Marine Mystery Solved: What Causes Sea Star Wasting Disease

Scientists identify the culprit behind billions of deaths from sea star wasting disease

By Jennifer S. Holland
Wildlife Science
Dec 17, 2025

On Calvert Island in British Columbia, Canada, a sunflower sea star succumbs to sea star wasting disease. Signs of infection progress from the stars ignoring food to contorting their arms to breaking out in lesions. Eventually, their arms fall off and, as each appendage functions independently, “you see single arms ‘walking’ away from the body. Finally, the whole thing melts into a pile of goo,” says marine biologist Alyssa-Lois Gehman of British Columbia’s Hakai Institute.

REMEMBER THE FIRST TIME YOU FOUND A STARFISH ON THE BEACH? The multiarmed, tide pool-loving creature may have seemed more mineral than animal, like something from a fantasy book. Starfish, also called sea stars, once were plentiful in tide pools along both U.S. coasts. But over the past decade, billions of the animals, representing some 20 species, have died of sea star wasting disease (SSWD)—most dramatically on West Coast beaches from Alaska to Mexico. Though scientists for years have searched for the pathogen that causes SSWD, the sea stars’ alarming deaths remained a mystery.

Until now. Last summer, a scientific team led by the Hakai Institute in British Columbia identified a novel strain of a bacterium, Vibrio pectenicida, as the sea stars’ killer. Abundant in marine ecosystems, Vibrio (a group of about 100 species) are known as barometers of climate change because they cause more infections in warmer waters. Discovery of the novel Vibrio strain slaying sea stars is giving scientists and conservationists hope that they finally can stop the deadly spread of SSWD.

Hardest hit by the disease’s West Coast rampage has been the sunflower sea star, a voracious predator of invertebrates—primarily sea urchins. Among the world’s largest sea star species, the animal can reach the diameter of a bicycle tire and sprout up to two dozen arms. Sunflower stars have lost more than 90 percent of their populations throughout their entire range, rendering them what scientists call functionally extinct. In 2020, the International Union for Conservation of Nature designated the sunflower star critically endangered, and it is proposed for listing as threatened under the Endangered Species Act.

Other sea stars suffering major losses include ochre, giant pink, rainbow, leather and mottled sea stars. Scientists don’t yet know why the bacterium affects some starfish species more than others, nor do they know where the novel pathogen originated.

Regardless, “it’s quite upsetting to see an infected population; they die a gruesome death,” says Hakai’s Alyssa-Lois Gehman, a marine biologist and one of the bacterium’s discoverers. She said the disease progresses from the stars ignoring food to contorting their arms to breaking out in lesions. Eventually, their arms fall off and, as each starfish appendage functions independently, “you see single arms ‘walking’ away from the body,” she says. “Finally, the whole thing melts into a pile of goo.”

An image of a pair of sunflower stars on rocky reef.

Of all starfish species, sunflower sea stars have been hardest hit by sea star wasting disease, with more than 90 percent of their populations killed off throughout their entire range, from Mexico to Alaska. Pictured above are healthy sunflower stars. University of British Columbia microbiologist Amy Chan (below) compares bacteria cultures from sick versus healthy sea stars.

Across the globe, reports of new wildlife diseases have mounted in recent decades, with problems such as habitat loss, pollution and climate change increasing the likelihood of pathogen emergence while simultaneously weakening animals’ natural immunities. Examples include chytridiomycosis in amphibians and white-nose syndrome in bats, both caused by fungal pathogens. SSWD has been equally devastating but unlike those other diseases, scientists have struggled to name the cause.

How the novel Vibrio strain finally came to light suggests that starfish scientists previously had been looking for it in the wrong places. Initially, researchers scoured infected sea stars’ surface skin and tube feet for possible pathogens, but the real culprit did not stand out among naturally occurring bacteria in these tissues.

An image of Amy M. Chan comparing bacteria cultures from a sick versus a healthy sea star.

It was when Gehman and her colleagues investigated a substance in the body cavity—or coelom—called coelomic fluid that they hit gold. “There was an aha moment when the coelomic data showed that 100 percent of the lab-exposed animals were infected with this strain of V. pectenicida and none of the controls were,” she says. “It’s rare to get such clean results in science, so we were surprised and thrilled.” The researchers published their findings last August in Nature Ecology & Evolution.

The discovery comes none too soon. Because of the crucial role they play in ecosystems, sea stars are what scientists call keystone species. Whenever a keystone species disappears from its habitat, a major shift—known as a trophic cascade—may ensue, completely reshaping ecosystem structure and function.

That is precisely what happened off the coasts of Northern California and Oregon where SSWD has taken its greatest toll on sunflower stars. After this critical top predator was wiped out, the stars’ sea urchin prey multiplied unchecked, and those many extra mouths devoured kelp at an unprecedented rate.

An image of purple sea urchins fleeing from the path of a leather sea star.

Both leather sea stars (pictured, with purple sea urchins) and ochre sea stars (below, on kelp) prey on sea urchins. When disease wipes out sea stars, urchin numbers explode, with devastating consequences for the kelp plants they feed on.

Compounding the problem, not only do sunflower stars control urchin numbers by eating them, the predators give off a chemical cue that suppresses urchin grazing behavior. Because kelp forests are crucial to marine intertidal zones—storing carbon, serving as fish nurseries, feeding animals such as seals and economically important rockfish and abalone, and helping control shoreline erosion—their loss has triggered an environmental and economic disaster in the areas most impacted by SSWD.

According to marine biologist Kevin Lafferty of the U.S. Geological Survey, finding “the smoking gun” behind SSWD is the first step toward stopping this devastating rampage. “Now we can move from guessing to acting,” he says.

Some researchers already are taking action. The Hakai-led team, for example, is looking closely at a remnant population of sunflower stars they discovered in fjord habitat along the central British Columbia coast. These sea stars seem to be fending off SSWD. “The temperatures there are cooler, so we’re asking, is temperature the deciding factor?” says Gehman.

Meanwhile, experts from Hakai and several other institutions, including the universities of Oregon, Washington and North Carolina, have begun seeking the source of the bacterium, exploring possible treatments for SSWD, investigating sea star immunity and breeding bacterium-resistant stars in the lab. They’ve also launched efforts to cryopreserve sea star sperm and eggs for future research as well as for breeding and, possibly, repopulating decimated areas.

“The information we have now, the identity of the pathogen, was for so many years the gate holding us back,” Gehman says. “We’re excited that the gate is now open, and we hold the key.”

Read about writer Jennifer S. Holland.