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Grisly sea-star deaths are continuing to litter the tidelands with decaying, ghostly goo, but scientists now believe they, at least, may have identified the culprit.

New evidence suggests a mysterious wasting disease killing sea stars by the millions may be the result of a virus that has been found in starfish since at least the 1940s, according to new research published Monday in the Proceedings of the National Academy of Sciences.

But it remains unclear if the pathogen’s current deadly spread is part of a complex natural cycle — or whether blame for this massive die-off is linked in some way to climate change, souring seas or other harm humans have inflicted on the ocean.

Either way, the gruesome deaths are still spreading, confounding scientists and threatening to fundamentally transform marine systems along thousands of miles of Pacific Ocean coastline.

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“This is the sea-star-removal experiment of the century. It’s pretty staggering,” said C. Drew Harvell, a marine epidemiologist from Cornell University based at Friday Harbor Labs on San Juan Island. “The ecological impact is going to be huge.”

And researchers still have no clue when the dying might end.

Sea-star wasting produces white spots on starfish and causes their limbs to curl, writhe and contort. Their bodies then appear to deflate or dissolve, their arms dripping away or breaking off until the animals disintegrate into goopy white smears on the seafloor.

This isn’t the first time a so-called wasting disease has struck the West Coast, but the outbreak that surfaced first in Washington in 2013 is on a scale unlike anything ever recorded. Previous mass die-offs of sea stars occurred during El Niño events in the early 1980s and late 1990s, leading scientists to presume there were links between ocean warming and the disease.

That’s why researchers had hoped, as this summer slid into fall and ocean temperatures cooled, the disease that has hit some 20 species of sea star might actually slow or halt its spread.

Instead, sea-star-rich coves off northern Vancouver Island that seemed unaffected just six weeks ago have since been hit one after another. The disease has spread to tidal areas as far north as Sitka, Alaska.

One remote swath of private shore south of Santa Barbara, Calif., appears so far to have escaped unscathed. But along the U.S. West Coast, “there really aren’t any big areas of coastline left unaffected at this point,” said Pete Raimondi, a marine evolutionary biologist at the University of California, Santa Cruz.

In fact, divers enlisted to monitor sea stars report places once home to thousands of the creatures now are often home to just five or six. Where divers used to see dozens of steering-wheel-sized 24-armed sunflower stars, they recently surveyed 17 sites and saw just two.

“In East Sound on Orcas Island alone there were probably 3,000 ochre stars at the beginning of the summer,” Harvell said. “There were maybe 10 at the end.”

Below the Coupeville wharf on Whidbey Island in August, diver Jan Kocian found the pasty remnants of melted stars — but also saw dozens of orange, brown and blue ones still wrapped snugly around pilings.

By last week, all those were gone, too.

“For divers seeing sea stars isn’t usually too exciting,” Kocian said. “They don’t move too fast, they don’t do too much and you’re used to seeing them. Now I go down and say, ‘Wow, I saw a sea star today.’ ”

A natural check?

Harvell and her colleagues, particularly Cornell microbiologist Ian Hewson, have spent more than a year investigating the phenomenon. The way the disease spread suggested early on that die-offs were somehow linked to an infectious agent.

So Hewson examined tissue samples and compared the genetic structure of sick stars and healthy stars. He found a type of parvovirus far more present in sick stars than in healthy ones. Researchers injected healthy stars with virus-sized particles from sick stars. After about two weeks, those stars, too, became sick.

Scientists collected hundreds of sea stars and found most of the stars that contained the virus also were sick. And they determined that the virus potentially could be spread through water, without requiring direct contact between starfish. Hewson even visited museums and took tissue samples from sea stars collected over decades and found evidence of the virus dating to 1942.

So if this is the virus potentially responsible for the outbreak, why is it wreaking so much havoc now?

Hewson said some reports had suggested that sea-star populations a few years ago seemed extraordinarily high. Crabbers had reported pulling mountains of them from their pots. Did the virus mutate from something relatively benign as the abundance of its sea-star hosts skyrocketed — a sort-of natural check on sea-star populations?

“Viruses do play a role in maintaining and controlling populations that undergo huge outbreaks,” Hewson said. “There are also decadal or half-century cycles.”

But it’s also true that environmental stress can make animals more susceptible to diseases they might have fought off under other conditions. The problem is, there doesn’t appear to be any clear signal on which stressor is playing a role.

Scientists have looked at pollution, marine discharges and shipping channels where invasive species may spread.

“Nothing categorically points to a single cause,” Raimondi said. “It’s just very cryptic.”

Initially scientists suspected there might be a link to a period of unseasonably warm ocean temperatures. Warm temperatures often help diseases spread and that appeared important in earlier outbreaks.

But it doesn’t always seem to be the case here. The areas where sea stars first began dying was close to shore, where marine temperatures weren’t extraordinary high. And the recent spread into Alaska came well after waters started to cool.

“These associations are super difficult to make,” Raimondi said. “You can sometimes find connections between warm water and the disease, but nothing that really makes sense in terms of the physiology.”

Some scientists also wonder if oxygen-depleted dead zones or ocean acidification — changes in marine chemistry as oceans absorb excess carbon-dioxide emissions — have somehow made sea stars more vulnerable.

“That’s not out of the realm of possibility … but we just don’t have the observations to make a case for it,” Raimondi said.

Ecosystem hit

While scientists continue to try and figure out how and why the disease is spreading, no one is exactly sure what to expect in the future.

Sea stars are such an important species to nearshore environments that their absence is already changing many areas. On Whidbey Island, green urchin populations are exploding. In Howe Sound, north of Vancouver, a similar phenomenon already is eliminating sea grasses typically used by young spot prawns.

“It’s going to have a dramatic impact on the ecosystem,” Hewson said.

In a few isolated sites, perhaps half a dozen, baby sea stars again are starting to grow, sometimes in very large numbers. But the good news is tempered by new surprises.

One of the places hit only moderately in 2013 — the Washington coast, just off Olympic National Park — is this fall being ravaged anew.

“We thought it was just one of those places that had escaped the real wrath of this,” Raimondi said. “But now it’s being hit very, very hard again, and that’s discouraging.”

Harvell hopes that as winter comes and water temperatures drop even more, the spread to the north — where scientists had hoped healthy sea-star communities would eventually help repopulate the West Coast — will finally slow. But even if that happens, she still worries about what spring will bring.

“Every day I wake up and wish I knew what was going to happen next,” she said.

Craig Welch: 206-464-2093 or cwelch@seattletimes.com Twitter @craigawelch