It’s the world’s biggest synchronised sex event that’s OK to teach your kids about.
Every year just after the full moon in late spring or early summer, the corals of the Great Barrier Reef release trillions of eggs and sperm into the water, usually over the course of about three or four nights.
And this year, that’s set to happen this weekend.
While it’s a spectacle most of us will never see first hand, the ABC will be streaming it as it happens on Friday and Sunday nights in a series called Reef Live on ABC TV and iView.
Despite the sheer scale of the event, scientists have only known about it since 1981. Before that, it was thought that corals brooded and released their fertilised embryos year round.
But in the past 40 or so years, they’ve learnt a lot, and we’ve asked them to catch us up.
So before the big night(s), here are a couple of answers to questions that you, or your kids, might have about nature’s greatest showcase of the birds and the bees.
What’s the full moon got to do with it?
A bunch of things influence the timing of coral spawning on the Great Barrier Reef.
Smaller-scale spawning takes place most months of the year, and that depends on things like location and species.
But there are three key triggers, called “proximate factors”, that kick off the mother of all spawning events in late November/early December, according to reef expert Bette Willis from James Cook University.
The first is water temperature. While the waters of the southern reef are much cooler than in the north, spawning is triggered reef-wide by a sharp rise in temperature as the weather warms toward the end of spring.
It’s the jump in temperature, not the absolute temperature, that is the trigger.
Because shallow water warms faster than deep water, some of the inshore reefs actually spawned after last month’s full moon.
The second trigger is moonlight, according to coral reproduction expert Peter Harrison from Southern Cross University.
As a PhD student studying in Townsville in the early 1980s, Professor Harrison was one of the first to discover that corals spawn.
“[Spawning] occurs somewhere between four and seven nights after the full moon in the spring and summer,” he said.
“Coral tissues are actually sensitive to moonlight and therefore the lunar cycle is entrained by periods of increasing moonlight up to full moon.”
Although increasing moonlight appears to be one of the triggers leading up to the event, actual spawning for most corals begins at night before the moon rises, Professor Harrison said.
“The final trigger is darkness. Some corals spawn right on dusk and others have a timer that starts half an hour to an hour after sunset,” he said.
“One of the advantages of spawning at night is that most of the visual predators are asleep.
“It’s possible that by spawning in that period just before the moonrise, when there’s very little light, they’re limiting their exposure to predators.”
Another theory for why corals spawn in the period a few days after the full moon is because this is when tidal action is the lowest, according to Professor Willis.
“About three or four days after the full moon is usually the time of the neap tides, [which is] where there’s minimal water movement,” she said.
Less water movement means there is more chance that the eggs and sperm will hang around and come into contact with one another.
It also means there is less chance that the new embryo will be quickly swept out to sea and away from suitable hard substrate that it can establish itself on when it’s ready to settle.
But how can we be sure it’ll happen this time?
Corals begin developing their eggs around nine months before spawning, and sperm around five months in advance.
Speaking from a research vessel on the way to Heron Island, Professor Harrison said as the eggs near maturity, they begin to change colour, with some species starting out pale and turning pink and then red.
“Some of my team have already sampled some of the corals at Heron [Island],” he said.
There are also visual cues on the actual night of the spawning too, according to Professor Willis.
Leading up to the spawning, the eggs and sperm are gathered together in bundles within the polyp.
Depending on the coral species, those bundles may contain 20 or more eggs.
“On the night of spawning you can actually see the bulge of the bundles sitting underneath [the polyp],” she said.
Despite these clues, corals can throw up some surprises too.
Because the lunar month (28 days) is shorter than our calendar months, the date of the full moon is different every year.
If coral spawning fell on the same lunar month each year, it would gradually get earlier and earlier until corals were spawning in winter when conditions are sub-optimal.
So every few years the corals have a correction called a “split spawning”, where, as the name suggests, they split their spawning between the earlier and later full moon, before fully shifting to the later full moon the following year.
So the sperm and egg meet, then what?
OK, time to step back for some quick coral biology 101.
The living part of a coral reef is called a polyp.
A polyp is a tiny animal, and coral reefs are home to billions (maybe even trillions) of them.
The coral of a reef is actually a calcium carbonate excretion made by the polyp — the coral itself is not alive.
Incidentally, the colour of a coral reef comes from algae (called zooxanthellae) that live within polyps.
As a polyp nears the time of spawning, the eggs and sperm it has been producing over the past several months (many corals are hermaphrodites) are gathered into bundles.
The bundles contain a high lipid content that causes them to float, according to Professor Willis.
When the bundles are released from the polyp, they head to the surface where the movement of the water breaks them apart, spreading the eggs and sperm far and wide.
Within 45 minutes of fertilisation — when a coral sperm and egg meet in the water — the first cell division happens.
Cell division continues around every 45 minutes throughout the night, and by the morning there are hundreds of cells in the new embryo.
Over the next few days, the developing embryo becomes cigar-shaped, with tiny hair-like cilia that allow it to very weakly propel itself through the water.
How long it continues to float depends on the species, but it can be as little as a couple of days before the embryo sinks and settles as larvae on a reef, Professor Harrison said.
“The larval period is usually between four to five to seven to eight days after spawning,” he said.
The beauty of such a mass-spawning event is that it helps to ensure genetic mixing between corals of the same species, sometimes over large distances.
This helps to stop inbreeding and enhances genetic diversity, which, in turn, can also help with evolutionary adaptation.
How many new corals will be born?
Despite literally trillions of eggs and sperm being released into the waters around the Great Barrier Reef, only a tiny proportion of those will go on to form larvae that settle and form new coral.
It could be as few as one in a million, Professor Harrison said.
Some eggs and sperm may not come into contact with their intended other, while many that do successfully fertilise will float into open water and fail to settle on a suitable, hard substrate.
And plenty will be eaten by predators like fish.
“I’ve seen little reef fish the next morning that could hardly swim because their stomachs were so distended,” Professor Harrison said.
Up until recently, the immense scale of the spawning meant that losing the vast majority of potential recruits didn’t matter.
But things are changing.
When polyps get stressed, they spit out their symbiotic zooxanthellae — the colourful algae that lives inside. When they do that, they appear to turn white or “bleached”.
But they need that algae to help them produce energy.
Last summer the reef was hit by significant coral bleaching. It was the third bleaching event since 2016 and the second worst ever in terms of severity.
Because corals begin developing their gametes so early, those damaged by bleaching last summer may not reproduce as well this year — if at all, Professor Harrison said.
“Depending on how badly stressed they were, sometimes they don’t produce strong enough eggs and sperm to reproduce,” he said.
“Some of the corals that survived the bleaching events this year may not be able to reproduce [at all].”
Professor Harrison said he hopes in the coming seasons the corals can get a break from bleaching. If not, time might be running out to witness the greatest (sex) show on earth.