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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.
acidic An adjective for materials that contain acid. These materials often are capable of eating away at some minerals such as carbonate, or preventing their formation in the first place.
algae Single-celled organisms, once considered plants (they aren’t). As aquatic organisms, they grow in water. Like green plants, they depend on sunlight to make their food.
biology The study of living things. The scientists who study them are known as biologists.
bleach A dilute form of the liquid, sodium hypochlorite, that is used around the home to lighten and brighten fabrics, to remove stains or to kill germs. Or it can mean to lighten something permanently, such as: Being in constant sunlight bleached most of the rich coloring out of the window drapes. Corals are said to bleach when the animals are stressed and expel their symbiotic algae, turning white.
breed (noun) Animals within the same species that are so genetically similar that they produce reliable and characteristic traits. German shepherds and dachshunds, for instance, are examples of dog breeds. (verb) To produce offspring through reproduction.
calcium A chemical element which is common in minerals of the Earth’s crust and in sea salt. It is also found in bone mineral and teeth, and can play a role in the movement of certain substances into and out of cells.
camouflage Hiding people or objects from an enemy by making them appear to be part of the natural surroundings. Animals can also use camouflage patterns on their skin, hide or fur to hide from predators.
cell The smallest structural and functional unit of an organism. Typically too small to see with the naked eye, it consists of watery fluid surrounded by a membrane or wall. Animals are made of anywhere from thousands to trillions of cells, depending on their size. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.
chemical A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O. Chemical can also be an adjective that describes properties of materials that are the result of various reactions between different compounds.
clone An exact copy (or what seems to be an exact copy) of some physical object. (in biology) An organism that has exactly the same genes as another, like identical twins. Often a clone, particularly among plants, has been created using the cell of an existing organism. Clone also is the term for making offspring that are genetically identical to some “parent” organism.
concentration (in chemistry) A measurement of how much of one substance has been dissolved into another.
conservation The act of preserving or protecting something. The focus of this work can range from art objects to endangered species and other aspects of the natural environment.
coral Marine animals that often produce a hard and stony exoskeleton and tend to live on the exoskeletons of dead corals, called reefs.
current A fluid body — such as of water or air — that moves in a recognizable direction. (in electricity) The flow of electricity or the amount of electricity moving through some point over a particular period of time.
dissolve To turn a solid into a liquid and disperse it into that starting liquid. For instance, sugar or salt crystals (solids) will dissolve into water. Now the crystals are gone and the solution is a fully dispersed mix of the liquid form of the sugar or salt in water.
diversity (in biology) A range of different life forms.
dynamite A type of explosive.
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
environment The sum of all of the things that exist around some organism or the process and the condition those things create for that organism or process. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature, humidity and placement of components in some electronics system or product.
extinct An adjective that describes a species for which there are no living members.
extinction The permanent loss of a species, family or larger group of organisms.
fertilize (in biology) The merging of a male and a female reproductive cell (egg and sperm) to set in create a new, independent organism. (in agriculture and horticulture) To provide basic chemical nutrients for growth.
fluorescent Capable of absorbing and reemitting light. That reemitted light is known as a fluorescence.
generation A group of individuals born about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans. The term also is sometimes extended to year classes or types of inanimate objects, such as electronics or automobiles.
genetic Having to do with chromosomes, DNA and the genes contained within DNA. The field of science dealing with these biological instructions is known as genetics. People who work in this field are geneticists.
genetic diversity The range of genes types — and traits — within a population.
graduate student Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).
greenhouse gas A gas that contributes to the greenhouse effect by absorbing heat. Carbon dioxide is one example of a greenhouse gas.
immune system The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.
infection A disease that can spread from one organism to another. It’s usually caused by some sort of germ.
innovation (v. to innovate; adj. innovative) An adaptation or improvement to an existing idea, process or product that is new, clever, more effective or more practical.
