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The bottom side of an adult sunflower sea star at UW Friday Harbor Laboratories.Dennis Wise/University of Washington
Sunflower sea stars are the largest sea stars globally: They can possess up to 24 arms and can reach the size of a bicycle tire.
Beginning in 2013, these animals, along with other sea star species along North America’s western coastline, perished in extensive numbers. The stars exhibited distressing symptoms: their arms twisted before completely detaching. Over the last decade, sea star wasting disease has eradicated billions of sea stars across approximately 20 species by essentially “melting” their tissues.
The affliction has decimated more than 90% of the formerly prevalent sunflower sea stars, particularly in the continental U.S., resulting in their placement on the International Union for Conservation of Nature’s Red List of critically endangered species. The decline of sunflower sea stars, which sustain kelp forests by consuming kelp-devouring sea urchins, has had widespread and enduring consequences on marine ecosystems.
Until recently, the cause of sea star wasting disease remained a mystery. However, on Aug. 4, an international collaborative research endeavor, including scientists from the University of Washington, unveiled the cause: a strain of the bacterium Vibrio pectenicida. Vibrio is a genus of bacteria that has ravaged corals and shellfish as well as humans (for instance, Vibrio cholerae is the pathogen responsible for cholera).
The researchers published this revelation in Nature Ecology & Evolution.
Drew Harvell holds a sunflower star at UW Friday Harbor Laboratories.David O Brown/Cornell University
“This is the revelation of the decade for me,” mentioned co-author Drew Harvell, an affiliate professor at UW in the School of Aquatic and Fishery Sciences and Friday Harbor Laboratories. “We have investigated both the cause and the effects of this illness throughout the entire epidemic. What’s astonishing is that the answer was right there in front of us. This Vibrio is a sly organism as it does not appear on histology like other bacteria do.”
“From initial assessments, we presumed the culprit was a virus,” Harvell added. “So discovering the pathogen in a more familiar group of bacteria was truly unexpected.”
The long-anticipated discovery identifying V. pectenicida strain FHCF-3 as the causative agent follows a four-year research cycle. Scientists examined numerous potential pathogens, including viruses. Initially, the team scrutinized sunflower sea star tissues before narrowing down on the high concentrations of V. pectenicida in the “blood,” or coelomic fluid, of afflicted sea stars.
“When we compared the coelomic fluid between infected and healthy sea stars, there was literally one distinguishing factor: Vibrio,” stated senior author Alyssa Gehman, a marine disease ecologist at the Hakai Institute and the University of British Columbia. “We were all filled with chills. We thought, ‘That’s it. We’ve pinpointed it. That’s what leads to wasting.’”
Harvell credits the team’s effectiveness to:
- Access to the appropriate facilities at the U.S. Geological Service with suitable quarantine and high-quality water circulation
- A skilled research team with expertise in pathology, virology, and bacteriology
- Availability of a source of suitable test animals, including sunflower sea stars bred in captivity by co-author Jason Hodin, a senior research scientist at UW’s Friday Harbor Laboratories.
Adult sunflower sea stars feeding on mussels at UW Friday Harbor Laboratories. The stars extract and consume the soft tissues of mussels, discarding the shells, which accumulate at the tank’s bottom. The sea star on the bottom, “Charlotte,” is a mother of the lab’s stars cultivated in captivity.Dennis Wise/University of Washington
Grace Crandall injects a sea star to expose it to wasting disease at the beginning of a new experiment.Grace Crandall/University of Washington
“I monitored and gathered health data on nearly every sea star twice daily during the majority of experiments over four summers,” shared co-author Grace Crandall, a UW doctoral candidate in the School of Aquatic and Fishery Sciences. “I have cherished sea stars and have been intrigued by diseases since I was a child. Being able to actively participate in research that intertwines both of these passions has been a dream fulfilled. I’m thrilled to engage in a project with such significant implications for the conservation of these vital sea stars: identifying both the cause of sea star wasting disease and enhancing our understanding of their immune response.”
To verify that V. pectenicida was indeed the cause, co-author Amy M. Chan, a research scientist at UBC, established pure cultures of V. pectenicida from the coelomic fluid of ill sea stars. The team subsequently injected the cultured pathogen into healthy sea stars, which then exhibited symptoms of sea star wasting disease — the ultimate proof.
“When we lose billions of sea stars, that significantly alters the ecological balance,” remarked lead author Melanie Prentice, an evolutionary ecologist at the Hakai Institute and UBC. “In the absence of sunflower stars, sea urchin populations increase,
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which signifies the disappearance of kelp ecosystems, impacting numerous marine organisms and humans dependent on them. Hence, the decline of a sea star extends well beyond the mere loss of that one species.
The group is photographed in the laboratory at the USGS Marrowstone Marine Field Station. From left to right: Alyssa Gehman, Grace Crandall, Melanie Prentice, and Drew Harvell.Grace Crandall/University of Washington
With researchers having pinpointed the pathogen responsible for sea star wasting disease, they can now investigate the factors contributing to the illness as well as signs of resilience. Investigations are especially focused on the connection between sea star wasting disease and the escalation of ocean temperatures. The disease seems to have a greater impact in elevated temperatures, and several species of Vibrio are also recognized to thrive in warm waters, Gehman noted.
Scientists and collaborators aspire that this finding will direct management and restoration initiatives for sea stars and the environments affected by their reduction.
“It’s truly heartbreaking to see them perish,” Harvell remarked. “Sunflower sea stars are fascinating creatures and are quite engaging. During feeding time, they approach you. If you toss clams toward the stars, they can catch them. It’s incredibly rewarding to finally have clarity.”
Additional co-authors of this study include Katherine M. Davis and Jan F. Finke at UBC and the Hakai Institute; Paul K. Hershberger at the U.S. Geological Survey; Andrew McCracken at the University of Vermont; Colleen T. E. Kellogg, Rute B. G. Clemente-Carvalho, and Carolyn Prentice at the Hakai Institute; along with Kevin X. Zhong and Curtis A. Suttle at UBC. The research received backing from The Nature Conservancy of California, the Tula Foundation, the Natural Sciences and Engineering Research Council of Canada Discovery Grant, the Canadian Foundation for Innovation and British Columbia Knowledge Development Fund Infrastructure award, and the U.S. Geological Survey, Biological Threats Research Program, Ecosystems Mission Area, and the Quantitative and Evolutionary STEM Traineeship.
For further inquiries, reach out to Harvell at [email protected].
Adapted from a release by The Hakai Institute.