A team led by researchers in British Columbia has solved the mystery of a gruesome disease that has killed billions of sea stars along the Pacific coast of North America, more than a decade after the die off.
Melanie Prentice, the lead author of a new study, recalls a moment of “not really believing it” when researchers found a strain of bacteria that was abundant in diseased sea stars and absent in healthy ones.
“My initial reaction was like, ‘Okay, so I’ve done something wrong,’” she said.
Prentice said the team spent months trying to disprove their findings, ultimately confirming they had cracked the code of the disease.
They found the bacterium Vibrio pectenicida is a clear cause of sea star wasting disease.
“(It’s) a question that researchers have been trying to answer for about 12 years, so we’re beyond thrilled,” said Prentice, a research associate at the Hakai Institute and the University of B.C. department of earth, ocean and atmospheric sciences.
The paper detailing the four-year research project and its findings were published online in the peer-reviewed journal Nature Ecology & Evolution on Monday.
Alyssa Gehman, who helped launch the project in 2021, described the disease as “gruesome,” causing sea stars to develop lesions, lose their arms and “disappear into mush” about a week or two after exposure to the pathogen.
It has been especially deadly for sunflower sea stars, killing about six billion of the species that can sprout 24 arms and span up to a metre.
The giant sea stars are now considered functionally extinct across much of their former range off the coast of the continental United States, with losses exceeding 87 per cent in the “northern refuges” where they still persist, the study said.
The collapse has had cascading impacts, including widespread losses of ecologically, culturally and economically important kelp forests.
“I think we didn’t really appreciate how important they were until we lost them,” Prentice said, describing the orange, purple or brown sunflower stars as a “keystone” species with an outsized impact on their ecosystem.
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The giant sea stars are top predators, striking fear into other invertebrates.
“Almost everything that lives on the ground underwater runs away from them when they’re coming,” said Gehman, a marine disease ecologist at the Hakai Institute and an adjunct professor at the University of B.C.’s Institute of Oceans and Fisheries.
They keep sea urchin populations in check, in turn ensuring the health of help forests that provide habitat and food for numerous other species.
The devastation of the sunflower sea stars has caused a “total ecosystem shift,” Prentice said, transforming biodiverse kelp forests into “urchin barrens.”
The bacterium that causes sea star wasting disease had remained elusive for more than a decade since sea stars were first observed dying in large numbers in 2013. The same bacterium has been known to attack scallop larvae.
Prentice said the breakthrough came after the research team switched from examining diseased tissues to focusing on the sea stars’ coelomic fluid, likening it to the blood of the sea star.
Earlier research had involved running the tissues through tiny membrane filters that would have excluded bacteria, which are typically larger than viruses, she explained.
The Hakai Institute team started by replicating the initial experiments, but they weren’t able to cause disease in healthy sea stars, she said.
“We were doing everything we could and we were just never ever able to cause disease, and so to us that suggested that the pathogen is larger than a virus.”
However, after pivoting to coelomic fluid, which Prentice described as “essentially sea water,” the researchers did trigger disease in healthy sea stars.
“That suggested that the pathogen was in that fluid, and so then we just end up working with a much cleaner, easier tissue type to investigate,” she said.
From there, Prentice created a list of all the different microbial species found in wasting sea stars and compare it against the healthy stars in the lab.
“I finally got to a place where I generated these different lists and it was very evident right away that there (were) tons of different Vibrio species within our wasting sea stars and we weren’t really seeing that in our healthy sea stars,” she said.
Prentice said she then filtered the genetic data to look at each strain of Vibrio bacteria, which led to their eureka moment with Vibrio pectenicida.
“We just saw it in every single wasting sea star sample, and then we looked at our controls and it was just not in any of them,” she said.
Prentice said other researchers had wished her “good luck” when she joined the project, but there was skepticism over whether they would solve the mystery.
It felt “incredible” to be part of a discovery that could help make a meaningful difference in the recovery of sea stars and their ecosystems, she said.
Gehman, too, said she wasn’t sure the project would result in a singular answer.
“I thought it would be complicated. I thought there would be multiple things relying on other things,” she said. “This was much clearer than I was expecting.”
The discovery allows researchers to turn their efforts to deeper questions, including the possible role of warming ocean temperatures and the potential to breed sea stars in captivity to promote disease resistance and spur recovery, she said.
The disease now appears to be seasonal, with outbreaks occurring in the warmer months, suggesting temperature may be a factor, said Gehman, adding she will soon conduct temperature experiments to investigate further.
“Does Vibrio pectenicida grow faster at warmer temperatures and the sea star can sort of survive at the growth rates at cooler temperatures, but when you get to warmer temperatures, they can’t, is that what’s happening?”
The findings could help researchers understand where sea stars may struggle or survive with climate change in the future, Gehman said.
Prentice said there are “remnant” populations of sunflower stars along the B.C. coast, and its “very possible” some could be more resistant to the wasting disease.
She said finding and selectively breeding sea stars with a higher capacity to fight off the disease could produce ‘superstar’ sea stars for reintroduction in the wild.
“It seems like science fiction sometimes, but people are working on it,” she said.
