Mass Coral Spawning:

Exploring National Marine Sanctuaries

By Rachel Plunkett

In the world of coral reproduction, timing is everything. At least, that’s what studies on coral spawning events in Flower Garden Banks National Marine Sanctuary have revealed. The famous discovery took place on August 13, 1990, after sunset in the dark waters of the Gulf of Mexico, over 100 miles offshore. As the divers shone beams of light through the water, they saw coral mounds covered in small, pale pink dots that appeared to be rising through the water column to the surface. The dots—gametes, or eggs and sperm—would undergo fertilization and develop into coral larvae that eventually drop back down to the ocean floor, settle on the reef, and become baby corals (if they’re lucky).

This wasn’t the first time a mass coral spawning event had been documented in the wild—it had been seen in the Pacific Ocean. But the spawning observed at the Flower Garden Banks may be the most spectacular coral spawning display in the western Atlantic due to the high density and cover of broadcast spawning species there.

Broadcast coral spawning is considered one of nature’s most awe-inspiring events, involving a combination of special cues such as changes in water temperature and ocean chemistry, and the rhythm of the lunar and solar cycles. Scientists still do not fully understand all the nuances of how multiple colonies of the same coral species are able to synchronize the release of their gametes so well.

Since the discovery in 1990, sanctuary researchers have continued to monitor the annual event to accumulate more precise data on the timing and species participation—and they’ve learned some interesting things! The spectacular display is highly predictable and typically occurs seven to 10 days after the full moon in August. It all starts one hour after sunset, and lasts for about four hours. On the eighth and tenth evenings, spawning activity peaks for certain species, and these peaks happen in a particular order—a symphony perfectly orchestrated by nature to maximize fertilization success and avoid mix-ups!

"Something we've found to be interesting is that if there happens to be two full moons in August—one at the beginning of the month and one at the end—sometimes we see a second spawning event happening in early September," said Michelle Johnston, research ecologist at Flower Garden Banks National Marine Sanctuary. "These split spawning events may act as an extra boost to the reef by generating an increased larval supply that year."

a coral with polyps that are releasing large white dots
Many broadcast spawning corals, like this boulder star coral (Orbicella faveolata) colony, are hermaphrodites, releasing gamete bundles containing both eggs and sperm. The majority of coral species are considered hermaphroditic broadcast spawners. Photo: G.P. Schmahl/NOAA
closeup of coral polyp with a milky white substance coming out of them
Unlike their hermaphroditic neighbors, great star corals (Montastrea cavernosa) are gonochoric, meaning they have separate male and female colonies. This male colony is releasing sperm into the water column. To increase success, some female gonochoric corals at Flower Garden Banks hold onto their eggs for about a half hour after the males spawn, only releasing them after they become fertilized. Photo: G.P. Schmahl/NOAA

Diving Deeper

It’s not just shallow-water corals that deserve all of the attention, however. Flower Garden Banks National Marine Sanctuary is known for its alluring mesophotic coral ecosystems and deep-sea coral communities as well.

Beginning right around the depth limits of recreational scuba diving (130 feet), mesophotic, or "twilight zone" habitats, experience low light conditions that several light-dependent species have adapted to live in. From here, the mesophotic zone extends into even lower light areas, where we begin to see deeper mesophotic reefs full of black corals, soft corals, sponges, algae, and algal nodules. The discovery of these coral reefs in deeper waters is one reason the sanctuary was first designated, and knowledge about the importance of these ecosystems as essential fish habitat through years of sanctuary research and discovery played a major role in the decision to expand the sanctuary in 2021 to include 14 additional reefs and banks.

a coral colony
These hermaphroditic boulder brain coral (Colphophyllia natans) colonies at McGrail Bank are now part of Flower Garden Banks National Marine Sanctuary, since the sanctuary’s boundaries were expanded in 2021. Photo: NOAA/UNCW-UVP

In recent years, observations of broadcast coral spawning at mesophotic depths have been made using remotely operated vehicles (underwater robots), adding to the body of knowledge around these reproductive events.

Because coral reefs around the world are facing major declines in health and coral cover due to stressors such as climate change, the long-term monitoring at Flower Garden Banks National Marine Sanctuary offers valuable information about coral reef health in other parts of the tropical western Atlantic where restoration efforts are underway. Stressed corals do not have the energy to reproduce; so the presence of a predictable and vibrant spawning event is a strong indicator of a healthy coral reef.

Rachel Plunkett is the writer/editor for NOAA’s Office of National Marine Sanctuaries