Mystery of what killed billions of starfish solved

More than a decade after a mysterious killer devastated sea star populations along the Pacific coast of North America, scientists believe they have finally found the culprit.

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. 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".

Calum Duncan, head of policy & advocacy at the Marine Conservation Society in the UK, said the overgrazing of kelp by sea urchins "underlines the delicate balance of the ecosystem". "The ocean is our biggest ally in tackling the climate crisis, and we should be doing all we can to boost marine ecosystem health." 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.

Governments are this week meeting in Geneva to negotiate a global plastics treaty to stop plastics from disrupting marine life, fishing and tourism. The UK government has called for a "bold and ambitious global agreement" to end plastic pollution by 2040.

Marine minister Emma Hardy said: "The UK is continuing to play a leading role in pushing for an effective treaty that ensures the sustainable consumption and production of plastics, tackles problematic plastic products, and paves the way to a circular economy.".