A wasting disease killed millions of sea stars. After years of searching, scientists just found a cause.

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'It was like a battleground,' Drew Harvell remembers. 'It was really horrible.'
She's reflecting on a time in December 2013, on the coast of Washington state, when she went out at low tide and saw hundreds of sick, dying sea stars. 'There were arms that had just fallen off the stars,' she says. 'It was really like a bomb had gone off.'
The stars were suffering from something known as sea star wasting disease. It's a sickness that sounds like something out of a horror movie: Stars can develop lesions in their bodies. Eventually, their arms can detach and crawl away from them before the stars disintegrate completely.
Harvell is a longtime marine ecologist whose specialty is marine diseases. And she was out for this low tide in 2013 because a massive outbreak of this seastar wasting had started spreading up and down the West Coast — from Mexico to Alaska — ultimately affecting around 20 distinct species of sea stars and wiping out entire populations in droves. In the decade since, some species have been able to bounce back, but others, like the sunflower sea star, continue to struggle. In California, for example, sunflower stars have almost completely died out.
The question in 2013 was: What, exactly, was killing all these stars? While marine ecologists like Harvell could recognize the symptoms of seastar wasting, they weren't actually sure what was causing the disease. From the very beginning, though, it was something they wanted to figure out. And so, soon after the outbreak started, they collected sea stars to see if they could find a pathogen or other cause responsible for the wasting. The hunt for the culprit of this terrible, mysterious disease was on.
Unfortunately, it was not straightforward.
' When this disease outbreak happened, we knew quite little about what was normal [in sea stars],' says Alyssa Gehman, who is also a marine disease ecologist. She says that when researchers are trying to do similar work to chase down a pathogen in, say, humans, they have an enormous trove of information to draw on about what bacteria and viruses are common to the human body, and what might be unusual. Not so for sea stars. ' We maybe had a little bit of information, but absolutely not enough to be able to really tease that out easily.'
Also, Gehman says, there can be a lag before the disease expresses itself, so some stars have the pathogen that caused the disease, but don't present with symptoms yet, making it harder for scientists to even distinguish between sick stars and healthy ones as they run their tests.
So even though a research team identified a virus that they thought might be associated with the wasting disease as early as 2014, over time, it became clear that it was most likely not the culprit, but rather just a virus present in many sea stars.
'The results were always confusing,' Harvell remembers.
In the decade since the initial mass outbreak, other researchers have proposed other theories, but none have brought them to a definitive answer either. And yet, it became increasingly clear that an answer was needed, because people started to realize just how important the sunflower stars they had lost really were.
' We actually learned a lot from losing so many of these animals at once,' Gehman says.
Before the outbreak, she says, they'd known that sunflower stars — giant sea stars that can be the size of dinner plates, or even bike tires — were skillful hunters and voracious eaters. They even knew that many things on the seafloor would run away from them. Gehman remembers taking a class on invertebrates back in college, where she learned that if you put even just the arm of a sunflower star in a tank with scallops, 'the tank would explode with scallops swimming everywhere trying to get away.'
But all that fearsome hunting was, it seems, pretty key to ecosystem health. In many places, she says, ' after the sea sunflower stars were lost, the urchin populations exploded.'
And so the die-off of the sunflower star and the explosion of urchins has been connected to the collapse of the Northern California kelp forests, a marine ecosystem that provides a home for a rich diversity of species.
A cross-state, cross-organizational partnership between the Nature Conservancy and a variety of research institutions is working hard to breed sunflower seastars in captivity in the hopes that they can be reintroduced to the coast and reassume their role in their ecosystems. But as Harvell remembers, she and Gehman knew that no recovery project would be successful if they couldn't find the cause of sea star wasting disease.
'You're not gonna be able to get these stars back in nature if you don't know what's killing them,' she says.
So in 2021, as part of the larger partnership, Harvell and Gehman, along with a number of their colleagues, launched into an epidemiological detective project. Their quest: to finally pin down the cause of seastar wasting disease.
'Really the work over the four years was done in the trenches by Dr. Melanie Prentice and Dr. Alyssa Gehman,' Harvell says, 'and then one of my students, Grace Crandall.'
It was an emotionally difficult project because it required Gehman and her colleagues to deliberately infect many stars with the disease.
'It feels bad,' she admits, and they would be open about that in the lab, 'but we also can remember that we're doing this for the good of the whole species.'
That work has paid off, though, and now, after four years of research, they've nailed their culprit in a paper out in Nature Ecology & Evolution today.
What follows is a conversation with Drew Harvell, edited for clarity and length, about what she and her collaborators found, how marine ecologists do this kind of detective work, and what identifying the culprit could mean for the future health of seastars.
How did you start the journey to figure out what actually had happened?
Well, we chose to work with the sunflower star because we knew it was the most susceptible and therefore was going to give us the most clear-cut results. So we set up at Marrowstone Point, which was the USGS Fisheries virus lab [in Washington state], because that would give us the proper quarantine conditions and lots of running seawater.
The proper quarantine conditions — what does that mean?
