Sea star wasting disease has affected over 20 species with sunflower sea stars to be the hardest hit. The mysterious syndrome is estimated to have killed billions since its 2013 emergence.Supplied
Sunflower sea stars were once everywhere along the Pacific coast. Big, colourful and many-armed, they could be spotted at low tide clinging to rocks as if painted there by Vincent van Gogh.
Then came a devastating sickness that turned healthy sea stars into mounds of decaying mush. Known as sea star wasting disease, the mysterious syndrome is estimated to have killed billions of the creatures since it emerged in 2013.
More than 20 species have been affected, with sunflower sea stars among the hardest hit. Their numbers have plummeted by 90 per cent, and the species is listed as critically endangered by the International Union for Conservation of Nature.
Now the cause of the contagion has been found: a bacterium – a member of the vibrio family – whose relatives include pathogens associated with seafood-borne illnesses and with cholera.
The breakthrough, reported Monday in the Nature Ecology and Evolution journal, marks a turning point in the quest to understand why sea stars are dying. It may also aid efforts to help populations recover.
“The exciting thing about it being a bacteria is that it makes it possible for us to isolate and grow it,” said Alyssa Gehman, an adjunct professor at the University of British Columbia and a marine disease ecologist at the B.C.-based Hakai Institute, whose lab spearheaded the work.
And since the strain can be cultured, it also means that Dr. Gehman and her colleagues have been able to report their discovery with an unusually high degree of confidence.
The report sheds light on the connection between sea star disease and climate change. As vibrio bacteria is usually more active in warmer water, rising ocean temperatures along the Pacific coast may explain the disease's growing intensity.Supplied
While the researchers employed the tools of modern molecular biology to arrive at their conclusion, their approach was essentially one developed by medical pioneers in the 19th century to pinpoint the cause of infectious disease.
“It’s very clean work, very thorough,” said Blake Ushijima, a marine microbiologist at the University of North Carolina Wilmington who was not involved in the discovery.
He said that, while the disease cannot be removed from the environment, knowing its cause raises the possibility that captive populations of sea stars can be protected from it and then successfully reintroduced.
The find also sheds light on a long-suspected connection between sea star disease and climate change.
Because vibrio bacteria tend to be more active in warmer water, rising ocean temperatures along the Pacific coast may account for why the disease has become so rampant.
The outcome has wreaked havoc on ocean ecosystems. Sea stars are voracious hunters. In their absence, sea urchin populations have exploded and led to severe overgrazing of kelp forests that provide crucial habitat for other species.
Dr. Gehman said that, when she first undertook the project in 2020 with funding from The Nature Conservancy of California, among other sources, she felt the complexity of the disease would likely preclude finding a definitive cause. But she thought she could help to move the field forward.
“We didn’t expect to get to the place we did,” she said.
Researcher Alyssa Gehman from the Hakai Institute first undertook the project in 2020 with funding from The Nature Conservancy of California and other sources. She did not expect the breakthrough she achieved.Supplied
Much of the work was conducted at the United States Geological Survey Marrowstone Field Station, located near the entrance to Puget Sound. The station includes facilities where sea stars can be kept in quarantine without risk of contamination or of infection being released back into the ecosystem.
It began by setting up a baseline group of specimens that were isolated long enough to be reliably disease free. These were subjected to a series of “challenge experiments” to determine under what circumstances disease would spread.
Dr. Gehman said that, from the outset, she and her colleagues considered all possible causes, including marine viruses and contagious forms of cancer. An important step forward came when the researchers learned they could spread the disease by injecting a body fluid from a sick sea star into a healthy one.
Known as coelomic fluid, the material ceased to be infectious once it was heat treated, pointing to the presence of a live pathogen. Infection was also blocked when the fluid was run through a filter that any viruses should have been able to penetrate.
By systematically combing through the genetic contents of the coelomic fluid, researchers eventually zeroed in on a bacterial cause. This was a Herculean task, since sea stars, like many organisms, host a vast and diverse microbiome. Sick sea stars can also be colonized by secondary infections that are not responsible for the disease but are present alongside it.
In January, 2024, Dr. Gehman and research scientist Melanie Prentice were on a Zoom call with Grace Crandall, a PhD student at the University of Washington who conducted the challenge experiments. As they reviewed their data, they noticed an abundance of vibrio DNA in the genetic readouts. It appeared to be from one particular strain of Vibrio pectenicida, a species known to attack scallop larva.
The researchers suddenly realized they might have hit upon the answer.
“It was a moment of eerie quiet as we were all looking at each other,” Dr. Prentice said. “Part of me was thinking, can we have some streamers coming from the ceiling or something?”
The bacteria, vibrio pectenicida, has been identified as the single definitive cause for the disease and is now cultured and available for further research.Supplied
But to confirm their suspicions, the team needed a diseased specimen to work with and provide infected fluid – not easy to find during the winter months. They quickly put out the call and heard back from a colleague who was dealing with an outbreak in his lab. It was a four-hour drive and a ferry ride away from their location at the University of British Columbia.
“So we just hopped in a car and drove straight there,” Dr. Gehman said. “We arrived at 9 p.m. and sampled the stars in the dark with headlamps.”
Using the sampled material, the group conducted a battery of controlled tests that confirmed their initial suspicions. The results consistently pointed to Vibrio pectenicida as the single definitive cause.
Laboratory cultures of the strain are now available for further research. That includes studies across different species of sea stars to look for variations in response to the disease and across individuals to search for signs of innate resistance.
All of this can inform recovery strategies for bringing back the threatened sea stars.
“This discovery has been so critical and so important,” Dr. Prentice said, “but the most exciting part about it is all the stuff that it’s allowing us to do next.”