Corals need to spawn within minutes of each other to reproduce optimally, but some are nowadays and even months out of sync. Trying to understand the reasons for this phenomenon, researchers are now pointing the finger at climate change.

Coral reefs release millions of tiny egg and sperm bundles simultaneously every year — but there’s a catch: they need to do it altogether to skip predators and stop sex cells from becoming too diluted in the water. Missing the window by minutes leads to reduced fertilization and missing it by a few hours or days can mean individual corals fail to fertilize at all.

While the link between global warming and coral bleaching and reef mortality is well-established, that’s not the case with its effect on coral spawning synchronization. We still don’t know how this process is affected by global heating, and Tom Shlesinger and Yossi Loya at Tel Aviv University in Israel decided to study the phenomenon.

The researchers, whose work was published in the journal American Association for the Advancement of Science, compared spawning behavior 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. They meticulously tracked signs of fertility among five of the most abundant coral species.

In order to develop, corals rely on many factors of the environment. 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.

“Coral spawning 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.”

The main breeding season of corals used to happen from June to September back in the 1980s, 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 from becoming hybrids. However, 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.

Now, each year the coral species would spawn over the space of several weeks, with different colonies spawning on different nights, instead of a synchronized mass spawning event. 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 fertilization 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 all likely to blame, the authors said.

Coral reefs harbor the highest biodiversity of any ecosystem globally and directly support over 500 million people worldwide, mostly in poor countries. Over the last three years, reefs around the world have suffered from mass coral bleaching events as a result of the increase in global surface temperature caused by anthropogenic greenhouse gas emissions

According to UNESCO, the coral reefs in all 29 reef-containing World Heritage sites would cease to exist by the end of this century if we continue to emit greenhouse gases under a business-as-usual scenario.