Mystery disease is killing Caribbean corals | Science News for Students

Mystery disease is killing Caribbean corals

Biologists race to learn what it is and maybe how to thwart it
Aug 29, 2019 — 6:45 am EST
maze corals with lesions from a new coral disease

Maze corals on Flat Cay reef near St. Thomas in the U.S. Virgin Islands. Lesions expose the corals’ stark white skeleton. A deadly new disease is to blame — and it's spreading through the Caribbean.

M. Brandt

Last January, divers were studying Caribbean reefs off the U.S. Virgin Islands. Suddenly, they noticed something alarming on one reef. Lesions were eating into the colorful tissues of hundreds of corals. By the next day, some were dead. Only stark white skeletons remained. Others took two weeks to die. Within four months, more than half of the reef near the island of St. Thomas was dead.

The prime suspect is a disease first discovered off Florida in 2014. Known as stony coral tissue loss disease, scientists have nicknamed it “skittle-D” — like the candy. But skittle-D is far from sweet. It is responsible for one of the deadliest coral disease outbreaks on record.

Stony corals are the building blocks of reefs. Skittle-D is now ravaging one third of the Caribbean’s 65 stony coral species. Scientists don’t know if the infection is due to a virus, bacterium or some mix of microbes.

Whatever the cause, “it’s annihilating whole species,” says coral ecologist Marilyn Brandt. She works at the St. Thomas campus of the University of the Virgin Islands. There, she heads a science team investigating the outbreak from many angles.

Other coral diseases near St. Thomas have cut coral cover by up to half within a year, she notes. But this new epidemic has done the same damage in half that time. It is spreading faster and killing more corals than any past outbreaks here.

“It marches along the reef and rarely leaves corals behind,” Brandt says. “We’re pretty scared.”

a series of photos showing the progression of stony coral tissue loss disease
Stony coral tissue loss disease, nicknamed skittle-D, can kill corals in just days or weeks. Here, a lesion eats away at a large brain coral over six weeks.
Sonora Meiling

A disease hot spot

Coral reefs cover less than 2 percent of the ocean floor. Still, they play an important role in ocean ecosystems. Reefs support about one-fourth of all marine species.

Sometimes mistaken for rocks or plants, corals are actually communities of tiny invertebrates. And they can get sick just like any other animal. Corals sometimes die from plagues. Other times, they can shake off milder illnesses, ones akin to the common cold.  

The first coral disease was discovered in the Caribbean more than 40 years ago. Since then, dozens more have emerged around the world. And the Caribbean is now considered a coral-disease hot spot.

Biologists know little about these illnesses and how they work. Many marine microbes don’t grow well in the lab, Brandt explains. That makes studying coral diseases tough.

Even the names given to the diseases are vague. There’s dark-spot syndrome. And white plague. Names are based only on the visual cues to an infection. And it doesn’t help that many diseases look similar. 

Florida scientists initially mistook skittle-D for white plague. Both infect stony corals. But scientists now know skittle-D is a new disease because it attacks stony coral species in a unique order. First hit are the brain corals. Then come the star corals and pillar corals.

Waters getting warmer

Climate change is providing an urgency to learn more about these diseases. Global warming is like a one-two punch for corals. Heat stress weakens coral defenses. At the same time, warming waters send microbes that cause disease into overdrive. Pollution, overfishing and other factors also can stress corals, making them even more vulnerable to disease.

Coral reefs just can’t catch a break,” Brandt says. Facing one challenge after another, she says, “I feel like we’re playing whack-a-mole.”

Oceans are warming 40 percent faster than what United Nations scientists had predicted in 2014, finds a January 11 analysis in Science. That warming trend will likely continue as oceans continue to soak up roughly 93 percent of the excess heat trapped by greenhouse gases.

As ocean temperatures rise, many corals have been hit by bleaching. This is where corals eject the symbiotic algae living inside the corals. Warming waters are also expected to make outbreaks of coral diseases more frequent and more severe. It’s something that researchers noted in a 2015 paper. And coral diseases will likely come to rival bleaching as a major source of coral declines.

