When Mya Breitbart heard that something was killing off sea urchins en masse, she thought: Oh, no, not again.

The long-spined sea urchin—a fist-size ball of black defensive spines—is a crucial and common part of the Caribbean’s coral reefs. In 1983, a strange affliction all but wiped them out. The urchins began behaving aberrantly, moving into dangerous open water instead of sheltering in crevices. Their spines, which they normally point at threats, became unresponsive and eventually fell off, making them easy targets for fish. Even if they weren’t attacked, they died within days of their first symptoms, as if they’d been eaten from the inside out. By 1984, up to 98 percent of them were dead—with disastrous consequences. By eating algae, which compete with corals for space, sea urchins allow reefs to flourish and expand. When they died, the algae gained the upper hand, and the corals receded. The urchin population was so badly hit that, despite active restoration efforts, they recovered by only 12 percent in the intervening 40 years. And no one ever worked out what originally killed them.

Then, in January 2022, urchins at St. Thomas island began dying again in the same gruesome way. By March, the affliction had spread to nine other islands. When reports reached Breitbart, who is a microbiologist at the University of South Florida, she feared that history would repeat itself. “It’s not that one dead sea urchin strikes fear in the heart of scientists, but corals are being assaulted and diseased on so many fronts that losing the urchins felt like another big blow to the reefs,” Breitbart told me. “I thought, We can’t let this one go.”

Breitbart quickly assembled a team of 48 scientists from 12 countries, including Ian Hewson, who had studied a similar degenerative disease in starfish, and Christina Kellogg, an expert on corals and their microbes. “Everyone dropped everything” to work on the case, Breitbart said, because they knew how quickly corals can suffer when urchins disappear. But they also knew that such mysteries usually take years or decades to get solved—if they ever are. At first, it looked like this case would be similarly frustrating: When the team collected tissue samples from urchins across the Caribbean and did a thorough genetic analysis to search for disease-causing microbes, they couldn’t find any of the usual suspects. No viruses or bacteria were responsible for the urchins’ plight. The only organism whose genes were present in the sick urchins and not the healthy ones was something unexpected—a previously unknown species of ciliate.  

Ciliates are microscopic, single-celled creatures that swim using a coat of beating hairs, or cilia—picture a furry, turbo-charged amoeba. They’re found almost anywhere there is water, including in the bodies of marine animals such as urchins. Ciliates are usually harmless, but there are a few documented cases of them acting as parasites, causing diseases in corals and even sharks. And one of those miscreant species is a close relative of the new one that Breitbart’s team found in the dying Caribbean urchins.

Still, a ciliate seemed like such an unlikely mass murderer that Breitbart wasn’t fully convinced. Fortunately, while the geneticists were working, team members Yasu Kiryu and Thierry Work were independently examining the urchin tissues under microscopes. Both investigations culminated at a single meeting, and when Team Gene announced that they had found the telltale sequence of a ciliate, Team Microscope pulled up their slides to reveal clear images of ciliates infesting the spines and bodies of the urchins. Both groups had independently identified the same unexpected suspect—and to seal the case, they grew the new ciliate in the laboratory and showed that it could kill healthy urchins within a week.  

That the researchers not only found the urchin killer, but did so in just three months, is an unprecedented feat. Stony corals in Florida and the Caribbean have been plagued by disease since 2014, and the perpetrator is still unknown. The same goes for sea stars in the Pacific Northwest, which have been hit by a degenerative illness for roughly the same amount of time. Breitbart credits her team, experienced members such as Hewson who had learned how to efficiently study these kinds of die-offs, and a “not insignificant amount of luck.” “I had a brand new graduate student working with me,” she said, “and I kept saying, ‘This isn’t the way it usually goes.’”

Many questions still remain. If the same killer ciliate was responsible for the 1983 die-off, why did it stay dormant for almost 40 years before striking again? If it’s a newcomer, where did it come from, and how does it spread? And perhaps most important, what can be done to stop it? Kellogg has also been looking for possible treatments that would kill the ciliate but spare the urchins, and she has some promising candidates.

Hewson thinks the team’s success bodes well: Every time they investigate a die-off of this kind, they get a little better at it—and they’ll need to be. Mass-mortality events among wildlife are becoming more common. Climate change is forcing animals to move to new ranges, allowing species that never previously coexisted to trade pathogens. Meanwhile, climatic upheavals and environmental degradation are also subjecting animals to more stress, weakening their immune system. “Things that maybe weren’t related to mass mortality in the past will start to cause new diseases,” Hewson said. The urchin die-off may not have started in exactly this way, but it’s still a harbinger of events to come. “I’d fully expect us to see more of this kind of thing in the future,” he said.

But 40-plus scientists can’t drop everything to investigate every new epidemic; there simply isn’t enough funding, expertise, or person-power to go around. In this case, doing so was clearly worth it, because the long-spined sea urchin is a keystone species—ones that play a disproportionately influential role in holding their ecosystem together. In this age of epidemics, such creatures may get triaged while others are neglected. “It’s going to be a matter of prioritizing, which is very hard,” Breitbart said.