SSWD is a starfish's nightmare. First, their limbs start going wacky and they tie themselves into knots (the technical term is, no joke, "pretzeling"). Then, their legs fall off one by one and crawl away from their bodies like little starfish zombie limbs. Finally, the starfish deflate (this is exactly what it sounds like), die, and melt into a pile of mush.
It's truly awful and it's been happening to starfish—technically called sea stars, since they're not fish—up and down the Pacific coast since last June, wiping out huge colonies. Evidence suggests it might be the largest known marine epizootic to date.
"Some of the inlets around Vancouver used to have, literally, avalanches of sea stars that would tumble onto divers who were on the bottom of the inlets," Ian Hewson, a microbiology professor at Cornell University who has been studying SSWD, said in a phone call. "They've gone from that situation to completely barren inlets. It's been very dramatic."
But the SSWD epidemic isn't just giving divers a better view of underwater rocks. It's starting to affect the ocean's ecosystem. Populations of urchins, oysters and clams, which starfish eat, are starting to grow and could eventually explode if their main predator continues to die off. Other organisms, like sea cucumbers, are showing signs of the disease, too.
Fortunately, Hewson and a team of fellow biologists think they've identified the virus that's causing SSWD.
The researchers identified the virus-sized organism on sick sea stars, but needed to test whether the two were connected.
First, they removed the virus from sick starfish and injected it onto healthy starfish to see if they got sick (they did). Then, they took the virus particles from those formerly-healthy starfish and injected those onto a new batch of healthy starfish to see if they got sick (they did, too).
Finally, the scientists isolated the virus particles, sequenced its DNA, reassembled the virus's genome and studied almost 500 sea stars to see if the sick ones had the virus DNA (yup).
"We feel that we have pretty strong evidence. It's being caused by a virus, definitely," Hewson said.
So, now what? As Hewson pointed out, diving into the ocean with a bunch of tiny syringes to inoculate sea stars isn't a feasible idea, though it is an adorable thought. And viruses are typically nature's way of evening out the scales to keep species in balance, he said. For example, sea star populations, particularly around Vancouver, had grown a lot in recent years.
However, now that they've identified the likely cause of the disease, they can investigate whether there are other factors involved—like climate change—that are increasing the prevalence of SSWD and use that information to prevent future, human-influenced outbreaks.
And in case you were worried, Hewson wanted to stress that there's no risk of the virus spreading to land and infecting humans.
"That's an important point," he said. "It's not like your limbs are going to start to crawl away from you."
Good to know.