Mystery of what killed billions of starfish solved, say scientists
The mass die-off, which began in 2013, wiped out an estimated five billion sea stars from Mexico to Alaska, and continues to affect over 20 species.
"It's really quite gruesome," said Alyssa Gehman, marine disease ecologist at the Hakai Institute in British Columbia, Canada, who helped identify the cause.
She said healthy sea stars - also known as starfish - have "puffy arms sticking straight out".
But the wasting disease that set in more than a decade ago caused them to grow lesions and "then their arms actually fall off".
Worst hit was the sunflower sea star species, which lost around 90% of its population in the outbreak's first five years.
After following numerous wrong turns and red herrings, scientists are now confident the killer was a bacterium that also infected shellfish.
The breakthrough, published in the journal Nature Ecology and Evolution, solves "a long-standing question about a very serious disease in the ocean," said Rebecca Vega Thurber, a marine microbiologist at the University of California, Santa Barbara, who was not involved in the study.
Previous efforts missed the real killer because they focused on the wrong type of virus or investigated samples of dead sea stars which no longer contained the bodily fluid that surrounds the organs.
But the latest study involved careful analysis of this so-called coelomic fluid, and found the pathogenic bacteria Vibrio pectenicida.
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The death of sea stars triggered a cascade of effects through coastal ecosystems.
Without predators like sunflower sea stars, which eat almost everything on the seabed, sea urchin populations exploded.
In Northern California, this has led to the destruction of roughly 95% of kelp forests over the past decade - important underwater habitats often described as the "rainforests of the ocean".
Scientists hope their discovery could help with work to save sea stars.
Researchers are investigating whether to relocate the animals, or breed them in captivity to later transport them to areas that have lost most of the population.
They can also test whether treatments like probiotics could boost immunity to the disease.
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