Photograph by Michaela Skovranova, National Geographic
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At the Keeper Reef in Australia's Great Barrier Reef, a type of coral called Acropora releases bundles of sperm and eggs from its gut cavity. The male and female cells float to the top of the water where they fertilize and become larvae.
Photograph by Michaela Skovranova, National Geographic

See 'Underwater Snowstorm' of Coral Reproducing

Under a full moon, corals release millions of tiny eggs. Whether or not those eggs fertilize is a leap of faith.

Illuminated only by a full moon, underwater photographer Michaela Skovranova and a group of divers parked in the Great Barrier Reef check the water around their boat for signs it's about to happen.

Perched on the edge of the vessel, periodically they scoop up water about every 15 minutes. In these samples, they're looking for tiny, sticky eggs recently released from the gut cavities of coral. Sometimes the eggs will produce what looks like a red slick on the water's surface.

When eggs finally turn up, it means a coral-spawning event is about to take place. When the divers see that, they quickly plunge into the water, in hopes of witnessing what can only be seen once a year, when waters are warm and the moon is full.

Hermaphroditic corals reproduce during these spawning events by releasing both male and female reproductive cells, which are called gametes. When these cells float to the water's surface, they pair up and combine. Once fertilized, the eggs develop into larvae, which eventually turn into polyps, or new coral colonies.

This process "is a leap of faith of nature," says Skovranova.

Risky Strategy

Releasing millions of these reproductive cells at the same time increases the odds that eggs will become fertilized by male gametes from other individual corals, thus ensuring genetic diversity of the offspring. Because the event happens only once a year, divers must be careful not to interfere with the process when swimming through the reef. Shining bright light on coral can prevent a spawning event from happening, but once it's in progress, the corals are unbothered by light.

See 'Underwater Snowstorm' of Coral Reproducing

To catch the spawning on camera, Skovranova dove into the darkness and tried to let her eyes adjust, instead of using a flashlight. Yet swimming through a dark reef can be risky. Much of the reef lies within enclaves where little light penetrates. Divers are careful not to injure the delicate coral or touch a crown of thorns, a large, poisonous species of starfish, that in cases of extreme allergic reactions can cause death.

During a full coral spawning, so many eggs are released at once that the event is often likened to an underwater snowstorm. But when Skovranova was diving this year, she witnessed only a split spawning, which means only some of the corals were releasing their gametes at that time. Corals that were not participating were saving their cells for a spawning event at a later date, most likely sometime in December.

Her photos of the reproductive event show how dense with eggs the waters can get, even when only some of the corals reproduce. Those eggs compete for space with a host of marine life, like zooplankton and clownfish.

Aiding Conservation

Coral spawning is a critical time of year for conservation scientists. It's during this time that coral eggs and sperm are collected to be studied and artificially grown in labs.

Artificially raised corals are becoming an increasingly important conservation tool. In 2016 and 2017, the Great Barrier Reef suffered massive coral bleaching events that damaged two thirds of the reef. Coral bleaching occurs when pollutants or abnormally warm water prompt coral to expel the algae that live symbiotically in their tissues, and which provide the polyps food through photosynthesis. When conditions don't improve, the algae don't return, and the corals eventually starve to death.

One study published by the United Nations this summer predicted all the world's corals could die off in 30 years if nothing is done to curb globally warming waters. This could have far-reaching impacts, because corals aggregate to become some of the world's largest living organisms, and they provide critical habitat to a host of marine life, from fish to invertebrates.

Scientists from the Australian Institute of Marine Science plan to study the coral cells collected during this year's spawning event to see how or if corals are adapting to warmer waters. There is some early indication that some corals are more resilient to warming than previously thought, giving some hope to conservationists.

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Afternoon light filters down to the Lodestone Reef at the Great Barrier Reef. Prior to looking for spawning corals under a full moon, divers scoped out the best spots to explore.