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Coral spawning on the Great Barrier Reef. (Credit: Tusa Dive)

Coral spawning: a rare natural wonder

  • BY Kara Murphy |
  • December 15, 2011

To catch the natural phenomenon of coral spawning, you need patience and just a little bit of luck.

EVERY YEAR, DIVERS AND snorkelers point their torches into the darkened waters of the Great Barrier Reef and Western Australia's reefs, hoping to witness the annual 'mass' coral spawning event - where many colonies and species of coral polyps simultaneously release egg and sperm bundles for external fertilisation. 

The spectacle resembles an underwater snowstorm, with a flurry of buoyant gametes, usually pink or white, slowly drifting upwards in a real-life version of a shaken snow dome.

Like heavy snowstorms, large-scale spawning is a mass of confusion and low visibility.

"If lots of coral colonies are spawning, trying to find which coral is releasing eggs becomes too difficult," says Stuart Ireland, a marine biologist and underwater photographer who has witnessed the event 12 times in the last 16 years. "But you can feel it. You run your fingers through (the water) and feel the eggs as they're going up. It's a little like if you're driving slowly in the rain, and you put your head out the window and feel the little droplets on your face...it's a strange feel."

The largest event Stuart witnessed was in 1996, and he says one difference between that "intense" event and other's he's seen is "how much you smell of coral spawn" when you leave the water.

"The smell isn't a bad one," says Stuart, explaining it's similar to the scent of coral mucus when the corals come out at low tide - just more pungent. "After the intense event, we actually had to use our regulators to blow holes through the (coral spawn) slick, it was so thick," Stuart recalls.

Witnessing the coral spawning is a worthy addition to anyone's bucket list - even if your hair does smell of coral spawn afterwards.

Coral spawning difficult to predict

While there are a few indicators for the initiation of these synchronised spawning events, predicting it is a tricky game. "You have to be in the right place at the right time," says Stuart.

What is known is that the phenomenon only happens at night, after rising water temperatures have stimulated the maturation of gametes (the egg and sperm) within the polyps. And they usually occur over several days to just over a week after a full moon. It's thought corals may be able to sense using primitive photoreceptors, although the time of year depends on location.

Thus far, the largest synchronised spawning events recorded have occurred on Queensland's Great Barrier Reef and in Western Australia's reefs. Large-scale spawning events on Western Australia's reefs tend to occur in autumn, with smaller but significant events also recorded in spring, while large events on the Great Barrier Reef appear to take place only in late spring or early summer.

These factors can help experts identify probable spawning dates, and local operators schedule coral spawning trips based on expert advice. However, because the exact timing is still a bit of a mystery, operators can't guarantee their clients will see the spawning.


A close-up view of coral spawning. (Credit: Tusa Dive)

Coral spawning clues arrive late

Closely examining the coral during daylight hours on these dates doesn't help experts, operators, or hopeful individuals pinpoint spawning nights either. This is because the coral doesn't reveal definitive visible clues until after sunset, when the spawning is imminent.

"Usually about a half an hour to a couple hours before spawning, you can start to see the egg or sperm bundles starting to be formed underneath the polyp, and the tentacles tend to contract," says Dr Peter Harrison, director of marine studies at Southern Cross University and a member of the research team at James Cook University who discovered the mass coral spawning phenomenon on the Great Barrier Reef in 1981.

"There's no indication that 'tonight's going to be the mass spawning night' until you're actually underwater, in the dark, and start checking the corals," he says.

Stuart agrees, saying he tends to see signs 20-30 minutes prior to spawning. "As the egg gets closer and closer to the mouth (of the polyp) then you can start to see more and more eggs. It's a bit like a pimple developing, I suppose. You start to see it getting really close to the surface and then they sort of start popping everywhere."

All corals in a reef don't spawn the same night

On 17 November 2011, seven nights after the full moon, divers travelling with Cairns-based operator Tusa Dive got lucky and observed about 35 minutes of spawning at Saxon Reef, one of 2900 on the Great Barrier Reef.

Stuart was among the divers; he saw spawning from five or six branching Acropora species, small plate corals (also Acropora), and boulder corals (a Porites species). 

"It was spectacular," Stuart raves, "probably the best spawning I've seen in five years."

Fellow dive operator Quicksilver's clients were fortunate also, witnessing spawning on Norman Reef. But while large-scale spawning took place on these two outer reefs that night, it didn't happen across the entire reef system.

Green Island, on the inner reef near Cairns, had a spawning event on November 16. And on Lady Elliot Island, a coral cay on the reef's southernmost tip where spawning tends to occur over a few nights, the action didn't begin until two days after the December full moon.

"That's hard to get across to people, that mass spawning doesn't occur on just one night each year throughout the whole Great Barrier Reef," Peter tells Australian Geographic.

"The periods after the full moon in October and November for the inshore reefs, and November and December for the offshore and northern and southern reefs have windows of opportunity where corals tend to synchronously spawn, but not all species spawn within those periods," he says.

Even in areas where researchers have studied coral spawning "it doesn't absolutely run to clockwork," Peter says. "In some years, changes in the environment that we don't yet understand sometimes cause some groups of corals to spawn outside their normal spawning window by a few days."

Plan for a return visit to see spawning

Witnessing synchronised spawning events can require tenacity. "You may not see it the first time, Stuart says. "And you might have to go out different nights over a series of different years to see the spectacular ones."

Although unsuccessful attempts can be frustrating, they can also bring rewards. Night diving grants a different perspective on the reef and the surrounding marine life. And witnessing even one coral colony spawn can be a profound experience. 

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