Christopher Harley, a marine biologist at the University of British Columbia, thought of himself as a reasonably dispassionate scientist. But one day in 2014 while out counting starfish, he thought he might cry.
Inspecting the tide pools along one of his favorite shorelines on Vancouver Island, Dr. Marley saw something new and alarming: sea stars in several stages of death, with their arms twisted or missing. The devastation went on and on. One of his favorite parts of the ecosystem — the big, colorful, “supremely weird” sea stars — was “dissolving away,” he recalled recently.
Since 2013, an epidemic of sea star wasting disease along the Pacific Coast of North America has caused billions of sea stars to twist and disintegrate. Sunflower sea stars, one of the world’s largest starfish, lost 90 percent of its population over the last decade. Populations of sea urchins, typically preyed on by sea stars, overran and wiped out kelp forests.
The cause of the disease remained elusive. But on Monday, a new study identified the killer: a sneaky bacteria known as Vibrio pectenicida.
“I have been waiting for this for a long time,” said Dr. Harley, who was not involved with the research.
A team of scientists led by Alyssa-Lois Gehman, a marine ecologist at the Hakai Institute in British Columbia, conducted experiments over four years to find the culprit, which was “hiding in plain sight,” Dr. Gehman said. The findings, published in Nature Ecology and Evolution, open many avenues of recovery for the sea star species that were previously unachievable without knowing the cause of the disease.
In the study, Dr. Gehman and her team collected animals from the field and quarantined them. If some became sick, the researchers placed them close to healthy sea stars to see if these became sick, too. The newly infected stars became the starting points: The researchers took samples of coelomic fluid from the animals’ bodies and inserted it into healthy stars. Coelomic fluid is the blood of sea stars, ferrying nutrients around internally.
Focusing not on tissue samples but on coelomic fluid was a “big breakthrough,” said Drew Harvell, a disease ecologist at the University of Washington and part of the research team.
In parallel, the researchers heated some samples of the coelomic fluid, to perhaps kill whatever was in it, and injected it into other healthy sea stars. For the duration of the experiments, none of the sea stars injected with the heat-treated fluid died or dropped an arm, Dr. Gehman said. In contrast, 90 to 100 percent of sea stars exposed to untreated fluid died.
A genetic analysis of the treated and untreated sea-star fluids revealed the presence of one key organism in the samples with the disease: Vibrio pectenicida. The team later isolated the bacteria and grew it in a culture in the lab. Subsequent experiments with it in the field confirmed that it was the culprit.
“To have one pathogen, V. pec, stand out so clearly as causing the disease was surprising and exciting,” Dr. Gehman said.
Dr. Harvell, called the finding “incredibly fulfilling and important” after so many years of effort. “For me, it’s the discovery of the decade.”
The disease ravaged populations of 20 sea star species along the Pacific Coast, and a number of efforts have been underway to restore these groups. A recovery program for sunflower sea stars, led by the Nature Conservancy, aims to restore the animal to its native habitat, and over a dozen aquariums are collaborating to captive-breed them. Knowing the cause should make it easier to refine and expand such efforts, experts said.
Lauren Schiebelhut, an evolutionary ecologist and geneticist at the Sunflower Star Laboratory in Moss Landing, Calif., said that one promising path involved selecting individual sea stars that showed the greatest resistance to the bacterium. Another involved probiotics, equipping sea stars with other microorganisms to help them deal with pathogens — a technique that has worked with corals.
“Those are just a few of the things that opened the door that we couldn’t do,” Dr. Schiebelhut said.
Many questions remain. How is the bacterium transmitted from one sea star to another? Is the bacterium native to the coasts of North America, or was it introduced? Is it accumulating in the food of the sea stars?
“Where did it come from?” said Kevin Lafferty, an ecologist at the U. S. Geological Survey. If it was introduced through aquaculture, researchers could work on preventing future spreads, he said.
When the wasting disease hit California in 2013, “it felt like a sea star apocalypse,” Dr. Lafferty said. He recalled snorkeling in a kelp forest and seeing hundreds of sea stars twisted, melting or reduced to piles of skeletal shards.
Now, he is excited when he sees even a few sea stars. “Maybe these are the lucky ones from the old saying, ‘Thank your lucky stars,’” he said.
Alexa Robles-Gil is a science reporter and a member of the 2025-26 Times Fellowship class, a program for journalists early in their careers.
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