Coral reefs are bustling underwater cities that lie beneath tropical, sunlit waves. Thousands of colorful creatures click, dash and dart, as loud and as fast-paced as the citizens of any human city.
Built up in tissue-thin layers over millennia, corals are the high-rise apartment buildings of this underwater Gotham. Their calcium skeletons represent generations of tiny invertebrate animals. Jacketing them is a living layer of colorful coral polyps. Their complex structures offer shelter. And for some 114 species of fishes, and 51 species of invertebrates, those coral skyscrapers are lunch.
Important as they are, corals are in jeopardy.
Warming oceans stress corals. This causes them to bleach, which means they turn white and become vulnerable. A prolonged spike in temperatures of just one to two degrees Celsius (1.8 to 3.6 degrees Fahrenheit) may be enough to kill corals. Greenhouse gas emissions acidify the water. That can dissolve the corals’ mountain of calcium skeletons.
Chemicals, too, can stress or kill corals or their larvae. Some of these chemicals wash off the land. Others, such as sunscreens, can wash off of people.
In some countries, fishermen catch fish by release a stunning dose of the poison cyanide. Others may detonate a blast of dynamite to catch fish, leaving behind coral rubble.
Today, about 60 percent of the world’s reefs are at risk of disappearing.
Threats to reefs have “dramatically escalated in the last few decades,” says Peter Harrison. He is a marine scientist at Southern Cross University in Lismore, Australia. He has studied corals for three decades. “In my time as a reef researcher,” he says, “I’ve seen it get worse, firsthand.”
Thirty years ago, massive coral die-offs were unheard of. But today, reefs are suffering through the third global bleaching event since 1998. High ocean temperatures have dragged on since 2014. This summer marked the longest and most widespread episode of worldwide coral bleaching on record. More than 80 percent of the northern part of Australia’s Great Barrier Reef, bleached white, according to an April report by the National Coral Bleaching taskforce.
As reefs take a nosedive, scientists around the world are scrambling to save corals while they still can. Approaches that were once radical are “now seen as necessary in some places,” says Ruth Gates. She is a coral biologist at the Hawaii Institute of Marine Biology in Honolulu.
In Florida, researchers are restoring reefs with tiny coral fragments. In Hawaii, Gates is scouring the water for stress-tolerant corals and experimenting in the lab to breed more of the hardiest individuals. At the 13th International Coral Reef Symposium in Honolulu, this past June, Harrison’s team reported early promising results of their effort to flood damaged reefs in the Philippines with tiny coral larvae.
There is no single answer. What works on one reef may not save another. So researchers are testing an arsenal of innovations to rescue coral reefs. They hope to find success before these underwater cities are gone for good.
A different story
In the early 1980s, Harrison was a graduate student at James Cook University in Townsville, Australia. He was working on the nearby Great Barrier Reef. It’s the world’s largest coral system. At the time, textbooks taught that most corals reproduce by brooding. In brooding, fertilization occurs inside the body. Babies are then released into the water to replenish reefs year-round. But Harrison witnessed something very different. For a few nights around the spring full moon, corals spawned. They spewed eggs and sperm into the water, to be fertilized in the water. The sea was covered in a pink, oily slick of coral eggs and sperm.
“We found the corals hadn’t read the textbooks,” Harrison says. Eggs and sperm were meeting outside of the coral bodies. The resulting young, called larvae, would develop while drifting in ocean currents.
That discovery “fundamentally changed our understanding,” Harrison says. It spurred a cascade of studies on coral reproduction. And those led to the modern understanding that many corals reproduce only once or twice a year, in coordinated mass releases of eggs and sperm. Most of the resulting larvae die or drift out to sea, Harrison says. Only a small fraction will survive to adulthood. Even so, mass spawns are “how reefs replenish themselves over time,” he says.
Soon after Harrison made his discovery, unusually warm ocean temperatures hit the Great Barrier Reef. Then other reefs around the globe got hit with the warm waters. Normally, tiny algae cells live inside coral polyps. The algae make sugars for the coral. They also give corals their characteristic bright colors. But during these temperature spikes, the algae turn toxic. Corals spit out their algal partners, which left them bleached white. And the corals can die if temperatures don’t cool down enough for the algae to return.
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Corals around the world began bleaching more often, and more severely, than had been recorded in the past. Scientists began to worry that reefs might vanish. Some researchers, like Dave Vaughan, took action. He manages the Coral Restoration program at the Mote Tropical Research Laboratory in Summerland Key, Fla.
In those days, Vaughan was a fish farmer. He raised saltwater fish species in captivity. He began growing corals for tropical aquarium tanks. At the time, all corals sold for aquariums had been taken from the wild, Vaughan recalls. He started growing coral species in captivity as an environmentally friendly alternative.
