Biologist Ian Hewson with a striped sun star. Photo courtesy Elliot Jackson.

Starfish from Mexico to Alaska’s Kenai Peninsula have been hammered by a wasting disease that causes their arms to melt into goo and fall off.

Researchers believe a virus called a densovirus is behind the widespread outbreak.

Cornell University microbiologist Ian Hewson has come to one of the last starfish strongholds, the Aleutian Islands, to suss out what makes this disease so virulent.

“It is definitely the most geographically extensive marine disease ever seen. It’s huge,” he said standing on a beach on Unalaska Island. “It’s wiped out millions and millions of sea stars. In some cases, up to 95 percent of the sea stars are gone—completely changed the coastal ecosystem structure. But fortunately, it hasn’t been out here yet.”

With help from local divers, Hewson has collected healthy sea stars from several bays around Unalaska.

Divers brought up sun stars, mottled stars, and other species from as deep as 130 feet underwater.

Hewson is shipping the healthy invertebrates back to his lab in Ithaca, New York.

He said the long trip to the Aleutians and the hassle of shipping water bags with live sea stars across the continent is worth it to find out what’s driving the disease.

”This is the only population that’s left, pretty much, that’s unexposed,” Hewson said.

Researchers have peered into the disease’s gruesome outcome as the sea stars slowly liquefy.

“They’re basically undergoing what’s known as programmed cell death,” Hewson said. “They’re killing themselves.”

Without healthy sea stars to study, scientists have little hope of understanding how the disease takes hold.

Many sea stars are considered "keystone species": remove them, and whole ecosystems can change dramatically. Hewson said in many areas hit by the wasting disease, sea urchins and mussel populations have exploded in the absence of a key predator.

Brenda Konar with the University of Alaska Fairbanks reports that the sea star wasting disease has been spotted in a couple of sunflower stars found off Adak in the western Aleutians this summer.

Konar said UAF graduate student Ben Weitzman saw hundreds of sea stars, of many different species, as he did sea urchin surveys on various islands in the Aleutian chain. None of the sea stars showed signs of disease, except for the two sunflower stars, which are normally only found hundreds of miles to the east.

“Historically, they weren’t found on Adak,” Konar said of the pizza-sized sunflower stars that have up to two dozen arms. Their once-dense populations have wasted away in the Lower 48 and British Columbia.

Before the outbreak, populations of the predatory sunflower stars were “just ridiculous” in the inland seas of Washington and British Columbia, according to Hewson.

“There were undersea mountains of sea stars,” he said. “Divers were afraid of avalanches of sea stars down slopes.” Crab fishing became a waste of time: Crab pots would come up overrun with sunflower stars.

But sunflower stars aren’t native or common out in the western Aleutian Islands. Konar thinks the few seen there may have arrived in the ballast water of a big ship. None of the hundreds of native sea stars spotted off Adak showed signs of the wasting disease.

According to Ian Hewson, citizen scientists have played a big role in keeping tabs on the underwater outbreak.

“So if you see diseased sea stars, by all means, take a photo of it,” he said.

Hewson said to send photos to a site like seastarwasting.org or to a local scientist, like Melissa Good of UAF in Unalaska, who works on ocean conservation.

“That will help us a lot in figuring out when it actually hits this island,” Hewson said. “Hopefully, it won’t hit for a while, but from what we know about this disease, it will probably sweep through here at some stage in the future.”

Scientists still don’t know whether other widespread changes, like climate change or ocean acidification, have fueled the rapid spread of this disease in the past two years. Konar said the virus does better in warmer water; Hewson said the disease appears to have spread to new areas in the winter, not the summer.

 “We know so little about the disease and how it spreads that it’s hard to predict,” Konar said.