intervention A planned effort to prevent or treat a disease or other condition relating to health or well-being.
invertebrate An animal lacking a backbone. About 90 percent of animal species are invertebrates.
juvenile Young, sub-adult animals. These are older than “babies” or larvae, but not yet mature enough to be considered an adult.
larva (plural: larvae) An immature life stage of an insect, which often has a distinctly different form as an adult. (Sometimes used to describe such a stage in the development of fish, frogs and other animals.)
marine Having to do with the ocean world or environment.
mature (noun) An adult individual. (verb) The process of growth and development that occurs as an individual moves toward adulthood.
microscope An instrument used to view objects, like bacteria, or the single cells of plants or animals, that are too small to be visible to the unaided eye.
millennia (singular: millennium) Thousands of years.
Pacific The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
Petri dish A shallow, circular dish used to grow bacteria or other microorganisms.
pH A measure of a solution’s acidity. A pH of 7 is perfectly neutral. Acids have a pH lower than 7; the farther from 7, the stronger the acid. Alkaline solutions, called bases, have a pH higher than 7; again, the farther above 7, the stronger the base.
plastic Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.
polyp A sedentary form of an animal, such as a coral, that has a fixed base, a body shaped like a column and an end with mouth and tentacles.
population (in biology) A group of individuals from the same species that lives in the same area.
proteins Compounds made from one or more long chains of amino acids. Proteins are an essential part of all living organisms. They form the basis of living cells, muscle and tissues; they also do the work inside of cells. The hemoglobin in blood and the antibodies that attempt to fight infections are among the better-known, stand-alone proteins. Medicines frequently work by latching onto proteins.
ratio The relationship between two numbers or amounts. When written out, the numbers usually are separated by a colon, such as a 50:50. That would mean that for every 50 units of one thing (on the left) there would also be 50 units of another thing (represented by the number on the right).
reef A ridge of rock, coral or sand. It rises up from the seafloor and may come to just above or just under the water’s surface.
rehabilitation The act of restoring something to its original state. Often called “rehab” for short, the term is used commonly for both physical injuries (such as regaining muscle strength after an accident, for example) and mental problems (such as addiction to drugs, alcohol or other substances).
resilience The ability to recover quickly from a setback.
runoff The water that runs off of land into rivers, lakes and the seas. As that water travels over land, it picks up bits of soil and chemicals that it will later deposit as pollutants in the water.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
seawater The salty water found in oceans.
skyscraper A very tall building.
spawn To release or fertilize eggs in an aquatic environment.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
sperm The reproductive cell produced by a male animal (or, in plants, produced by male organs). When one joins with an egg, the sperm cell initiates fertilization. This is the first step in creating a new organism.
stress (in biology) A factor, such as unusual temperatures, moisture or pollution, that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional, or behavioral reaction to an event or circumstance, or stressor, that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative. (in physics) Pressure or tension exerted on a material object.
symbiosis (adj. symbiotic) A relationship between two species that live in close contact. A species that lives this way, offering substantial help to the other species, is sometimes called a symbiont.
threatened (in conservation biology) A designation given to species that are at high risk of going extinct. These species are not as imperiled however, as those considered “endangered.”
tissue Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues. And brain tissue will be very different from bone or heart tissue.
toxic Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.
transplant (in medicine) The replacement of a tissue or an organ with that from another organism. It is also a term for the material that will be transplanted. (verb) To move something from one spot and plant or place it in another.
tropics The region near Earth’s equator. Temperatures here are generally warm to hot, year-round.
Report: P. Harrison et al. Coral reef restoration using mass coral larval reseeding. Australian Centre for International Agricultural Research. Published June 20, 2016.
Journal: A.J. Edwards et al. Direct seeding of mass-cultured coral larvae is not an effective option for reef rehabilitation. Marine Ecology Progress Series. Vol. 525, April 9, 2015, p. 105-116. doi: 10.3354/meps11171.