One day, Philippe Cousteau came to tour the farming operation. Cousteau is the grandson of legendary ocean explorer Jacques Cousteau. He has continued in the family business, working to educate people about the ocean and conservation. When the young Cousteau saw that Vaughan was raising corals for aquariums, “he shook his head,” Vaughan remembers. He said, “Dave, if you could do this for the aquarium trade, you can do this for the reef.”
In those earliest days, most scientists were tackling small-scale reef damage caused by dropped anchors or boats that ran aground, Vaughan explains. To repair reefs on those scales, scientists began transplanting corals. Reef managers would break 3- to 5-centimeter (1 to 2-inch) fragments from healthy corals on a neighboring reef. Then they would transplant those chunks to damaged spots.
Cousteau’s visit convinced Vaughan to try restoring reefs. While doing this, 11 years ago, Vaughan made a game-changing discovery. Usually he transplanted pieces just 3 to 5 centimeters in size. But he also transplanted tinier fragments, ones less than half the size of the traditional pieces. These coral “microfragments” repair themselves remarkably fast, he found: 25 to 40 times faster than scientists had ever recorded corals growing.
Today Vaughan’s team is spreading many genetically identical microfragments — clones — over the surface of dead coral skeletons in the Florida Keys. As those bits fuse together, they create a fast growing “skin” over an otherwise dead reef.
The hope is that thousands of pieces could potentially carpet a small reef in two to three years, says Chris Page. This biologist teamed up with Vaughan at Mote Marine Laboratory. Just two or three years is super fast, he says. “There’s no way that’s happening in nature.”
Vaughan’s team is cultivating 17 species for microfragmentation today. The corals grow in large troughs on land, with seawater running through them. A half dozen or so of the corals are slow-growing species that create the structure of the reef. Some can mound into boulders the size of a truck.
The first 200 microfragments plunged into the ocean three years ago. They were introduced at two sites in a nearshore coral reef off Big Pine Key, Fla. The colonies are now six to eight times larger than they were at planting. And they have begun to fuse into patches about the size of a 5-gallon bucket lid. Since then, Vaughn and Page have planted close to 10,000 microfragments in the wild.
“People were looking for some glimmer of light,” he says. “And restoration is turning out to be that in a big way.”
Seeds of reefs
Fragmentation and the newer microfragmentation, are both time and labor-intensive. That makes them very expensive. And, Harrison says, they rely on cloning.
When a single coral is broken into fragments, to be fattened up and then planted around a reef, each chunk is genetically identical. All those pieces have the same DNA blueprints to do things like fight infection and cope with stress. But being clones, each of these corals will share the same strengths — and the same weaknesses. That doesn’t happen much on natural reefs. There, individuals are genetically different and have different strengths and weaknesses.
“People have spent years growing coral gardens only to have them wiped out by the next bleaching event,” Harrison says. With more diversity, some of those corals might have survived, he says. In a warmer world where bleaching and disease will probably become more common, “genetic diversity equals resilience.”
To address the diversity issue, Vaughan and Page are raising 20 to 30 genetic variants of each coral species. A mix of them will be planted around the reef. They are also collecting eggs and sperm from wild colonies of four coral species to grow on Summerland Key.
Harrison has been thinking about genetic diversity ever since the early 1980s, when he saw coral spewing sperm and eggs into the ocean. Few of the resulting larvae would survive. Many would drift away and most would die. At the same time, he saw reefs in decline. What if, Harrison wondered, scientists could take millions of those diverse baby corals and help them settle onto reefs to replenish injured ecosystems?
Other researchers asked themselves the same question. In the late 1990s and from 2007 to 2009, two projects released coral larvae onto healthy reefs in mesh tents pitched over the seabed. The studies took place in Australia and in Palau, an island nation in the Western Pacific. In both projects, thousands of coral babies settled under the tents. This was many more than scientists would have seen naturally.
But those early results may have been misleading. Most early coral settlers in Palau died within 30 weeks. Flooding the reef with larvae hadn’t made a lasting difference in coral numbers. Maybe, the researchers speculated, settlers had been too crowded. In that case, swamping reefs with larvae made no sense.
The idea was to find a badly damaged reef, where the worst problems, such as blast fishing, had stopped. Harrison would take a few mature corals to the lab and convince them to release sperm and eggs in aquarium tanks. He would then take more than a million of their larvae back out onto the reef. The plan was to saturate the environment with settling babies. This would mimic what adult corals would have done in healthier days.
In 2013, Harrison’s team, led by student Dexter dela Cruz, began a small pilot experiment in the Philippines. A reef called Magsaysay had been ravaged by blast fishing. This is when fishermen throw explosives to stun or kill fish for an easy catch.
Blast fishing is “like hitting the reef with a sledgehammer,” Harrison says. The large corals that had formed the foundation of Magsaysay’s reef were blasted to bits. A once-vibrant city was now a wasteland.
