(CMR) Divers monitoring coral reefs off St. Thomas in the U.S. Virgin Islands in January noticed something alarming: Big white lesions were eating into the colorful tissues of hundreds of stony corals. Some corals were dead by the next day — only their stark white skeletons remained. Others languished for up to two weeks. Within four months, more than half of the reef suffered the same demise.
What’s killing the corals is far from clear, but the prime suspect is stony coral tissue loss disease, sometimes referred to by its initials SCTLD or by the nickname “skittle-D.” This infection, discovered off Florida in 2014, is responsible for what some scientists consider one of the deadliest coral disease outbreaks on record.
In the Caribbean, the disease is now ravaging about a third of the region’s 65 reef-building species, scientists estimate. Yet researchers aren’t even sure if the disease is viral, bacterial or some other microbial mix. Whatever the cause, “it’s annihilating whole species,” says coral ecologist Marilyn Brandt, who is leading a science team trying to tackle the outbreak from multiple research angles.
Past outbreaks of other coral diseases near St. Thomas have cut coral cover by up to 50 percent over a year, says Brandt, of the University of the Virgin Islands. But this new disease has done the same amount of damage in half that time — spreading faster and killing more corals than any past outbreaks in the area.
“It marches along the reef and rarely leaves corals behind,” Brandt says. “We’re pretty scared.”
Coral reefs occupy less than 2 percent of the ocean floor. But they play a crucial role in the ecosystem, sustaining an estimated quarter of marine species. Sometimes mistaken for rocks or plants, corals are actually collectives of coral polyps, tiny invertebrates that get sick just like any other animal. Corals sometimes succumb to deadly plagues. Other times, they can shake off milder maladies akin to a common cold.
Since the first coral disease was documented in the 1970s in the Caribbean, researchers have identified dozens more around the world, with the Caribbean now considered a coral disease hot spot. But scientists still know little about these illnesses and how they work. Many marine microbes don’t grow well in petri dishes and test tubes, so studying coral diseases is tough, Brandt says.
Even the names given to the diseases are vague, based only on the visual cues of an infection, such as yellow-band disease, dark-spot syndrome and white plague. And it doesn’t help that many look similar. Stony coral tissue loss disease, which first attacks brain corals before moving on to other stony corals, was initially mistaken for white plague.
Off southeast Florida, the outbreak has persisted for five years. In that time, the disease has affected almost all of a 580-kilometer stretch of reef, including the Florida Keys, says marine biologist Karen Neely of Nova Southeastern University in Fort Lauderdale, Fla. Such a prolonged assault surprised scientists. Coral disease outbreaks typically burn out after a few months.