U.S. scientists believe they've at last solved the mystery of what killed more than 5 billion sea stars. A strain of the bacterium Vibrio pectenicida has been attacking the sea stars—often known as starfish—in a decade-long epidemic on the West Coast of North America.

Marine researchers have been searching for the cause of what has become known as sea star wasting disease since large numbers of them perished in 2013 from Mexico to Alaska. The epidemic has devastated more than 20 species and continues today. Worst hit was a species called the sunflower sea star, which lost around 90% of its population in the outbreak’s first five years.

The bacterium has also infected shellfish, according to a study published in the journal Nature Ecology and Evolution.

Alyssa Gehman, senior author and marine disease ecologist at the Hakai Institute, says the die-out was a gruesome affair.

So sea stars, when they're looking healthy, have sort of puffy arms, they're straight out, they might curl like turn a bit. But when they're sick, they start to sort of like curl back on themselves, so they look very twisty. They get lesions, so you can see what the lesion looks like on their surface. Then their arms actually fall off and walk away. So it's pretty noticeable when it happens. And that first disease outbreak in particular, it hit sunflower stars which they are, they seem to be our most susceptible ones, and we sort of had the huge losses. So they're now listed as critically endangered. We've lost over 90% of their population from Mexico to Alaska, which is over 5 billion sea stars, might be over 6 billion, it's so many, says Gehman.

It took more than a decade for researchers to identify the cause of the disease, with many false leads and twists and turns along the way.

Having identified the cause of the epidemic, scientists now have a better shot at intervening to help sea stars, a scientist said. That’s not only important for sea stars themselves, but for entire Pacific ecosystems, because healthy sea stars gobble up excess sea urchins.

This article was provided by The Associated Press.