The Spectacular Synchronized Sex of the Great Barrier Reef

Americans aren't the only ones to indulge in an annual glutinous feast in November. On the other side of the globe, millions of marine species, from small worms to giant fish, satiate themselves on the torrents of fatty, juicy eggs and sperm set forth by the corals of the Great Barrier Reef. Following the full moon of November, millions of tiny coral polyps, no bigger than a pencil eraser, coordinate an impressive synchronized sex act that stretches across more than 1000 miles—fueling the next generation of corals and feeding the current generation of reef residents.

For those lucky enough to be diving on the reef this time of year, the experience is akin to swimming through a blizzard, only the snowflakes are bright pink and rise upwards, rather than falling down. Because most of the action happens at night, the floating bundles of eggs and sperm can also create the illusion of outer space, as if the salty depths suddenly swapped places with the starry heavens above.

The effect stems from hundreds of coral species all releasing their gametes within a few minutes of one another, timing their once-a-year climax perfectly with a few thousand of their nearest neighbors. Even more impressive, each species goes off at a different time than other species throughout the night, and over the course of several days. This separation of spawning in time helps to reduce the chance of inter-breeding. This allows the eggs from one species meet and mingle with the sperm from that same species and not another (listen to this Stuff to Blow Your Mind podcast for more details on how this all works). Such impeccable timing maximizes the fertilization success of each species, and minimizes the risk of forming hybrids—which in general are bad news (but not always).

(Pause here for a moment and just imagine trying to perfectly coordinate that final release with your own partner after an entire year of build-up...and then having to match the timing with everyone in your home town. Corals accomplish this remarkable feat every year—and they do it without the aid of a brain.) 

Coral releasing egg-sperm bundles. Photo credit:  Auscape UIG/Getty  via

Coral releasing egg-sperm bundles. Photo credit: Auscape UIG/Getty via

After release from the bottom-dwelling parent colonies, the bundles float to the surface where they break apart, allowing sperm from one colony to mix with eggs from a neighboring colony. This is how corals date and mate, without ever having to move.

Fertilization happens, and a few weeks later, the larval coral sinks down from the surface to settle atop existing reef. The young coral cements itself to its ancestors' old skeleton and  goes on to build the next layer of living reef. 

The grandeur of the event is only matched by its importance: this is the only chance corals have to reproduce each year. It's a small window of opportunity to establish the next generation of corals—an accomplishment that is harder than ever for corals to achieve.

Turns out that John Oliver can count on corals to join the rest of us in bidding a fond farewell (NSFW) to the crap year 2016 was. Starting in February, a mass bleaching event killed more corals than ever recorded in modern history. In the northern part of the reef especially, the death toll reached over 95% on some reefs, with a loss of over two-thirds on many reefs. 

Image Credit: Terry Huges via Twitter

Image Credit: Terry Huges via Twitter

Such devastation is a double whammy for corals: first, there are fewer adult colonies left to spawn; second, those colonies that are left are spaced farther apart, making it harder for eggs and sperm to meet and mingle. The jury is still out on how this mass bleaching event will affect reproduction overall but initial studies show that in the northern reef, spawning was indeed less than in previous years. Similar results were found in previous research looking at bleaching impacts on coral reproduction in the Caribbean.

Such effects make this year's mass spawning even more impressive—a natural phenomena that is truly phenomenal, and worthy of celebration. But we must do more than marvel at this spectacle; we must also re-double efforts to protect what is left. That means not only fighting climate change, but also reducing local impacts from land-based activities such as mining. 

In addition to helping to protect what is left, scientists are also working to assist coral ecosystems with adapting to anticipated climate change.  Such "designer reefs" are not yet reality, but the approach offers one potential pathway forward for future reefs.

Scientists, such as Dr. Kristen Marhaver at CARMABI research station in Curacao, are also working on a kind of IVF for endangered corals—helping them grow more babies in order to repopulate reefs. 

Coral reefs can also benefit from a boost in their numbers provided by coral farming. Because they are colonial animals, a fragment from a coral colony can successfully regenerate into a new, independent, healthy colony of its own. The process is similar to taking a cutting from a bush to grow a new plant and can take many forms.

Staghorn coral farm. Photo credit:  The Nature Conservancy

Staghorn coral farm. Photo credit: The Nature Conservancy

For instance, The Nature Conservancy has launched an ambitious effort to restore one million corals in the US Virgin Islands and Florida. Working in partnership with Mote Marine Lab, the effort will help restore corals and further research into coral strains that have natural resistance to climate change. Meanwhile, on the other side of the globe, the Karya Segara venture off Indonesia grows corals to sell to the aquarium trade, providing artisanal fishers supplemental income in exchange for giving up destructive fishing practices, such as dynamite fishing. Corals are also out-planted onto destroyed reefs. The coral farms have also attracted tourists who will pay to swim among the baby coral "crops."  

Farming corals offers a way to rebuild degraded reefs at a faster pace than nature can do on its own. And while farming could never suffice to restore a reef at the scale of the Great Barrier Reef, it can work to help rebuild populations of locally endangered species. 

I remain hopeful regarding the future of coral reefs, but it is an optimism laced with a healthy dose of reality that time is running out. This year, the corals of the Great Barrier Reef showed they are doing all they can to provide for the future of the largest living structure on the planet—now it's our turn to do the same.