Flat Cay reef off of St. Thomas, where Brandt’s team first noticed the skittle-D lesions, had been considered resilient. It rebounded from a major bleaching event in 2005 and from back-to-back hurricanes in 2017. But the new outbreak has killed all of the reef’s maze corals, a type of brain coral. And pillar corals could be next, Brandt says. Skittle-D “seems to be capable of changing the face of coral reefs as we know it.”

Performing reef triage

Brandt hopes to get ahead of the St. Thomas outbreak by using lessons learned from Florida. The skittle-D outbreak off the state’s southeast coast has persisted for five years. It now affects almost all of a 580-kilometer (360-mile) stretch of reef, including the Florida Keys, notes marine biologist Karen Neely. Such a lengthy assault surprised scientists. Coral disease outbreaks typically burn out after a few months.

a map showing locations in the Caribbean where there are suspected and confirmed cases of stony coral tissue loss disease
Stony coral tissue loss disease appeared off Florida in 2014. Since then it has spread across the Caribbean. Confirmed cases (red) have shown up in reefs off Mexico, Jamaica, St. Maarten, the Dominican Republic and St. Thomas in the U.S. Virgin Islands. Suspected cases are shown in orange.
E. Otwell; Source: P.R. Kramer, L. Roth and J. Lang/AGRRA 2019 (Data source: AGRRA) 

Neely works at Nova Southeastern University in Fort Lauderdale, Fla. She and others are trying to save Florida’s stony corals by moving hundreds of healthy ones to tanks. There they can be studied, bred and protected from the outbreak. Meanwhile, divers have been slathering an antibiotic paste over sick corals still on the reef. Neely estimates that Florida researchers have treated nearly 1,200 colonies in the first half of 2019.

With the antibiotic, “we are seeing about 85 percent success,” Neely says. Though the paste seems to heal lesions, it doesn’t stop new ones from popping up. For now, she says, as treatments go, this paste “is the best we can hope for.”

The antibiotic’s success suggests the disease could be bacterial, Brandt says. But the disease might have viral origins. By that, she means, a viral disease might weaken corals so that a bacterial infection can step in. If true, the paste might be treating a symptom, not the cause.

Because the St. Thomas outbreak is just getting started, Brandt’s team is trying a different approach: removing sick corals and leaving the healthy ones behind. That could reduce the amount of pathogen in the water. In theory, that will make it more difficult for the disease to spread, she says. But her team won’t know if its strategy works until later this year.

a photo of Marily Brandt surveying coral underwater
Coral ecologist Marilyn Brandt surveys a grooved brain coral. It is pocked with skittle-D lesions. In Flat Cay reef, off St. Thomas, this outbreak has killed all maze corals, a type of brain coral.
Howard Forbes Jr./Univ. of the Virgin Islands

Hunting a coral killer

To find out what might cause skittle-D, Brandt’s team is looking at the microbes that live in and around corals. Building a list of suspects requires first sorting out what normally belongs on healthy corals.

Marine ecologist Amy Apprill works at Woods Hole Oceanographic Institution in Massachusetts. She and others are studying microbes on sick corals. They are also looking at microbes living in sediments and the water swirling around St. Thomas’ reefs. Comparing those data with some from Florida corals may uncover similarities between the two outbreaks. That might help narrow the list of culprits, says Apprill.

The team is also focusing its microscopes on samples of brain and star corals taken just as lesions popped up. The corals came from a healthy reef in St. Thomas. They caught the disease during an experiment in which these specimens were placed near infected corals in an aquarium. “We might be getting a look at what ‘early’ disease looks like,” Apprill says.

She doesn’t expect to find just one pathogen. Many scientists think it may be groups of microbes that cause coral diseases like skittle-D.

More clues about skittle-D are coming from a research team led by Julie Meyer at the University of Florida in Gainesville. It found five types of bacteria dominate Florida corals infected with skittle-D. At least one type thrives in the low-oxygen conditions of dying tissue. And all these germs have been linked to outbreaks of coral disease elsewhere. The team shared its findings this past May on bioRxiv.      

