Spawning a Solution

As the planet’s coral reefs vanish, the race is on to rescue and restore what remains.

  • Paul Tolmé
  • Conservation
  • Jun 01, 2021

Colorful soft corals (above) provide food and shelter for a school of Diana’s hogfish on a productive and diverse reef in Egypt’s Red Sea. A hermit crab (below) peers out from hump coral in the Solomon Islands. (Photo above by Birgitte Wilms/Minden Pictures)

SWARMING WITH COLORFUL LIFE-FORMS that can dazzle the eyes and overwhelm the senses, the coral reefs of Palau are home to some 700 coral species and 1,400 species of fish. “You find yourself surrounded by fish of all varieties going every which way, ducking in and out of the corals in an ebb and flow of life that is incredible to watch,” says Stephen Palumbi, a marine biologist at Stanford University who has spent eight years studying and diving amidst the reefs of this Pacific island nation.

Hermit Crab in Hump Coral 30 feet deep, Solomon Islands.

Such species richness, along with their massive structures and kaleidoscopic colors, have earned coral reefs the nickname “tropical rainforests of the sea.” Limited primarily to a band of shallow tropical and subtropical waters straddling the equator, coral reefs are found in the continental United States only off the coast of Florida, where the 360-mile-long Florida Reef Tract makes up the third largest coral barrier reef system on Earth. Some divers still recall exploring those reefs many decades ago and experiencing a sense of wonder not unlike Palumbi’s.

But that is no longer possible. Like many of the planet’s reefs, Florida’s have long been in decline due to human activities. According to Erinn Muller, coral restoration program manager for Mote Marine Laboratory, new results from long-term monitoring estimate that only about 2 percent of Florida’s total coral cover remains.

Now a mysterious waterborne illness dubbed stony coral tissue loss disease is rapidly spreading through the reefs, with nearly half of the tract’s 45 coral species susceptible. “The speed at which it travels is scary,” says National Oceanic and Atmospheric Administration (NOAA) marine biologist Jennifer Moore. Once infected, coral colonies usually die within weeks or months, she adds.

Facing potential disaster, Moore and other federal and Florida state officials recently launched the largest coral rescue effort ever undertaken. The goal is to collect up to 4,400 specimens from vulnerable coral species so they can be stored on land to protect their genetic diversity, then propagated in tanks so their offspring can be returned to the ocean. “This has never been done before on such a large scale,” Moore says of the roughly $14 million project. Currently in its third year, the effort is highlighting the hope that advances in coral rescue, research and restoration can help ailing reefs—as well as the grim outlook for those reefs due to mounting threats.

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“Whitsunday Islands, aerial view, Great Barrier Coral Reef, Queensland, Australia.

Made up of thousands of individual reefs, Australia’s 133,000-square-mile Great Barrier Reef (above) is the world’s largest coral reef ecosystem. In recent years, this global treasure has suffered an unprecedented number of coral bleaching events (below) triggered by rising ocean temperatures.

Womb of the world’s seas

Related to jellyfish and sea anemones, reef-building corals are a natural wonder of symbiosis—part animal, part plant, part mineral. Coral polyps are tiny carnivorous animals whose bodies shelter photosynthetic algae, called zooxanthellae, that provide the polyps with energy from the sun in exchange for carbon dioxide and shelter. Polyps produce a skeleton of calcium carbonate, or limestone, to protect their soft bodies. Over time, the polyps replicate themselves through asexual reproduction, producing genetically distinct colonies that aggregate into reefs.

Damselfish group in bleached Stony Coral, Heron Island, Great Barrier Reef, Australia.

If the world’s seas have a womb, it is these coral reefs—the most biodiverse of all ocean ecosystems. About a quarter of marine species depend on coral reefs for food or shelter. Just as high-rise buildings and cities create more habitat for humans to live in dense populations, reefs form dynamic structures that provide niches for many life-forms. This diversity of life creates a food web where plankton and algae are consumed by mollusks and herbivorous fish, which are then eaten by successively larger fish up to sharks, barracuda and other apex predators—including humans. Beyond providing fish for human consumption, reefs protect coastal communities from storm surges and support valuable recreation and tourism industries. NOAA estimates that Florida’s reefs alone generate $4.4 billion in local sales, $2 billion in local income and 70,400 jobs, primarily through tourism and fishing.

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“A shiver of Caribbean reef sharks swim over a coral reef with Common sea fans and sea plumes.

Caribbean reef sharks (above) swim over a reef in Cuba. A mushroom coral (below) releases sperm on the Great Barrier Reef.

Coral reefs in collapse

Scientists believe that today’s reef-building corals originated approximately 250 million years ago, and they have survived four mass extinctions. Absent outside forces, “they are essentially immortal,” Palumbi says. “We know of no corals that have died of old age.”

Despite their longevity, coral reefs today are collapsing. The greatest single threat: Rising ocean temperatures resulting from greenhouse gas emissions are causing a condition known as bleaching. Bleaching occurs when coral polyps eject their colorful and life-giving symbiotic algae and turn white. If warm water persists too long, the corals starve and die, leaving behind only their skeletons. While some bleached corals can recover if temperatures return to normal, others never bounce back.

First observed during the 1980s, large-scale bleaching events are becoming more frequent. On Australia’s Great Barrier Reef, for example, mass bleaching once happened every 27 years but is now reported much more often, including catastrophic back-to-back events in 2016 and 2017. According to a 2019 National Academy of Sciences (NAS) report, the majority of the world’s reefs will be exposed to bleaching conditions every year by 2050 if ocean temperatures continue to rise.

Mushroom Coral releasing sperm at night during spawning, Great Barrier Reef, Australia.