By 2013, the blast fishing had stopped. But Magsaysay wasn’t recovering on its own. So Harrison’s team brought in new recruits. These larvae had been collected from a nearby, healthier reef. They came from a species of fast-growing, purple-tipped coral, called Acropora tenuis. The scientists released more than a million larvae into floorless mesh tents that had been pitched underwater over the reef. After five days, Harrison’s team removed the mesh enclosures.
Over the next six months, most of the tiny coral settlers died. But three months after that, the remaining populations were now stable. Scientists expected more of the juvenile corals to die. But “incredibly and extraordinarily,” Harrison says, none have. At three years old — and the size of dinner plates — the juvenile corals had reached sexual maturity. Dela Cruz presented his team’s findings this past June at the meeting in Honolulu.
For slower-growing corals, Harrison’s approach will take extra patience. But for fast-growers like A. tenuis, reseeding larvae could be a quick and affordable way to help severely damaged reefs bounce back.
Getting more larvae onto damaged reefs is the first step, Harrison says. But some individuals are stronger and more stress-tolerant than others. As they grow up, these “winners” stand out by surviving.
Across the Pacific from Magsaysay, Gates is studying such winners. Rows of indoor and outdoor aquariums gurgle in her lab on Coconut Island, off the Hawaiian island of Oahu. Those tanks are full of Montipora capitata. This is a local and fast-growing coral. It was collected from the patchy reefs surrounding the island.
In 2014 and 2015, unusually warm water hit Hawaii. Under stress, many corals rejected their symbiotic algae. They then blanched from a healthy brown to white. Some died.
Gates’ team patrolled the reefs around the island during and after the bleaching. The scientists looked for hardy M. capitata individuals that stayed brown, even in hot water. They are also interested in M. capitata that bleached white, but then recovered. Gates equates the work to professional sports scouting “at high schools, looking for the best athletes.”
When she finds top performers, Gates brings them back to her lab to run them through their paces. She exposes each pro-performer to different temperatures and pH levels in seawater tanks. Some conditions re-create today’s oceans. Others mimic the warmer and more acidic seas expected in the future.
Today, Gates is breeding the strongest corals. (Her first batch of babies was born this summer.) She hopes that top performers will have “extremely talented kids,” that inherit their parents’ strengths. It’s too soon to tell how the new babies will do once they’re planted on a reef.
“We’re trying to give corals a leg up,” she says. Reefs healthy enough to survive without human intervention are the ultimate aim. In the next five years, the researchers plan to branch out from M. capitata. They will look for super corals of all five species found in the bay surrounding Coconut Island.
It would be ideal to find the super corals before the next big bleaching happens But for that, the researchers need another sign of resilience. That sign, Gates says, could be hidden in the way that corals glow.
Corals and their symbiotic algae are loaded with fluorescent proteins. These proteins absorb incoming light, then spit it back out by glowing. It’s unclear what fluorescent proteins do for corals. They may be a kind of sun block, protecting corals from intense light in shallow seas. Or they could be some form of camouflage — even part of the immune system.
Stress affects corals’ glowing proteins, and changes their fluorescence patterns. Consider the Pacific and Indian Ocean species Acropora yongei. In 2013, researchers reported in Scientific Reports that the level of green fluorescent protein fell with temperature stress. That caused the coral to glow less intensely. An earlier study looked at the endangered Caribbean coral Orbicella faveolata. That prolonged high temperatures change the ratio of green to orange fluorescence.
Gates expects that under stress, super corals will keep their healthy fluorescence patterns much longer than corals that are bleaching. One next step, she says, is to stress out tiny pieces of coral and watch what happens under a very powerful microscope. She’ll expose nubbins of coral to acidifying water or to warming temperatures inside a Petri dish. The microscope will pick up the fluorescence of the nubbins. That should let her see which corals stay healthy the longest.
Once scientists can identify the strongest corals, they can combine selective breeding with other rehabilitation techniques. Approaches like microfragmentation could help super colonies mature super fast. Then, Gates says, “We would have a strategy to get the reef producing its own offspring quite quickly.”
No two approaches to saving reefs are the same. That is probably a good thing. Coral fragmentation, reseeding and selective breeding each have their pros and cons.
“The assumption that one size will fit all is completely flawed,” Gates says. Worldwide, reefs are populated by different species that face different threats. Some problems, like warming oceans, are global in scope. Others, like pollution running off of roads and farms, overfishing and dynamite fishing, tend to be more local. With all of these differences, the approaches to rehabilitation must also vary. The program used might depend on the type and severity of damage, and how the mix of local coral species might respond.
Like far-flung cities, each reef has different needs and priorities. What might work on the Florida coast wouldn’t necessarily work in the Pacific. Across the globe, “will the things that we do be different?” Gates asks. “Absolutely.”
Rather than competing, Gates, Vaughan and Harrison are three crusaders among many working toward a common goal. They dream of a day when corals no longer need human help. Their hope is to find the right mix of approaches while there are still reefs left to save.