Synchronised sex

By Kara Murphy 9 May 2016
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The annual mass coral spawning is one of the highlights of the Great Barrier Reef’s biological calendar.

This article appears in the Australian Geographic Tropical North Queensland Special Edition. Get your copy of the full issue, on sale now. 

EVERY NOVEMBER, SNORKELLERS visiting the reef hope to witness an amazing natural phenomenon, when many different colonies and species of coral polyps flood the surrounding waters with egg and sperm bundles all within seconds of each other.

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. And similar to 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 more than 12 times. “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…driving slowly in the rain, and putting your head out the window and feeling the little droplets on your face.”

Coral spawning

Tiny egg and sperm bundles from Goniastrea retiformis, a stony coral, begin to rise into the current. In the hours before release, the bundles appear at the mouth of each polyp and can be seen spiralling there just prior to the moment they are ejected. (Image: Kevin Deacon)

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, who adds that 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. But although there are a few indicators for the initiation of these mass spawning events, predicting it is a tricky game. “You have to be in the right place at the right time,” says Stuart. The phenomenon only happens at night, after rising water temperatures have stimulated the maturation of gametes (the egg and sperm) within the polyps. And this usually occurs from 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.

Coral spawning

A sea cucumber or trepang (below) adopts a spawning stance. It’s believed this position assists the eggs and sperm to enter the water column and ride the current. (Image: Kevin Deacon)

Thus far, the largest synchronised spawning events recorded in Australia have occurred on the Great Barrier Reef (GBR) and on reefs off Western Australia. Large-scale spawning events in WA tend to occur in autumn, with smaller but still significant events also recorded in spring. Large events on the GBR appear to take place only in late spring or early summer, after sunset, one or two days after the November full moon. However, if the November full moon occurs early in the month, the spawning may not begin until after the December moon. In the southern GBR around Heron and Lady Elliott islands, the spawning might not occur until January or even as late as February.

Local tour 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 spectacle. Closely examining the coral during daylight hours around these dates also doesn’t help experts, operators, or hopeful individuals pinpoint spawning nights; the coral doesn’t reveal definitive visible clues until after sunset, when the spawning is then imminent.

“Usually about a half an hour to a couple of hours before spawning, you can 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 that 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 under water, 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), 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.”

Witnessing even one coral colony spawn can be a profound experience.

Dr Glen Burns, a resident marine biologist with Quicksilver cruises, witnessed the spectacle at Norman Reef on 29 November 2015 during a special evening excursion for the company’s biologists and dive crew aboard Silversonic. “There’s an anomalous tide towards the end of November where you only get three tides in 24 hours instead of four, so that was the night we expected the spawning to happen,” says Glen. “But the boat was only available the night before. We went anyway.” he says.

Their first dive was just after dark, at about 7.30pm, with about 30 crew and scientists in the water, but nothing was happening. “So we had a bit of a break and then dropped back in the water for the second dive,” says Glen. “After about 20 minutes or so we started to see some little flecks of spawn and we honed in on one particular colony, but then we came around the corner and the water was just thick with spawn – it was going off in a really big way,” he says. “It was really good to see but it was all the Porites and we thought that perhaps the next night may have been the spawning of the Acropora, because everyone likes to see the branching stag corals and the plate corals spawning,” says Glen.

“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 Harrison says. “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 adds.

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.”

Witnessing synchronised spawning events can require tenacity. “You may not see it the first time, Stuart says. “And you might have to go out on 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.

How to see coral spawning

Spawning events can’t be guaranteed but a number of tour operators run night dives if the conditions are thought to be favourable.

Visit the following sites for more information:
Quicksilver; Tusa Dive; Deep Sea Diver’s Den

Additional reporting by Chrissie Goldrick.


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This article appears in the Australian Geographic Tropical North Queensland Special Edition. Get your copy of the full issue, on sale now.