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Diseased starfish
Sea star wasting disease can cause starfish to turn white, lose their limbs and disintegrate in a matter of days. (Credit: Kevin Lafferty / USGS)

The mass die-off of starfish off the West Coast is becoming a little less mysterious: Scientists say the starfish, also known as sea stars, fell prey to a one-two punch of virus infection plus unusually warm sea water.

The die-off started in 2013, reached a peak in 2014 and continued last year. Infected sea stars developed lesions that gradually dissolved the creatures from the outside, causing the arms to break away and leaving only whitened piles of starfish goop.

The outbreak has virtually wiped out ochre stars in the coastal waters of Puget Sound, the San Juan Islands and the Olympic Peninsula. More than 20 other species have suffered from Mexico all the way north to Alaska.

In a study published this week by the Philosophical Transactions of the Royal Society B, scientists concentrated on what happened to the ochre stars. They already knew that the sea star wasting disease was linked to a densovirus – a pathogen that the scientists say apparently caused more limited outbreaks of the disease decades earlier. But what made the virus more virulent this time?

In an effort to find out, the researchers conducted controlled experiments on sea stars at the University of Washington’s Friday Harbor Laboratories. They determined that the risks of disease and death were significantly greater when the water temperature rose above 54 degrees Fahrenheit (12 degrees Celsius).

That meshed with the circumstances of the die-off. “The outbreak occurred during a period of anomalously warm sea water, and stars in the San Juan Islands had a higher disease risk at warmer sites,” study co-author Drew Harvell, a professor of ecology and evolutionary biology at Cornell University, said in a news release.

The findings add to the concern about climate change and warming seas. “Consistent with some other marine diseases, results from this study suggest that changes in ocean temperature might hasten outbreak progression and the overall severity of impacts,” the researchers wrote. They said starfish at some coastal sites in the Puget Sound region, where the water can get warmer than 70 degrees F, may be “especially susceptible” to the disease.

There’s no known cure for sea star wasting disease, but Harvell said “not moving stars around is a key recommendation.”

“Future work should investigate whether survivors may have some natural resistance to the disease that might be exploited,” she said.

Although the evidence points to warmer seas as a factor in sea star wasting disease, the case of the disintegrating sea stars isn’t yet closed: Even when temperatures are taken into account, the outbreak is more serious than the models would have predicted. “It is unknown yet whether the shifts could be due to new, more virulent pathogen strains, changing environmental conditions, altered structure of host populations or some combination,” the researchers wrote.

In addition to Harvell, the authors of “Ochre Star Mortality During the 2014 Wasting Disease Epizootic: Role of Population Size Structure and Temperature” include Morgan Eisenlord, Maya Groner, Reyn Yoshioka, Joel Elliott, Jeffrey Maynard, Steven Fradkin, Margaret Turner, Katie Pyne, Natalie Rivlin and Ruben van Hooidonk.

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