In the journal Nature Ecology & Evolution, a group of researchers reveals the culprit behind sea star wasting disease, a marine epidemic that has decimated sea star populations along the west coast of North America. Understanding the cause is essential for the recovery of sea stars and their kelp forest ecosystems.
AUGUST 4, 2025 - Today in Nature Ecology & Evolution, a group of researchers reveals the cause of sea star wasting disease (SSWD). This discovery comes more than a decade after the start of the marine epidemic that has killed billions of sea stars—representing over 20 different species from Alaska to Mexico. SSWD is considered the largest marine epidemic ever documented in the wild.
The challenging four-year investigation eventually pinpointed the microbial culprit behind SSWD: a strain of the bacterium Vibrio pectenicida.
Vibrio is a genus of bacteria that has devastated coral and shellfish as well as humans; Vibrio cholerae is the pathogen that causes cholera. A strain of V. pectenicida has previously been shown to wipe out the larvae of several species of scallops with a "swift and dramatic" course of infection.
In the case of sea stars, infection with the V. pectenicida strain FHCF-3 initiates a grim disease that begins with exterior lesions and ultimately kills sea stars by "melting" their tissues, a process that takes about two weeks after exposure. Afflicted individuals often become contorted and lose their arms.
The international research effort was led by scientists from the Hakai Institute, the University of British Columbia (UBC), and the University of Washington—and conducted in collaboration with The Nature Conservancy, the Tula Foundation, the U.S. Geological Survey's Western Fisheries Research Center, and the Washington Department of Fish and Wildlife.
Over 90 percent of sunflower sea stars (Pycnopodia helianthoides)—which are capable of sprouting 24 arms and growing to the size of a bicycle tire—were wiped out by the disease in the past decade, landing them on the International Union for Conservation of Nature's Red List of critically endangered species . The loss of sunflower sea stars, which support kelp forests by feeding on kelp-eating sea urchins, has had widespread and lasting effects on coastal ecosystems.
"When we lose billions of sea stars, that really shifts the ecological dynamics," says Melanie Prentice, the first author on the study and an evolutionary ecologist at the Hakai Institute and UBC. "In the absence of sunflower stars, sea urchin populations increase, which means the loss of kelp forests, and that has broad implications for all the other marine species and humans that rely on them. So losing a sea star goes far beyond the loss of that single species."
Kelp forests provide habitat for thousands of marine creatures and contribute millions of dollars to local economies through fisheries, recreation, and tourism. They're also culturally important for coastal First Nations and tribal communities, they sequester and store planet-warming carbon dioxide, and they protect coastlines from storms.
"Understanding what led to the loss of the sunflower sea star is a key step in recovering this species and all the benefits that kelp forest ecosystems provide," says Jono Wilson, the director of ocean science for The Nature Conservancy's California chapter.
Identifying the disease in afflicted sea stars was impossible without a known pathogen, as sea stars can respond to other stressors and diseases with similar visual signals of contortion and loss of arms. The long-awaited result showing V. pectenicida as the causative agent comes after a four-year research process. The team of scientists explored many possible pathogens, including viruses, first looking in sea star tissues before homing in on the high levels of V. pectenicida in sea star "blood," or coelomic fluid.
"When we looked at the coelomic fluid between exposed and healthy sea stars, there was basically one thing different: Vibrio," says Alyssa Gehman, senior author of the study and a marine disease ecologist at the Hakai Institute and UBC. "We all had chills. We thought, That's it. We have it. That's what causes wasting."
Amy M. Chan, a marine microbiologist in the Aquatic Virology and Microbiology Lab at UBC, then created pure cultures of V. pectenicida from the coelomic fluid of sick sea stars. Researchers then injected the cultured pathogen into healthy sea stars, and the ensuing rapid mortality was final proof that V. pectenicida strain FHCF-3 causes SSWD.
The research was supported by UBC, The Nature Conservancy, the Tula Foundation, and several other institutions, and conducted at the Aquatic Virology and Microbiology Lab at UBC, and at the Marrowstone Marine Field Station in Washington State, run by the U.S. Geological Survey's Western Fisheries Research Center.
Now that scientists have identified the pathogen that causes SSWD, they can look into the drivers of disease and resilience. One avenue in particular is the link between SSWD and rising ocean temperatures, since the disease and other species of Vibrio are known to proliferate in warm water, Gehman says.
"Those patterns of Vibrio in general suggest that we really should look down that road to see how temperature dependence matters."
Researchers and project partners hope the discovery will help guide management and recovery efforts for sea stars and the ecosystems affected by their decline.
"This finding opens up exciting avenues to pursue and expands the network of researchers able to develop solutions for recovery of the species," says Wilson. "We are now actively pursuing studies looking at genetic associations with disease resistance, captive breeding of the animals, and experimental outplanting to understand the most effective strategies and locations to reintroduce sunflower sea stars into the wild."
