A research diver checks on a sunflower sea star that was raised at Friday Harbour Labs on San Juan Island, Washington, and introduced to the waters of Puget Sound for a few weeks. Photo / Ralph Pace, The Nature Conservancy
It starts with a twist. One arm pretzels in on itself. Then another. Then another.
Before long, the writhing arms detach from the body and begin crawling away zombie-like on their own.
The skin festers with lesions and internal organs ooze out from the inside. By the end, the starfish is nothing more than a puddle of goo.
This scene worthy of a horror movie has replayed billions of times along the Pacific Coast.
Over the past decade, a mysterious wasting disease has ravaged some 20 species of starfish, also called sea stars, from Alaska in the United States to Baja California in Mexico.
Since the start of the outbreak in 2013, scientists have struggled to pinpoint the cause.
However, a group of researchers said today that they had finally identified a culprit: a bacterial cousin of the pathogen behind cholera.
“I’ve wondered and thought about this, and felt haunted by the fact that we did not know what the agent was,” said Drew Harvell, a researcher at Cornell and the University of Washington who co-wrote a paper in the journal Nature Ecology and Evolution on the discovery.
“So it’s personally incredibly fulfilling to me to have such a solid answer after all this time,” she said.
Knowing the cause of the wasting syndrome is key to conservation efforts, including plans to breed a galaxy of disease-resistant starfish and restore the ecosystems in which they live.
Fewer starfish has led to a surge in sea urchins upon which they prey. The urchins, in turn, have mowed through stands of kelp that are needed to help sequester carbon and guard against coastal erosion.
“The lack of understanding what the disease is has really been a pretty major impediment to being able to move forward with all the kinds of restoration strategies that we’d like to be able to do,” said Jason Hodin, a senior research scientist who breeds starfish at the University of Washington who also co-authored the paper.
Yet not every starfish scientist is convinced the strain of Vibrio pectenicida bacteria identified in the study is really behind the die-off, considered the largest recorded marine epidemic in the wild.
Researchers have got it wrong before: incorrectly attributing the wasting disease to a virus.
“It’s absolutely critical not to jump the gun,” Cornell marine biologist Ian Hewson said.
For the most recent study, the researchers focused on the sunflower sea star, a voracious predator as wide as a hula hoop that corrals and consumes clams and sea urchins with up to two dozen limbs.
Since 2013, more than five billion have died of the wasting disease. By 2023, the situation was so bad that federal officials proposed protecting it under the Endangered Species Act.
In the research experiments, which lasted for four years, healthy starfish died after being exposed to untreated fluids extracted from sick stars, while those exposed to fluids that were heated were fine, suggesting a microbe is behind the disease.
A genetic analysis of the deceased starfish revealed the first hints that a strain of Vibrio was responsible. The microbe is in the same genus as the bacteria that cause cholera, as well as certain foodborne illnesses.
The researchers found the same genetic markers of the pathogen in wild sunflower sea stars wasting away off the coast of British Columbia in Canada, and Alaska.
The most convincing piece of evidence came after the researchers isolated and grew a pure culture of the Vibrio bacteria. The vast majority of sunflower sea stars exposed to that strain in the lab melted away while healthy controls lived on.
Melanie Prentice, an evolutionary ecologist at the Hakai Institute and the University of British Columbia who led the work, said her team spent years trying to poke holes in their theory that the bacteria was causing the wasting.
“Eventually, we had to be like, ‘Okay, maybe we’re right here.’ But it was a long process.”
Hewson, who was not involved in the study, cautioned against overinterpreting the results.
He noted that the team experimented on one type of starfish, meaning other species hit with wasting may have fallen victim to a different pathogen or changes in the environment.
Injecting starfish with bacteria, which the team did in some of its experiments, may induce a response not seen in the wild.
And the gruesome process of dissolving into goo, he added, may be a stress response to many different factors.
He said: “It’s really, really hard to do these type of experiments, and particularly the interpretation of it is somewhat difficult”.
In 2014, a team of researchers that included Hewson and Harvell published a paper identifying a kind of virus called a densovirus as the cause of the mass starfish mortality. In subsequent experiments, some of those same researchers were unable to replicate the results.
This time, Alyssa-Lois Gehman, a marine disease ecologist also at the Hakai Institute and University of British Columbia and a study co-author, is confident they got it right because the team was able to grow colonies of the Vibrio bacteria outside of the sea stars and use them to reinfect healthy starfish.
In the past, the researchers weren’t able to grow a pure culture of the densovirus in the lab and complete that last step.
In some of its experiments, the team showed that the disease can spread not just through injections but also when starfish simply share the same water.
“We knew we were working in a system where the answer was not correct the first time, and so we were very careful,” Gehman said.
“We spent a lot of time confirming everything that we did. Every experiment was rerun. We didn’t rely on one trial to give us answers.”
Now, scientists want to find potential reservoirs of the bacteria in the open ocean, test other species of starfish in the lab to see how they respond to the pathogen and see if warming waters fuel outbreaks.
The ultimate goal is to breed sunflower sea stars that can tolerate the wasting disease and use them to repopulate the Pacific.
“This discovery is huge to the success of that programme,” Harvell said. “It was just not going to be possible if we didn’t know what was killing it.”