Researchers have identified a virus as the most likely cause behind millions of sea star deaths along the Pacific coast.
A study published Monday in the Proceedings of the National Academy of Sciences names Sea Star Associated Densovirus (SSaDV) as the culprit in the largest marine die-off ever recorded.
"It's the first time we've linked this particular group of viruses to a mass mortality event," said Ian Hewson, who led the team of Cornell University scientists that made the discovery.
Back in the summer of 2013, deflated-looking sea stars with white lesions and curled, shrunken limbs started appearing in the Pacific Northwest. Within a matter of days they would dissolve into skeletal piles, dying from something called sea star wasting syndrome. In some areas, as much as 98 percent of sea stars would be wiped out.
This wasn't the disease's first outbreak — it struck off the coast of Maine in 1972 and in the Gulf of California in 1978. But the scope of previous outbreaks paled in comparison to the 2013 epidemic, which quickly spread as far north as Alaska and as far south as Baja California, hitting San Diego sea stars earlier this summer.
No one knew what was causing the disease, though some researchers suspected warming waters could be playing a role. Hewson thought so too, until the disease began spreading through aquariums along the West Coast.
"That pointed to it not being so much an environmental condition like acidification or warming," he said. "Because they actually maintain their aquariums at certain temperatures and do their own pH balance. So it was more something that was coming in from the outside."
Another clue: aquariums that treated their incoming water with virus-killing UV light were free of sea star wasting syndrome.
Hewson said further research is needed to understand how factors like ocean warming could be aiding the disease's spread. But he's confident his lab's experiments show that a contagious virus is at the heart of the problem.
The researchers sequenced the DNA of viruses found in sick and healthy sea stars. Through comparison, they zeroed in on a virus in the densovirus genus, known to affect arthropods and insects — but not sea stars.
The researchers verified this unexpected finding by injecting the isolated virus into healthy sea stars. Sure enough, the subjects developed wasting symptoms within 10 days.
Having identified their prime suspect, Hewson and his colleagues wanted to know how long the virus had been present in sea stars. So they mined museums for samples, and found SSaDV in specimens dating back as far as 1942.
Hewson doesn't yet have a clear answer as to why a virus that's been lurking in the environment for at least 72 years has only now become so devastating. But he does have a hunch.
Hewson said in affected ares, "sea stars were truly overpopulated," making them a vulnerable target.
"When a pathogen does take hold, it wipes them out very rapidly. There's a lot of transmission, and along with that, you get high rates of mutation of the virus. Which might result in it becoming more virulent."
There's a lot more Hewson wants to know about the disease, like which species serve as a host for the virus. There's evidence sea urchins might carry it, meaning sea stars could have picked up the disease from their dinner.
One thing is clear, though: ecosystems once dominated by sea stars are now in for major changes. Sea stars act as keystone predators, meaning they play an outsized role in maintaining populations. Mussels and sea urchins are likely to explode in numbers where sea stars have been decimated.
But Hewson said as bad as things have become for Pacific coast sea stars, it's important to remember that viruses play a natural role in these communities too.
"Although it's very dramatic, and people are obviously very concerned, we probably will see a resurgence of other resistant species of sea stars, as well as other types of invertebrates. It will sort of balance out in the long-run."
