Concern around this year’s Reef coral spawning

One of the people who discovered “coral spawning” and changed our understanding of reef life, is worried that global warming might increase the temperature of the ocean so much that this year’s spawning might be impacted.

Described as an “oceanic orgy”, “one of Earth’s great biological spectacles” and “a symbol of regeneration and hope”, coral spawning occurs when multiple coral species synchronise the release of their eggs and sperm to make coral babies.

No-one’s exactly sure when it will happen, beyond saying that on the Great Barrier Reef (GBR), it’s usually after the full moons in the months of October to December.

But with 2023 being the hottest year on record, surpassing the previous record set in 2016, reef scientists are watching closely to see if rising ocean temperatures will affect the timing of the coral spawning.

Bureau of Meteorology Oceanographer Dr Grant Smith says Australia experienced its warmest winter on record in terms of mean temperature since 1910, while the GBR’s sea surface temperatures have warmed between 0.56 and 0.84 degrees since 1950.

“Sea surface temperature forecasts for summer (2023/2024) in the Great Barrier Reef are likely to be warmer than average,” Smith says.

Sea surface temperature forecasts for summer (2023/2024) in the Great Barrier Reef are likely to be warmer than average.

Dr Grant Smith

CSIRO Senior Research Scientist Dr Christopher Doropoulos has been working on coral spawning related ecology since 2009.

“As a general rule of thumb, development happens faster in warmer conditions,” Doropoulos says.

But beyond sea surface temperatures, there are other environmental drivers of coral spawning, including lunar phase, and the daily light cycle, with time after sunset another important cue.

He says that while it’s theoretically possible that corals could spawn earlier than usual, there’s a surprising dearth of scientific literature exploring this possibility.

“It’s a bit hard to say exactly whether we’ll have an earlier spawning or not,” he says.

“There are some shorter-term experiments, and they might suggest that it might happen, but it’s usually just a few days rather than say the month before.”

A thornier problem is the prospect of coral bleaching, which is already happening in the northern hemisphere.

Last month, scientists from the National Oceanic and Atmospheric Administration (NOAA) provided a briefing that revealed a large-scale and coral bleaching event was underway, impacting 2 ocean basins and multiple countries.

The picture doesn’t bode well for the GBR this summer, according to Professor Peter Harrison, a coral ecologist and Director of the Marine Ecology Research Centre at Southern Cross University.

“We’re all really anxious about it,” he says.

Harrison was part of the team that first discovered mass coral spawning on the GBR in 1981.

Prior to this, it was believed that coral reproduction involved brooding of tiny coral larvae within the polyps after sperm from neighbouring colonies was released and drawn into the polyps of other colonies to fertilise their eggs.

It was supposed that the embryos developed in the polyps over many weeks, were released back into the water at an advanced stage of development and settled soon after release.

“That led to the incorrect idea that the coral larvae were available year-round, and didn’t disperse very far,” he explains.

Back then, reefs were also thought of as separate entities, with the Great Barrier Reef conceived of as a series of isolated reefs, rather than a highly interconnected living super-structure that’s visible from space.

“The discovery of the mass coral spawning showed … that the vast number of coral species did things very differently and instead of brooding larvae they spawned eggs and sperm for external fertilisation and development of planktonic larvae,” he says.

While a few species are known to brood larvae or spawn twice a year, Harrison says that more than 90% of GBR corals studied to date are “broadcast” spawning coral.

Most are hermaphrodites, and produce both eggs and sperm in their polyps and release them in bundles that float to the surface, creating “slicks” that can be kilometres long.

“They contain literally billions or trillions of eggs and sperm,” he says.

Harrison and colleagues first wrote about of coral spawning in Science in 1984.

But the jubilation that accompanied the 1981 discovery was soon shaken when Harrison visited corals he’d tagged on the shallow fringing reefs of Magnetic Island, offshore from Townsville, just months later.

… half of them had turned white and eventually many of them died.

Professor Peter Harrison

“Just after Christmas, I went back to my research sites where I had hundreds of corals that I’d carefully tagged so I could monitor when they were spawning, and half of them had turned white and eventually many of them died,” he says.

He’d never seen such a thing. Conversations with colleagues confirmed that this was part of a significant coral bleaching event which happened at the start of 1982.

Coral bleaching occurs when tiny coral polyps, stressed by spikes in sea temperatures, expel the microscopic photosynthetic algae called zooxanthellae which otherwise live symbiotically within their tissues and give reefs their colour.

“That’s lived with me ever since, because we had this elated feeling about discovering this new coral spawning event and then a few months later, half the corals bleached,” he says.

Harrison says the most recent events – such as the back-to-back mass bleaching which occurred in 2016/2017 and severe bleaching in 2020 which collectively affected two-thirds of the GBR – were “at scales way beyond those early bleaching events”.

“Unfortunately, none of this is surprising,” he adds.

“All of the early climate models were pointing to increasing bleaching, increasing severity, increasing frequency, and increasing catastrophic outcomes as the base level of the sea temperatures continues to rise, and then you build in El Niño peaks on top of that.”

While corals construct huge limestone skeletons, this highlights their fragility, Harrison says.

In response to the 1982 mass bleaching, Harrison conceived the idea of culturing and using millions of coral larvae to restore damaged areas of reef.

However, at that time, the GBR was seen as so large as to be virtually indestructible, so funding for restoration research was not available.

More recently, working with his then PhD student Dexter dela Cruz, Harrison pioneered the process of larval restoration.

This involves harvesting coral eggs and sperm during mass spawning events, growing the larvae in floating enclosures on the reef or in culture tanks, and releasing them onto dead coral areas to boost the rate of recruitment of juvenile corals.

“We’ve now done lots of reef trials and shown that supplying millions of coral larvae can rapidly increase settlement and recruitment leading to restored breeding populations within a few years even on a highly degraded reef system,” he says.

Early work focussed on reefs in the Philippines due to the greater availability of research funding.

But as the tide turned following the mass bleaching in 2016/17, Harrison’s team conducted research at Heron Island on the Southern GBR which demonstrated that the higher numbers of larvae placed on reef patches that you’re trying to restore, the higher number of successful colonies grow and survive.

We can’t delay restoring degraded reefs in the hope that global emissions will be controlled quickly enough to protect vulnerable corals and reefs in the next couple of decades.

Professor Peter Harrison

A couple of years ago, these so-called IVF coral babies came of reproductive age.

“In the last 10 years, that’s pretty much all I’ve been focused on – looking at how to optimise these methods and then starting to employ them in different regions,” Harrison explains.

Corals with higher heat tolerance are more likely to survive bleaching events and can be used as broodstock to create the next generations of restored coral.

“This can buy more time for reefs to recover and protect remaining genetic diversity among surviving corals, while the world decreases reliance on energy production from fossil fuels and starts to decrease greenhouse gas emissions,” Harrison says.

“We can’t delay restoring degraded reefs in the hope that global emissions will be controlled quickly enough to protect vulnerable corals and reefs in the next couple of decades.

“So we need to intervene now to restore and protect the most important source reef areas that will supply coral larvae to other reefs downstream in future and enable larger scales of recovery.”

Corals that are better adapted to higher heat tolerance may survive bleaching events.

However, bleaching survivors may be sterile during a subsequent breeding event, because the development of eggs was cued to start during the period when they were bleached.

“They had no capacity to put any energy or resources into growing eggs or sperm because they were in survival mode to keep their tissues alive,” Harrison says.