shlesinger.

Image details

In-situ spawning of some studied species. Scale bars indicate ~1 cm. (A) At ~30 minutes prior to spawning, Acropora eurystoma colonies are in a “setting” mode, in which egg-sperm bundles are already visible at the mouth-opening of each polyp. (B) Synchronised whole-colony spawning of Acropora eurystoma. (C) Partial spawning of Platygyra lamellina, without the formation of egg-sperm bundles, in contrast to (D), synchronised whole-colony spawning of egg-sperm bundles. (E) Acanthastrea echinate. (F) A male Galaxea fascicularis colony.

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