Five years after a mysterious virus wiped out millions of starfish off the western coast of North America, causing them to lose legs, dissolve into fleshy goo and taking various species to the brink of disappearance, scientists have announced a remarkable reversal.

In what the authors of a new study say may be an example of evolution in action, a species appears to have evolved genetic resistance to a virus decimating it. After the peak of the epidemic, there was a 74-fold increase in the number of juveniles surviving among ochre stars, one of the species hardest hit by the sea star wasting disease, the scientists report in the Proceedings of the National Academy of Sciences.

The cause of the outbreak, which they call “one of the largest marine mass mortality events on record”, is still somewhat mysterious. Researchers theorize the disease is a densovirus, which has existed in the region for decades, but may have proliferated in response to climate change impacts such as warmer waters or ocean acidification.



In the early summer of 2013, scientists first started noticing sea star deaths, and soon many tidal areas up and down the west coast once crawling with purple and orange stars were nearly empty. Over 80% of ochre stars fell victim to the epidemic. The trend was particularly disturbing because the species is a keystone of intertidal areas along the west coast, and its disappearance could have a profound effect on ecosystems.

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“The sick ones tend to just fall apart in front of your eyes. An arm will actually break off and crawl away,” the Vancouver Aquarium biologist Jeff Marliave told a Seattle radio station in 2013.

The new paper’s authors compared DNA of sea stars from before and after the outbreak, and found the juveniles who are succeeding in coastal ecosystems today share a gene that resists the virus, suggesting that the virus catalyzed a process of natural selection.

“When you’ve removed a whole bunch of them, you’ve shifted the whole genetic diversity of that population,” said Chris Mah, a researcher at the Smithsonian Institution and a starfish expert. “In other words, to put it in human terms, if you wiped out a huge chunk of the human species, you would change the genetic makeup of humans.”

While the new research indicates that the ochre stars have evolved resilience to the consequences of climate change, other species may face greater difficulties. “The concern is that marine disease, extreme environmental events, and the frequency of those are on the rise,” said the lead author, Lauren Schiebelhut. “If we have too many extreme events in a row, maybe that becomes more challenging for species to respond to.”