Dusk settles across Trinity Inlet as we head out to sea on a mission to catch one of the Great Barrier Reef's most elusive and mesmerising sights - mass coral spawning.

More than 50 dive and snorkel enthusiasts are along for the ride with TUSA Dive for an adventurous evening in search of the natural phenomenon, where corals reproduce under the romantic glow of a waning full moon.

Scientists believe corals spawn a few nights after the full moons in spring and summer, but the research community is still unravelling the exact science behind the impressive spectacle.

What they do know is one of the main cues for parent corals to spawn is the warming ocean temperature in spring and summer.

Coral polyps then synchronise the release of tiny pink egg and sperm bundles with the lunar cycle.

November is typically an ideal month to spot the reproductive ritual, where many colonies and species of coral polyps simultaneously release millions of egg and sperm bundles for external fertilisation.

But corals are a fussy species and the ocean needs to be warm enough to encourage spawning, which means there is no guarantee the spectacle will follow the lunar cue in each of the spring and summer months as anticipated.

TUSA Dive skipper Sam Appleton is hopeful the Reef will “go off” tonight and steers the boat towards Flynn Reef, a remote dive spot situated on the edge of the continental shelf about 60km northeast of Cairns, which is renowned for its impressive coral cover and minimal current.

Sam has witnessed the coral spawning phenomenon every year for the past four years. Rather than luck, he chalks it up to his dedication and patience to head out diving several nights in a row following the full moons in spring and summer.

The thrill of the chase never gets old for Sam.

“Sometimes I’ve only seen the edges of it,” he says.

“Other times I’ve gone out for several nights and seen it fully go off.”

Darkness descends during the one-and-a-half-hour boat ride to Flynn Reef and we excitedly gear up on the dive deck to try our luck with Mother Nature.

Sinking beneath the inky water, the glow of torchlight from other divers illuminates the underwater habitat.

I flash my torch across the coral and multiple pairs of electric red shrimp eyes beam back at me, while a crab scurries out of sight and massive humphead maori wrasse cruises past.

A brightly hued parrotfish rests on the ocean floor underneath a coral, ensconced in a clear mucous cocoon, which offers protection from nocturnal predators as well as a morning meal.

Resting on my knees on the sandy bottom, I stare intently at a blue-tipped acropora coral, urging it to spawn. But it’s clearly not ready and as the 45-minute dive winds down it becomes more likely the Reef won’t put on its dazzling display tonight.

Maybe we were too early? Maybe the corals would spawn later in the evening? Maybe they would spawn next month? 

TUSA dive instructor Vili Baleilevuka says it’s an astonishing spectacle if you’re in the right place at the right time.

Diving on the Reef during mass coral spawning is likened to being in an upside-down snow storm, with millions of tiny pink gametes the size of couscous swirling to the surface and bouncing off divers.

A colony the size of a boogie board can produce half a million eggs alone.

“It feels like rain,” says Vili, who has experienced coral spawning every year for the past six years.

He’s no scientific expert, but Vili says one Reef creature typically provides an unusual clue when the coral is preparing to spawn, and he is still waiting for it to happen this year.

“The sea cucumber starts to put its head up,” he says.

After corals spawn, the eggs float to the ocean’s surface – creating a colourful pink carpet – where they wait to be fertilised by sperm from nearby corals.

They continue to float at the surface for a couple of days and then the oval-shaped larvae begin swimming towards the ocean floor.

After about five days the larvae are ready to find a place to attach and develop into a single polyp, or a juvenile coral.

Amazingly, the larvae can travel up to 5km before settling on coralline algae to form new coral.

Scientists believe their ability to travel such a distance is Mother Nature’s way of re-seeding nearby reefs that have been destroyed by bleaching, cylcones or crown-of-thorns starfish.

We may have missed the incredible spectacle tonight, but divers and snorkellers climb back aboard the boat and swap tales of our underwater adventures, content with an incredible night dive and the thrill of the chase.

After all, it wouldn’t be such an exciting adventure if coral spawning was easy to catch.

Maybe we’ll be in the right place next time.

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