“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........

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