Corals are no longer spawning in sync Tom Shlesinger

Corals need to spawn within minutes of each other to reproduce optimally, but some are now days and even months out of whack. Climate change is to blame for this previously unknown threat, researchers say.

Each year, whole coral reefs release millions of tiny egg and sperm bundles simultaneously – turning the ocean into a bright underwater blizzard that can sometimes be seen from space.

They need to spawn en mass to outfox predators and stop the sex cells from becoming too diluted in the water. Missing the window by minutes leads to reduced fertilisation and missing it by a few hours or days can mean individual corals fail to fertilise at all, according to Tom Shlesinger and Yossi Loya at Tel Aviv University in Israel.


Links between global warming and coral bleaching and reef mortality are well-established, but its effect on coral spawning synchronisation is not as well studied.

Spawning behaviour

To examine this impact, Shlesinger and Loya compared spawning behaviour on a reef in the Red Sea over recent years with historical data from the 1980s.

Between 2015 and 2018, the two researchers conducted 225 night-time surveys, each lasting between 2.5 and 5.5 hours. They meticulously tracked signs of fertility among five of the most abundant coral species.

Coral spawning relies on environmental cues: temperature and daylight patterns can help them work out when to get ready to spawn, while the exact night spawning occurs is thought to be triggered by lunar cycles and the exact hour is cued by the sunset.

Back in the 1980s, the major breeding season occurred from June to September, with one coral species having a slightly different breeding season to another based on the lunar cycle. This is thought to help prevent nearby corals becoming hybrids.

Out of sync

However, the researchers found that, in recent years, three out of the five species studied had lost their tightly aligned spawning windows and had no consistent pattern relative to the phase of the moon, sea temperature or wind speed.

Instead of a synchronised mass spawning event, each year the coral species would spawn over the space of several weeks, with different colonies spawning on different nights. One species, Acropora eurystoma, spawned near a full moon one year, and near a new moon the next.

This appears to have reduced the reproductive success of the coral populations, as rates of fertilisation also declined over the years.

Climate change, thermal stress, light pollution and an influx of hormones such as testosterone and progesterone in the water are most likely to blame, the authors wrote.

Corals are under increasing stress, and without the genetic diversity provided by sexual reproduction, it will be difficult to create new, stress-tolerate genotypes that can better cope with climate change, Nichole Fogarty and Kristin Marhaver wrote in an accompanying editorial.

“We could easily overlook the possibility that species that appear to be abundant may actually be nearing extinction through reproductive failure,” Shlesinger and Loya wrote

Journal reference: Science, DOI: 10.1126/science.aax0110