Great Barrier Reef: Coral Spawning Climax

If you happen to be in north Queensland, Australia around the November full moon, look out for the biggest mass sexual event of the year. The Great Barrier Reef corals are mating . . . by James Teo.

Cairns, Queensland, 28 November 2018. As if being instructed by the queen of fertility, hard and soft corals on Moore Reef off the coast of Cairns have begun releasing eggs and sperm as part of what marine biologists say is one of the biggest mass coral spawnings on the Great Barrier Reef (GBR).

 Prof Peter Harrison is committed to regenerating healthy Great Barrier Reef coral growth. Courtesy SCU.

Prof Peter Harrison is committed to regenerating healthy Great Barrier Reef coral growth. Courtesy SCU.

Veteran coral chasers Stuart Ireland, Gareth Phillips and Pablo Cogollos base themselves at the Moore Reef pontoon, 47 km off the coast of Cairns in readiness for the unique underwater event, predicted to be over several days of the full moon in late November-early December.

Mass mating game

What happens is that mature corals already growing on the reef release all their eggs and sperm (their gametes) at the same time. The timing is crucial, because the gametes are viable (capable of mixing) for only a few hours. As a result, a blizzard of billions and billions of coral gamete flakes and blobs swirl into the sea at the same time, making it more likely that fertilisation will occur.

 Millions of corals release eggs and sperm at the same time - and nobody quite understands how. Courtesy GBR.

Millions of corals release eggs and sperm at the same time - and nobody quite understands how. Courtesy GBR.

Once eggs and sperm have been released, the gametes are swept away by the ocean currents to new spawning grounds, where the fertilisation can be more useful to the establishment on new colonies. Because the gametes are made up partly of fatty substances called lipids, they float to the surface when the coral eggs and sperm can join together to form an embryo, then a coral larva called a planula.

At this stage, they are a tiny would-be coral and float about for a few days until they drop down to the seafloor and if they are lucky, attach themselves to the existing structures to grow into a new coral colony. That will take years, but the sheer numbers of eggs and sperm mean the coral reefs mostly keep growing. Unless climate change decrees otherwise, that is.

Although scientists still don’t understand exactly how, the timing is so accurately synchronised, it works. First discovered on the Great Barrier Reef in the 1980’s by Southern Cross University’s Prof. Peter Harrison and colleagues, mass spawning has changed scientific understanding of how and when corals reproduce.

Reseeding

Ireland, Phillips and Cogollos will be on hand to film a group of coral scientists who will undertake the most ambitious coral reseeding project ever undertaken.

Led by Prof. Harrison, the team will harvest millions of coral eggs and sperm to grow new coral larvae which will be released back onto damaged reefs in the Vlasoff and Arlington Reef area off Cairns.

“This is the first time that the entire process of large-scale larval rearing and settlement will be undertaken directly on reefs on the Great Barrier Reef,” said Prof. Harrison. “Our team will be restoring hundreds of square meters with the goal of getting to square kilometres in the future, a scale not attempted previously. It could be described as IVF for the reef.” Harrison believes that a successful spawning will prove the GBR is resilient and has potential to recover from the back-to-back bleaching events of 2016 and 2017.

This year marks the 22nd time that photographer Ireland has captured the GBR reefs as they erupt into a new cycle of life. “The main event will be when Acropora, Montipora and other stony corals will mass spawn, ejecting tiny pink and mauve balls of either eggs or bundled egg and sperm masses as well as cloud-like white slicks known as “smokers” representing individual male colonies,” he said. “It’s like being in an upside-down snow storm.”

Hopefully, all those billions and billions of eggs and sperm will successfully mix, with a few millions or so caught in nets set out by a team of James Cook University researchers.

Nurseries

 The Larval Restoration Project (LRP) sets up giant nets to catch millions of potential new coral larvae for planting on nearby depleted reefs. Courtesy LRP.

The Larval Restoration Project (LRP) sets up giant nets to catch millions of potential new coral larvae for planting on nearby depleted reefs. Courtesy LRP.

“Once we have caught them in big sieve-like nets, they will clump together and fertilise to form larvae, over about 7 hours,“ said lead researcher Katie Chartrand. “They grow over the next 5-7 days in our massive coral nurseries, then when they are mature we can take them and put them back on the reef where they can find somewhere to settle.”

Chartrand said many of the larvae would be settled on the more degraded reefs areas that need repopulating by other volunteers from the Larval Restoration Project (LRP). The LRP is a collaboration between researchers Peter Harrison (Southern Cross University), James Cook University, David Suggett (University of Technology Sydney), the Great Barrier Reef Marine Park Authority, Queensland Parks & Wildlife Service, and other tourism and industry partners.

If you miss this year’s spawning, look out for it next year. It takes place every year over two to six nights after the October / November full moon. The corals get itchy and ready to spawn when there is a very small variation between low and high tides, and when temperature are around 27-28C. Keep your eye on those tide tables and weather reports.