Scientists identify culprit killing sea stars in the Pacific Ocean

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More than a decade after a mysterious sickness began killing billions of sea stars off the Pacific Coast, scientists say they've identified the bacteria that causes the deadly disease.

A team of at least 15 scientists from a half-dozen organizations collaborated on the research, hoping to figure out what had killed the sea stars. Solving that riddle would allow work to begin on recovering the species and the ecosystems harmed by their decline.

After four years of testing the creatures and scrutinizing the results of their DNA analyses, the researchers found a bacteria always present on the sick sea stars that wasn't on healthy ones.

The study raises hopes for a brighter future for sea stars, for potential treatments and for possibly restoring the kelp forests that relied on them to keep sea urchins under control, said two of its co-authors, Melanie Prentice and Alyssa Gehman, colleagues at the Hakai Institute in British Columbia and the University of British Columbia. The study announcing the project's results was published Aug. 4 in the journal “Nature Ecology and Evolution.”

Although most people think of the star-shaped animals as starfish, and some of them are even named starfish, scientists call them “sea stars” because they’re not fish. They're a group of animals, including sea cucumbers and sea urchins, called echinoderms.

What happened to sea stars?

From a tide pool in Olympic National Park, along the Pacific coast of Washington state, the first case of sea star disease was reported in June 2013.

Soon, sea stars fell ill in Sitka, Alaska, then off the coast of British Columbia and San Diego, and eventually as far south as Mexico. Sea stars that contracted the disease became lethargic, developed lesions, lost their arms and disintegrated into gooey masses within days, according to the National Oceanic and Atmospheric Administration.

The pathogen could kill a healthy group of sea stars in 24 hours, Paste BN reported in 2013.

Between 2013 and 2017, the pathogen killed more than 90% of sunflower sea stars in one of the largest marine wildlife disease outbreaks on record, NOAA concluded. These sea stars can measure more than 3 feet from tip to tip and appear in a range of colors.

Once one of the more abundant and recognizable members of the sea star family on the Pacific coast, sunflower sea star populations plummeted offshore south of Washington State and disappeared almost entirely from the coast of Southern California. Although they are the most susceptible to the pathogen, the disease has been identified in more than 20 sea star species.

Vast kelp forests in the ocean also suffered, because it turns out the sea stars were helping to control the voracious sea urchins that, left unchecked, decimated the kelp.

What did the researchers do?

A large group of researchers met in February 2020, just before the COVID-19 pandemic gripped the planet, and strategized on the science necessary to try and recover sunflower sea stars, said Prentice and Gehman, in a joint interview with Paste BN.

Their meeting and subsequent collaboration also included The Nature Conservancy, the Tula Foundation, and researchers from the University of Washington and the U.S. Geological Survey.

The scientists collected wild sunflower sea stars in at least six locations in British Columbia and Washington between 2021 and 2024, then conducted a set of controlled experiments.

In a fluke of history, the scientists were working to stop the spread of a deadly virus in sea stars at the same time the world was dealing with the pandemic. As they were quarantining sea stars they had collected from the wild, team members who traveled were quarantining and isolating themselves to prevent the spread of COVID-19.

Sea star mystery unveiled

Was there ever an "aha" moment? Yes, Prentice said. One day, she was looking at data on microbes they’d collected from healthy sea stars and comparing them to data sets of microbes from wasting sea stars. Just as she was about to walk into a meeting with Gehman and Grace Crandall, another colleague and study co-author, she noticed there were “tons” of vibrio pathogens in the ailing sea stars.

In their meeting, she opened her computer, showing Gehman and Crandall what she’d learned, and they started breaking down the genetic sequences of the vibrio they were seeing. The culprit was identified as Vibrio pectenicida.

"It was apparent immediately that it was in all of our wasting samples and none of our healthy samples," Prentice said.

They spent another year, working to back up their conclusions and kept getting more evidence that it was the pathogen causing the disease.

What is vibrio?

Vibrio is one of a vast array of species in the marine environment, and many are known to be pathogens. One kind of vibrio kills oysters and can be deadly to people, commonly referred to as “flesh-eating" bacteria. Another causes cholera.

One thing that was particularly shocking about their moment of discovery, said Gehman, was that the group of scientists had assumed finding the answer would be complicated.

“It turns out that the pattern was visual and we could really see it," she said. "That was surprising, and was the moment we realized we might be able to solve this question."

Kevin Lafferty, a marine disease ecologist and senior scientist with the U.S. Geological Survey, who was not a part of the study group, wrote a companion piece in the journal lauding the group's work.

"The research is really amazing and took so much careful, hard work," Lafferty told Paste BN. Marine diseases can be difficult to diagnose "we just don't have a lot of expertise in marine disease.

"That's a huge handicap and they overcame that with a lot of hard work and modern molecular tools," he said. "The fact that you can take an eyedropper of sea water and identify 100 species of bacteria is a huge leap forward for understanding microbial diseases, especially in the water."

What happens now for sea stars?

This discovery "opens up a door that wasn’t available to us because we didn’t know what was causing the disease," Gehman said.

"There's a lot of people who are trying to save this species," she said. "There's a lot of work that can immediately take advantage of what we are doing."

Researchers already are working on a diagnostic test akin to a COVID test that could be “really important” for testing sea stars and the water when they consider transplanting sea stars to try to recover populations, Prentice said. “It’s going to be really, really helpful in the way we think about managing the species.”

Questions remain, wrote Lafferty, the USGS scientist. The origin of the pathogen is unknown, he said. Could it be transmitted from the mollusks that sea stars eat, the way people get sick after eating oysters infected with a vibrio pathogen? Could it be transmitted from sea star to sea star or spread through aquaculture? These questions and more are expected to be part of the future research possibilities now open.

Because there are remnant healthy populations of sea stars in Alaska and British Columbia, and a few in Washington, armed with the information that identifies the disease, Gehman and Prentice said researchers hope to be able to bring some of the healthy sea stars into captive breeding programs to raise animals that can resist the pathogen.

Other possibilities include finding a probiotic that could help sea stars fight off disease, similar to a method scientists use to pretreat some corals, or identifying a naturally occurring marine virus that only attacks specific kinds of bacteria, such as the virus that researchers are using to help recover abalone populations off the southern California coast.

“Hopefully one day we’ll actually be out planting sea stars back into the wild where we’ve lost them," Prentice said.

(This story has been updated to correct a typo.)

Contributing: Elizabeth Weise, Paste BN