Profiling the victims

While some researchers hunt for suspects, Laura Mydlarz wants to know what happens to coral during an infection. A coral immunologist, she works on Brandt’s team. She is trying to figure out why some species of stony corals are more vulnerable than others are to diseases such as skittle-D.

a photo of healthy coral transplants
In Florida, scientists are “rescuing” healthy corals in the path of skittle-D and moving them into tanks (shown). The team hopes to transplant the corals back to their home reef once the outbreak ends.
Ananda Ellis/FWC

Mydlarz’s lab is at the University of Texas at Arlington. There, her team has shown that the immune systems of corals that tend to die from infections work differently than do those in corals that typically survive.

Her team tricked corals into thinking sickening bacteria were infecting them. The immune systems of species that fought off the fake infection and lived went into a cell-recycling mode. Those that died instead had gotten stuck in a cell-death mode. Their cells died and were sloughed off. Her team reported its findings two years ago in Proceedings of the Royal Society B.

Mydlarz suspects something similar might happen in corals prone to skittle-D. That’s because the species in her study that favored cell-death mode are among those hit hardest by the new outbreak.  

A disease on the move

Researchers are trying to keep up with skittle-D’s spread. Germs may have traveled from Florida to St. Thomas in the ballast water of ships, says Dan Holstein. He’s a coral reef ecologist. Skittle-D also has shown up in reefs off Mexico, the Dominican Republic, Jamaica and other Caribbean islands.

Holstein is a member of Brandt’s team who works at Louisiana State University in Baton Rouge. He has been looking at ocean currents and other factors in hopes that he might be able to forecast where skittle-D will show up next. The team’s early results point to Puerto Rico.

In May, divers confirmed that the ongoing outbreak is inching toward the Puerto Rican island of Vieques. Star corals about 17 kilometers (11 miles) offshore and 40 meters (130 feet) deep are already pocked with lesions, notes Tyler Smith. He oversees the reef-monitoring program at the University of the Virgin Islands in St. Thomas.

The discovery was disheartening, Smith says. Scientists knew that star corals in shallower waters were at risk. They had hoped those living in deeper waters might be spared. He likens the deep reefs to a powder keg. They are made up of hundreds of millions of densely packed colonies where disease can jump easily from coral to coral. If skittle-D hits them, “the spread of [the disease] might pick up very rapidly,” Smith says, “even more than it is now.” 

Brandt’s group continues to monitor reefs in the U.S. Virgin Islands. A June survey of 270 sites around St. Croix spotted new coral infections. “It was a moment of panic,” Brandt says. In the end, it turned out these corals had the less-severe white plague. None had skittle-D. That, she says, offers a glimmer of hope — at least for now.

Power Words

(more about Power Words)

antibiotic     A germ-killing substance, usually prescribed as a medicine (or sometimes as a feed additive to promote the growth of livestock). It does not work against viruses.

bacteria     (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals). Bacteria are one of the three domains of life on Earth.

bacterial     Having to do with bacteria, single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside animals.

ballast water     This is water that ships take on to keep them low enough in the water that they remain stable. Tanker ships often take on ballast to replace the weight of goods they have just off-loaded in a port. This water, sucked in from the local river or coastal region, will contain local organisms too, that may later be released half a world away. Ballast water is a leading source of the transport and release of non-native marine species into new ecosystems.

Caribbean     The name of a sea that runs from the Atlantic Ocean in the East to Mexico and Central American nations in the West, and from the southern coasts of Cuba, the Dominican Republic and Puerto Rico down to the northern coasts of Venezuela and Brazil. The term is also used to refer to the culture of nations that border on or are islands in the sea.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

climate change     Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.

coral     Marine animals that often produce a hard and stony exoskeleton and tend to live on reefs (the exoskeletons of dead ancestor corals).

current     A fluid — such as of water or air — that moves in a recognizable direction.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

defense     (in biology) A natural protective action taken or chemical response that occurs when a species confront predators or agents that might harm it. (adj. defensive)

ecosystem     A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.

epidemic     A widespread outbreak of an infectious disease that sickens many people (or other organisms) in a community at the same time. The term also may be applied to non-infectious diseases or conditions that have spread in a similar way.

factor     Something that plays a role in a particular condition or event; a contributor.

global warming     The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

greenhouse gases     Gases that contribute to the greenhouse effect by absorbing heat. Carbon dioxide and methane are two examples of such gases.

habitat     The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

host      (in biology and medicine) The organism (or environment) in which some other thing resides. Humans may be a temporary host for food-poisoning germs or other infective agents.

hurricane     A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.

immune     (adj.) Having to do with the immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.