The buildup of atmospheric carbon dioxide is also making oceans more acidic, which weakens and dissolves the limestone skeletons of stony corals. One study found that ocean acidification can result in a 20 percent loss in the density of coral skeletons, weakening reefs and making them more vulnerable to breakage. A 2018 report in Science concluded that before the end of this century, oceans may reach a tipping point—where increasing acidity levels cause coral reefs to dissolve faster than they can regenerate.

In addition, climate change is increasing the frequency and intensity of hurricanes and typhoons that can harm reefs by inundating them with sand, snapping off coral branches and overturning entire colonies. Meanwhile, a myriad of long-standing threats to coral reefs—from overfishing to pollution by agricultural pesticides and fertilizers, sewage and coral-smothering sediments—are exacerbating the damages caused by climate change.

“To have any hope of saving coral reefs, we need to reduce the carbon pollution that is heating up and acidifying our oceans,” says National Wildlife Federation Chief Scientist Bruce Stein. To buy reefs some time, though, “there are immediate steps we can take to reduce stresses on corals.”

Beyond reducing pollution and halting destructive fishing practices, these steps include creating protected coral sanctuaries, especially on reefs buffered from the worst temperature extremes due to their remoteness or specific locations. “Let’s focus on protecting and not messing up coral sanctuaries where reefs have the best chance to survive climate change,” says Tim McClanahan, a marine biologist with the Wildlife Conservation Society. McClanahan is working with the governments of Kenya and Tanzania, for example, to establish a transboundary marine sanctuary spanning the countries’ border, where a deep basin of stable water dilutes warm currents that come across the Indian Ocean during bleaching events.

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Marine biologist Liv Williamson (above) “outplants” lab-spawned staghorn coral on a reef being restored off the coast of Miami. At The Florida Aquarium in Tampa, pillar coral (below) is induced to spawn in a tank by overhead LED lights that replicate the full moon.

Coral restoration scales up

Other scientists, including authors of the recent NAS report, emphasize the need to restore reefs. In the past, restoration focused on small areas, such as portions of a reef harmed by ships running aground or an oil spill. But on the critically imperiled Florida Reef Tract, coral restoration efforts are finally, and dramatically, scaling up.

Three years ago, as the new coral disease continued spreading through the tract, coral experts from more than 40 organizations met to identify interventions that could slow or stop the disease and save priority species. Among their recommendations was removing some healthy coral from the reef for storage on land. This large task fell to the Florida Coral Rescue Team, a group of scientists and divers from state and federal agencies as well as nonprofits and the private sector. In 2019, the team’s divers began using hammers and chisels to remove healthy coral fragments from the sea floor. By the end of 2020, they had collected some 2,000 specimens from 22 species. Weighing about 10 pounds each and roughly the size of saucers, the corals were flown to aquariums around the country, including four in Florida and 18 in other states, from Colorado and Connecticut to Michigan, Texas and Iowa.

The next challenge was “figuring out how to produce the millions of corals needed to jump-start the repropagation of the reef,” Moore says. At The Florida Aquarium’s Center for Conservation, Senior Scientist Keri O’Neil and her team took on that challenge, implementing a technique for getting corals to sexually reproduce in the laboratory. On the reef, the animals spawn on summer full-moon nights. In the lab, scientists use computer-controlled LED lighting to replicate the rising and setting of both the moon and sun. In 2019 and again in 2020, the team achieved a milestone by successfully inducing some of the rescued corals to spawn. “It was the biggest feeling of both excitement and relief,” O’Neil says. Nicknamed “Coral Mama,” she has since overseen multiple spawning events.

A Diploria spawn tank at the Florida Aquarium.

When they spawn, corals release tiny bundles containing both sperm and eggs that float to the surface and burst, creating a slurry of fertilized eggs. O’Neil and her crew collect the eggs and raise them for about five days, when they transform into tiny larvae. The larvae are transferred to containers holding algae-coated tiles, where they settle during the course of several days and metamorphose into coral polyps.

Researchers then transfer the polyps to tanks containing symbiotic algae that are taken in by the animals after several weeks. These genetically distinct “coral babies” now begin asexually reproducing—essentially cloning themselves—the first stage in forming a new colony. As of late April, when this issue went to press, O’Neil’s team had produced more than 30,000 coral polyps from 10 species. “We then raise and grow them to a size where they can be returned to the reef,” she says.

Last November, University of Miami divers ferried the first batch (approximately 150) of the center’s captive-bred corals to three sites on the Florida Reef Tract. These initial “outplantings” constitute a pilot project—proof of concept from which researchers hope to learn best practices and expand their efforts. O’Neil, meanwhile, is working to increase the rate at which corals can be propagated in the lab and successfully moved to the reef. “We are trying to make this process faster and more efficient,” she says. Other lab scientists are racing to find treatments for the disease or to discover corals with genetic resistance to it. In the field, researchers are scoping out reef locations that remain disease-free so they can be targeted for future outplantings.

But none of this will be enough without aggressive steps to combat climate change, Stein says. “With continued inaction on climate, we are consigning most of the world’s reefs to almost certain death.” And there’s no more time to waste, he adds. “Sick coral reefs are an indication of sick oceans, and our oceans are the basis of all life on Earth.”


Seeking relief for coral reefs

For decades, the National Wildlife Federation has supported policies and programs that benefit coral reefs, especially in U.S. waters off Florida, Hawai‘i and Puerto Rico. Such efforts include advocating for reductions in water pollution and public education for responsible boating and diving. Today, NWF focuses especially on lowering carbon dioxide emissions, which fuel the latest and most-dire threats: ocean warming, sea level rise and ocean acidification. To learn more, visit

Paul Tolmé is a writer based in Seattle, Washington.

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