A silent killer is spreading throughout the waters of the Caribbean. Known as stony coral tissue loss disease, it afflicts at least 22 species, including some of the largest, oldest, and most important reef-building corals. Infected colonies develop white patches that slowly enlarge, draining the color and life from the animals. In the most susceptible species, such as pillar, brain, and star corals, infected colonies usually die within weeks or months.
It’s the “worst thing I’ve ever seen,” says William Precht, a coral disease specialist in Florida.
Stony coral tissue loss disease, abbreviated as SCTLD, was discovered in the fall of 2014 in corals off Miami. The disease, likely spread by a bacterium or virus or some combination thereof, has already expanded throughout Florida’s coast and much of the northern Caribbean. It’s now present in at least 20 countries, from Mexico to Honduras to St. Lucia. And in May 2021, corals became infected with the disease in Florida’s Dry Tortugas National Park, a hot spot for coral diversity.
Researchers, including Precht, are racing to discover what causes the condition, how it spreads, and how to help treat it. Though the disease propagates slowly via water currents, new research suggests that it may also be spread by commercial shipping vessels at major ports, such as in the Bahamas.
Caribbean corals—which form the basis of their reef ecosystems—are already threatened by warming waters due to climate change, pollution, nutrient runoff, and more, which makes solving the mystery all the more pressing.
The summer of 2014 was bad for coral in Florida. A heat wave caused the water temperature to jump to a record high off the coast, causing a massive bleaching event. This happens when stressed corals expel the symbiotic algae, called zooxanthellae, that keep them alive. Corals can recover from bleaching, but it can weaken and leave them susceptible to disease. (Read more: Acidification threatens Florida’s coral reefs.)
At the time, Precht, the chief scientist of a Miami-based environmental consulting company, Dial Cordy and Associates, was running a series of monitoring stations on coral reefs around Miami to keep track of any impacts caused by a dredging project in the area.
In October, one of the company’s divers, named Ryan Fura, saw a few corals that looked “a little funky” on a reef a short distance from the outflow of the Miami-Dade County water treatment plant, Precht recalls. Over the next few weeks, the as-yet-unknown ailment seemed to spread rapidly. Precht visited the affected reef in early November to check it out himself.
More than half of the corals appeared to be infected, and some were already dead. “I couldn’t believe my eyes,” he says. “It was absolutely sickening.”
The disease’s appearance elsewhere has often been likewise sudden—and devastating. In October 2019, the disease had not yet arrived in the Bahamas, in part because the prevailing ocean current runs northward up the Florida coast. That month, marine ecologist Craig Dahlgren and colleagues surveyed about 60 miles of reef and found no sick coral. Yet by November, the team was getting reports that corals near Freeport had an unknown infection—which soon proved to be SCTLD.
During another extensive survey in March 2020, Dahlgren, with the Perry Institute for Marine Science, once again surveyed more than 60 miles of reef—and found infected corals in every site, particularly brain and pillar corals. Within months, the vast majority of the infected colonies were dead.
Left: A colony of grooved brain coral—a species highly susceptible to SCTLD—is seen in a laboratory near Tampa, Florida. Researchers are studying such corals in a race to find the cause of the disease and how to treat it.
Right: Spiny flower coral is one of at least 22 species that can be infected by SCTLD. Brain and pillar corals are also particularly vulnerable to the disease. PHOTOGRAPH BY LUCAS JACKSON/REUTERS
Many of the affected corals form the dominant structures of reefs—such as the large, striking columns of pillar corals—and can live for centuries.
“Colonies that took hundreds of years to grow can be wiped out in a matter of weeks,” he says.
In July 2021, Dahlgren and co-authors published a study showing the disease radiated from the commercial ports of Freeport and Nassau. A reasonable explanation for this pattern is that commercial shipping vessels are spreading the disease, Dahlgren says. One possibility is that the pathogens are being carried in commercial ships’ ballast water, which is held in tanks to stabilize these huge vessels. However, more research needs to be done to confirm this hypothesis, Precht says.
The disease also arrived suddenly in the U.S. Virgin Islands in January 2019, near the commercial shipping port of Crown Bay, on the Island of St. Thomas. It then gradually spread around the island and to neighboring St. John, jumping suddenly to two separate locations in St. Croix—both near commercial shipping ports, says Marilyn Brandt, who studies coral at the University of the Virgin Islands, on the island of St. Thomas.
The disease has been devastating to Virgin Islands reefs, which in some places have lost between a half and three-quarters of their coral within two years of the first infection, she says.
“Everything I’ve seen in the past pales in comparison to this,” says Brandt.
Brandt fears the loss of coral will harm fish populations, as well as negatively impact the economy, which depends heavily on coral reef snorkeling and diving tourism.
The coast guards of various countries, such as the Bahamas, have issued recommendations to ships to not exchange ballast water within ports, but so far, few enforceable laws have been passed to stop the practice.
To prevent the disease from spreading between islands, ships need to be more careful about how and when they exchange ballast water, and avoid releasing it near ports and coral reefs, Dahlgren says. (Learn more: Window to save world’s coral reefs closing rapidly.)
Nobody knows for sure yet what causes the disease—but dozens of researchers are working to identify it.
One tantalizing clue emerged in a case report published online this fall by U.S. Geological Survey researcher Thierry Work. While peering at infected coral cells with an electron microscope, he noticed that that these corals’ zooxanthellae cells appeared to be full of holes. Within the degraded cells, he found curious strand-like particles—“like packed spaghetti,” Work says.
These strands turned out to be unidentified viruses, similar in size and shape to plant viruses in the family Flexiviridae. Work can’t prove that these viruses are causing the disease, but he suspects that they are playing an important role, and several researchers are following up on this finding.
But there are reasons for skepticism. For one, infected corals respond well to antibiotics, which kill bacteria, not viruses. On the other hand, antibiotics can have stimulatory effects on the immune system that cause effects beyond their intended targets. Preliminary work also shows some infected corals respond favorably to antiviral treatments.
Secondly, seemingly healthy corals that Work examined also had viral particles in their zooxanthellae. But Work thinks that these corals might not have been as healthy as assumed—and that perhaps they, too, were likely to get sick in the future—or had asymptomatic disease.
Some researchers suspect a bacterium is a more likely culprit. Brandt and Erinn Muller, a biologist with the Mote Marine Laboratory in Sarasota, looked at what types of bacteria were most prevalent in diseased corals in the Virgin Islands and Florida, respectively.
Precht agrees the cause is likely a bacteria, perhaps similar to one that causes a known coral disease called white plague.
There is likely no single culprit, either. For one, heat-stressed corals are more likely to be infected by any pathogen. In addition, coral diseases are often caused by more than one pathogen.
“It has to be complex, because there’s not a clear signal that’s come out of any of our studies,” says Amy Apprill, a biologist with Woods Hole Oceanographic Institution who has studied the disease. She suspects there’s a complicated interplay between one or more pathogens—perhaps even including bacteria and viruses—and the microbiome of the coral.
Julie Meyer, a marine microbiologist at the University of Florida, agrees that it’s likely a disease caused by multiple microbes. As part of her research, she’s sequenced the genome of all microbes present in the coral to look for clues about a cause.
One reason the research is so challenging is that “the ocean is basically a bath of bacteria and viruses,” Meyer says. Furthermore, not much is known about coral diseases in general, or the intricacies of coral immune systems—let alone the immune systems of the 22 different affected species.
On the plus side, there’s a massive amount of research going on right now, with multiple papers submitted for publication every week.
“This is a huge crisis,” Brandt says. “The whole community is throwing everything they have at this.”