All of the outflow water has to be cleansed of any potential virus or bacterium, and so all of the water has to be run through virus filters and also actually bleached in the end, so that we're sure that nothing could get out.
We did not want to do this work at our lab, Friday Harbor Labs, or at any of the Hakai labs in Canada because we were really worried that if we were holding animals with an infectious agent in our tanks without really stringent quarantine protocols, that we could be contributing to the outbreak.
So you have these sea stars. They're in this quarantined environment. What is the methodology here? What are you doing to them or with them?
So the question is: Is there something in a diseased star that's making a healthy star sick? And that's like the most important thing to demonstrate right from the beginning — that it is somehow transmissible.
And so Melanie and Alyssa early on showed that even water that washed over a sick star would make healthy stars sick, and if you co-house them in the same aquarium, the healthy ones would always get sick when they were anywhere near or exposed to the water from a diseased star.
There's something in the water.
That's right. There's something in the water. But they wanted to refine it a little bit more and know that it was something directly from the diseased star. And so they created a slurry from the tissues of the disease star and injected that into the healthy star to be able to show that there really was something infectious from the disease star that was making the healthy star sick and then die.
And then you control those kinds of what we call 'challenge experiments' by inactivating in some way that slurry of infected disease stuff. And in this case, what they were able to do was to 'heat-kill' [any pathogens in this slurry] by heating it up. And so the thing that was very successful right from the beginning was that the stars that were infected with a presumptive disease got sick and died, and the controls essentially stayed healthy.
You do that control to make sure that it's not like…injecting a slurry into a star is what makes them sick?
That's right. And you're also having animals come in sick, right? So you want to know that they weren't just gonna get sick anyway. You want to be sure that it was what you did that actually affected their health status.
So you have a slurry — like a milkshake of sea star — and you know that within it is a problematic agent of some kind. How do you figure out what is in that milkshake that is the problem?
The real breakthrough came when Alyssa had the idea that maybe we should try a cleaner infection source and decided to test the coelomic fluid, which is basically the blood of the star. With a syringe, you can extract the coelomic fluid of the sick star and you can also heat-kill it, and you can do the same experiment challenging with that. And it was a really exciting moment because she and Melanie confirmed that that was a really effective way of transmitting the disease because it's cleaner.
It's cleaner, like there's less stuff than in the tissue? Like blood is just like a simpler material?
Right. So, that was really the beginning of being able to figure out what it was that was in the coelomic fluid that was causing the disease.
So basically it's like: … So it seems like it might be ingredient B that's causing the problem here because it's consistent across all samples?
Yeah, that's exactly it. And so then that was very, very incredibly exciting. Wow. There's this one bacterium — Vibrio pectenicida — that's showing up in all of the diseased material samples. Could it be that?
We weren't sure. We sort of thought, after 12 years, this is gonna be something so strange! So weird! You know, something alien that we've never seen before. And so to have a Vibrio — something that we think of as a little bit more common — turn up was really surprising.
Then one of our colleagues at the University of British Columbia, Amy Chan, was able to culture that particular bacterium from the disease star. And so now she had a pure culture of the presumptive killer. And then last summer, Melanie and Alyssa were able to test that again under quarantine conditions and find that it immediately killed the stars that were tested.
How did you all feel?
Oh, we were definitely dancing around the room. It was — just such a happy moment of fulfillment. I really do like to say that at the beginning of the task that Nature Conservancy handed us — to figure out the causative agent — we told them again and again that this is a very risky project. We can't guarantee we're going to be successful.
So yeah, we were incredibly elated when we really felt confident in the answer. It was just hundreds and hundreds of hours of tests and challenge experiments that came out so beautifully.
What does it mean to finally have an answer here? What are the next steps?
This was the part of it that really kept me awake at night because I just felt so worried early on at the idea that we were working on a roadmap to recovery of a species without knowing what was killing it, and I just felt like we couldn't do it if we were flying blind like that.
We wouldn't know what season the pathogenic agent came around. We wouldn't know what its environmental reservoirs were. We didn't know what was making stars susceptible. It was going to be really hard, and it wasn't going to feel right to just put animals out in the wild without knowing more.
And so knowing that this is one of the primary causative agents — maybe the only causative agent — allows us to test for it in the water. It allows us to find out if there are some bays where this is being concentrated, to find out if there are some foods the stars are eating that are concentrating this bacterium and delivering a lethal dose to a star.
Now we'll be able to answer those questions, and I think that's going to give us a really good opportunity to design better strategies for saving them.
It feels like you now have a key to use to sort of unlock various pieces of this.
We totally do. And it's so exciting and so gratifying because that's what we're supposed to do, right? As scientists and as disease ecologists, we're supposed to solve these mysteries. And it feels really great to have solved this one. And I don't think there's a day in the last 12 years that I haven't thought about it and been really frustrated we didn't know what it was. So it's particularly gratifying to me to have to have reached this point.
Drew Harvell is the author of many popular science books about marine biology and ecology, including her latest, The Ocean's Menagerie. She also wrote a book about marine disease called Ocean Outbreak.
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