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The suspected infection causes some sea stars’ arms to fall off, while others seem to inexplicably dissolve.

Vancouver Aquarium

When a Vancouver Aquarium diver visited the society's rockfish study site off Bowyer Island this summer, she noticed something strange: sea stars dying in alarming numbers, some with limbs falling off and others apparently melting in place.

What she saw was the apparent beginning of a mass die-off of the echinoderms, commonly referred to as starfish, that has been observed all along the West Coast, from California up to Alaska. Months later, scientists are still scratching their heads over what is causing them to waste away.

"The die-off has been so abrupt and so complete here," said Jeff Marliave, the Vancouver Aquarium's vice-president of marine science. "It appears like a contagion because it spreads to adjacent populations: It spread up Burrard Inlet and it spread up Howe Sound over the [span] of a month. Then it's gone up the Sunshine Coast and it took a couple of months for it to hop across to Vancouver Island."

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A time-lapse video of a diseased sunflower sea star at the Vancouver Aquarium, quarantined in a hospital habitat, is a curious sight: Over the span of seven hours, the sea star moves throughout the tank, its arms falling off one by one and wiggling away.

"It's normal for a sea star to drop an arm if a predator has a good hold on one arm," Mr. Marliave said. "The technical term is autotomize; they just lop off an arm, like a lizard deliberately losing its tail so it can live to grow a new one. But what's happening here is that the tissue is just going necrotic."

Other sea stars seem to inexplicably dissolve, turning into white goo en masse. When healthy sea stars eat these remains, they appear to become infected and die as well. The process is so abrupt that scientists are unable to culture the bugs to study the presumed infectious agent.

"You have to isolate and reinfect, but when everything's gone, you don't have a good population to work with," Mr. Marliave said.

Some marine biologists have feared the deaths could trigger a chain reaction. Sea stars are what scientists call a keystone species, playing a critical role in maintaining the relationship of an ecosystem. If sea stars are wiped out, shellfish populations – such as mussels, sea urchins and clams – that sea stars prey on could grow out of control.

For now, Mr. Marliave is not concerned. "It's possibly nature's way of bringing communities back into balance, because echinoderms do go through real population explosions," he said. "Around Vancouver, it was really bad for a good decade, where you would have whole walls covered with nothing but these gigantic sunflower sea stars."

One theory is that water-borne radiation from the Fukushima nuclear disaster in March, 2011, is behind the die-off. Researchers have dismissed this based on the fact that, if true, the transoceanic radiation would have affected the outer coast first, rather than inland areas such as Howe Sound and the Burrard Inlet.

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Another is ocean warming and climate change. But researchers dismiss this as well. "Those are long-term trends, whereas this is something very sudden and very isolated. This is spread in a fashion that probably does not link to that sort of thing," Mr. Marliave said. "It may just wind up being one of these mysteries."

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