For the last dozen years, scientists have been on the hunt for a killer that has claimed billions of lives. They’ve finally found it.

In 2013, researchers from Olympic National Park reported what looked like a sea star massacre: ochre sea stars with limbs that had split off from their decaying bodies. It was the first of what soon became a coast-wide underwater epidemic stretching from Mexico to Alaska.

Within years, the mysterious condition, dubbed sea star wasting disease, had wiped out billions of sea stars. It was declared the largest known disease outbreak in the open ocean. The effects were both devastating and gruesome for more than 20 species. Sea stars broke apart, their arms crawling away seemingly in a failed attempt to escape before dissolving into goo.

Many-armed sunflower sea stars as big as bicycle wheels were some of the hardest hit, declining by 99% in U.S. coastal waters and earning the designation of “critically endangered” from the International Union for Conservation of Nature.

Scientists struggled to figure out what was behind this devastation. Initial suspicions of a kind of virus proved wrong. Warming waters appeared to play a role, but that in itself couldn’t explain it.

Starting in 2021, Canadian and U.S. scientists mounted a massive, 4-year hunt to find the culprit. Last week, they announced the results in Nature Ecology & Evolution: a bacteria called Vibrio pectenicida, part of a family of particularly nasty pathogens known to cause everything from cholera to scallop-killing outbreaks.

The discovery is a critical first step in figuring out how to protect or restore sea stars, which are linchpins of many coastal ecosystems such as kelp forests. Those forests are in decline partly because they are being devoured by sea urchins, once prey to sea stars. “Now that we’ve identified the disease-causing agent, we can start looking at how to mitigate the impacts of this epidemic,” said Melanie Prentice, a scientist at the University of British Columbia involved in the research.

The sleuthing involved years spent painstakingly narrowing down the possible causes of the disease, much of it at a U.S. Geological Survey laboratory in Washington state equipped to handle waterborne diseases.

First, scientists tried different ways of exposing healthy sea stars: they put them in tanks with infected ones; added water from tanks with sick sea stars; and injected the sea stars with tissue from infected ones. All approaches proved deadly. Of 50 healthy sea stars, 46 succumbed.

The researchers zeroed in on a substance called coelomic fluid, likened to sea star blood. When sea stars were injected with the fluid from an infected individual, they grew sick. But when they received a version that had been heat-treated to kill live organisms, they remained healthy.

When the DNA of the contents of coelemic fluid from healthy and sick sea stars was scrutinized, the sick ones contained a lot of DNA from the Vibrio bacteria.

“When we looked at the coelomic fluid between exposed and healthy sea stars, there was basically one thing different: Vibrio,” said Alyssa Gehman, a marine disease ecologist at the Hakai Institute and the University of British Columbia. “We all had chills. We thought, ‘That’s it. We have it. That’s what causes wasting.’”

As a final test, they refined a pure sample of the bacteria, then injected it into 6 sunflower sea stars, while another 6 received doses inactivated by high heat. The ones with the live bacteria all died, while the others all survived.

“This is the discovery of the decade for me,” said Drew Harvell, an ecologist with the University of Washington and author of several books about ocean life “What’s crazy is that the answer was just sitting right there in front of us. This Vibrio is a sneaky critter because it doesn’t show up on histology like other bacteria do.”

Other factors, such as heat, might still play a role. It’s not known how the disease first reached sea stars on this coast. But Vibrio bacteria generally thrive in warmer conditions. In fact, scientists have called them a “barometer of climate change.”

The new discovery doesn’t mean scientists will be able to find a “cure.” But it can help guide their work to find sea stars that are resistant to the disease. And researchers can now monitor for outbreaks in the wild by taking water or tissue samples. That might help them decide where to release lab-raised sea stars to give them the best chance of surviving.

“This finding opens up exciting avenues to expand the network of researchers able to develop solutions for recovery of the species,” said Jono Wilson, head of ocean science for The Nature Conservancy’s California chapter, which helped fund the research. “We are actively pursuing studies looking at genetic associations with disease resistance, captive breeding and experimental introduction of captively-raised stars back into the wild.”

Prentice, et. al. “Vibrio pectenicida Vibrio pectenicida strain FHCF-3 is a causative agent of sea star wasting disease strain FHCF-3 is a causative agent of sea star wasting disease.” Nature Ecology & Evolution. Aug. 4, 2025.

Image: Wasting cookie sea star near Calvert Island. courtesy of Grant Callegari/Hakai Institute