A decade after the outbreak of sea star wasting disease (SSWD) — regarded as the largest epidemic ever recorded in the wild — scientists have pinpointed the microbial cause: a strain of the bacterium Vibrio pectenicida.
Over the past 10 years, the pathogen has devastated populations of sunflower sea stars (Pycnopodia helianthoides) along North America’s western seaboard, wiping out an estimated 5.8 billion individuals since 2013; about 90% of the global population. The species is now listed as critically endangered on the IUCN Red List.
Earlier research that tested tissue samples failed to yield conclusive results. However, by examining the sea stars’ coelomic fluid—which functions much like blood—researchers were able to confirm V. pectenicida’s role, owing to its high concentration in afflicted animals.
Infection with the V. pectenicida strain FHCF-3 begins with external lesions, progressing to twisted limbs, tissue disintegration and ultimately death, leaving the animal reduced to a white, mucus-like paste. Diagnosing the disease in the field was previously difficult, as sea stars display similar outward signs in response to other stressors such as low oxygen levels, fluctuations in salinity or extreme heat. The link between rising ocean temperatures and SSWD remains a subject of investigation, as Vibrio species are known to thrive in warmer waters, particularly during seasonal surges and marine heatwaves.
The findings, published in Nature Ecology & Evolution, were led by Dr Melanie Prentice and Dr Alyssa Gehman of the Hakai Institute in British Columbia, in collaboration with the University of British Columbia, the University of Washington, the Nature Conservancy and several international partners, following a four-year study.
The collapse of sunflower sea star populations has far-reaching consequences beyond the loss of a single species.
“Identifying the cause of SSWD is incredibly impactful,” Prentice said. “In the absence of sunflower stars, [kelp-eating] 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.”
Kelp forests—which rely on sea stars as predators of sea urchins—are crucial ecosystems, providing habitats for thousands of marine species, supporting fisheries and tourism, holding cultural significance for First Nations and tribal communities, stabilising sediments, shielding coastlines from storms, and sequestering carbon.
Although the epidemic continues, researchers hope that identifying the pathogen will support recovery and conservation strategies for sea stars and the ecosystems they underpin. Approaches under consideration include captive breeding to select for resistant individuals and the development of probiotic treatments that could be introduced into affected marine environments.
“Now that we have found the causative agent of disease, it makes me hopeful that we might actually be able to do something for sunflower sea stars,” says Gehman. “We can be really targeted in how we work with them, and I think that’s going to help us move a lot faster and to try to tackle SSWD.”
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