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.

immunology     The field of biomedicine that deals with the immune system. A doctor or scientist who works in that field is known as an immunologist.

infect     To spread a disease from one organism to another. This usually involves introducing some sort of disease-causing germ to an individual.

infection     A disease that can spread from one organism to another. It’s usually caused by some type of germ.

invertebrate     An animal lacking a backbone. About 90 percent of animal species are invertebrates.

lesion     A tissue or part of the body that shows damage from injury or disease. Lesions come in all shapes and sizes, both inside the body and on its outside. A pus-filled wound on the skin is one example. Cells with holes in them or missing parts due to disease represent a totally different class of lesions.

marine biologist     A scientist who studies creatures that live in ocean water, from bacteria and shellfish to kelp and whales.

microbe     Short for microorganism. A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

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.

monitor     To test, sample or watch something, especially on a regular or ongoing basis.

outbreak     The sudden emergence of disease in a population of people or animals. The term may also be applied to the sudden emergence of devastating natural phenomena, such as earthquakes or tornadoes.

oxygen     A gas that makes up about 21 percent of Earth's atmosphere. All animals and many microorganisms need oxygen to fuel their growth (and metabolism).

pathogen     An organism that causes disease.

plague     (noun) Any horrific infection that spreads easily and kills many people (or other valued organisms), usually quickly. Best known are the infections caused by the bacterium Yersinia pestis. Indeed, they are commonly referred to simply as the plague. (verb) A common term for being beset by unpleasant conditions, events or circumstances — ones that cause serious impacts.  

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.

resilient     (n. resilience) To be able to recover fairly quickly from obstacles or difficult conditions.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

sediment     Material (such as stones and sand) deposited by water, wind or glaciers.

slough     (n.) A swamp or similar type of wetland. (v.) The process of having a layer of skin or some other material fall off. This is usually said to "slough off."

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

strategy     A thoughtful and clever plan for achieving some difficult or challenging goal.

stress     (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem.

survey     To view, examine, measure or evaluate something, often land or broad aspects of a landscape.

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.

symptom     A physical or mental indicator generally regarded to be characteristic of a disease. Sometimes a single symptom — especially a general one, such as fever or pain — can be a sign of any of many different types of injury or disease.

syndrome     Two or more symptoms that together characterize a particular disease, disorder or social condition.

tissue     Made of cells, it is any of the distinct types of materials that 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.

unique     Something that is unlike anything else; the only one of its kind.


Journal:​ ​​J.L. Meyer et al. Microbial community shifts associated with the ongoing stony coral tissue loss disease outbreak on the Florida Reef Tract. Posted online May 3, 2019.

Journal:​ ​​L. Cheng et al. How fast are oceans warmingScience. Vol. 363, January 11, 2019, p. 128. doi: 10.1126/science.aav7619.

Journal:​ ​​L.E. Fuess et al. Life or death: disease-tolerant coral species activate autophagy following immune challengeProceedings of the Royal Society B. Vol. 284, June 14, 2017. doi: 10.1098/rspb.2017.0771.

Journal:​ ​​W.F. Precht et al. Unprecedented disease-related coral mortality in Southeastern FloridaScientific Reports. Vol. 6, August 10, 2016. doi: 10.1038/srep31374.

Journal:​ ​​J. Maynard et al. Projections of climate conditions that increase coral disease susceptibility and pathogen abundance and virulenceNature Climate Change. Vol. 5, May 4, 2015, p. 688. doi: 10.1038/nclimate2625.

Further Reading