The underside of an adult sunflower sea star at UW Friday Harbor Laboratories.Dennis Wise/University of Washington
Sunflower sea stars are the largest sea stars in the world: They have up to 24 arms and grow to the size of a bicycle tire.
Starting in 2013, these creatures and other sea star species along the west coast of North America died in epidemic proportions. The stars had harrowing symptoms: Their arms contorted before falling off completely. Over the past decade, sea star wasting disease has killed billions of sea stars in up to 20 species by effectively "melting" their tissues.
The disease has wiped out more than 90% of the once-common sunflower sea stars, most critically in the continental U.S., 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.
Until now, no one knew what caused sea star wasting disease. But on Aug. 4, an international research effort including scientists from the University of Washington has finally revealed the cause: a strain of the bacterium Vibrio pectenicida. Vibrio is a genus of bacteria that has devastated coral and shellfish as well as humans (for example, Vibrio cholerae is the pathogen that causes cholera).
The researchers published this finding in Nature Ecology & Evolution.
Drew Harvell holds a sunflower star at UW Friday Harbor Laboratories.David O Brown/Cornell University
"This is the discovery of the decade for me," said co-author Drew Harvell, a UW affiliate professor in the School of Aquatic and Fishery Sciences and Friday Harbor Laboratories. "We have studied both the cause and the impacts of this disease for the entire epidemic. 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."
"From initial studies, we thought the culprit was a virus," Harvell continued. "So it was a surprise to find the pathogen in a more common group of bacteria."
The long-awaited result showing V. pectenicida strain FHCF-3 as the causative agent comes after a four-year research process. Scientists explored many possible pathogens, including viruses. At first, the researchers looked in sunflower sea star tissues before they homed in on the high levels of V. pectenicida in sick 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," said senior author 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.'"
Harvell attributes the team's success to:
- Having the right facilities at the U.S. Geological Service with proper quarantine and high-quality water flow
- A talented research team that had pathology, virology and bacteriology experience
- Having access to a source of the right test animals, including sunflower sea stars raised in captivity by co-author Jason Hodin, UW senior research scientist at Friday Harbor Laboratories.
Adult sunflower sea stars feeding on mussels at UW Friday Harbor Laboratories. The stars suck out and ingest the soft tissues of mussels, then discard the shells, which collect at the bottom of the tank. The sea star on the bottom, "Charlotte," is the mother of the lab's stars grown in captivity.Dennis Wise/University of Washington
Grace Crandall injects a sea star to expose it to wasting disease at the start of a new experiment.Grace Crandall/University of Washington
"I observed and collected health data on nearly every single sea star twice a day for the majority of experiments for all four summers," said co-author Grace Crandall, a UW doctoral student in the School of Aquatic and Fishery Sciences. "I have loved sea stars and have been fascinated by diseases since childhood. To get to participate so actively in research that combines both of these interests has been a dream come true. I'm excited about getting to work on a project with such consequential findings for the conservation of these important sea stars: to find both the cause of sea star wasting disease, and to better understand their immune response."
To confirm that V. pectenicida was the culprit, co-author Amy M. Chan, a research scientist at UBC, created pure cultures of V. pectenicida from the coelomic fluid of sick sea stars. The team then injected the cultured pathogen into healthy sea stars, which developed symptoms of sea star wasting disease - the final proof.
"When we lose billions of sea stars, that really shifts the ecological dynamics," said lead author Melanie Prentice, 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."
The team poses in the lab 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
Now that scientists have identified the pathogen behind sea star wasting disease, they can look into the drivers of disease and potential hallmarks of resilience. Researchers are particularly interested in studying the link between sea star wasting disease and rising ocean temperatures. The effects of the disease seem to be stronger in warmer water, and other species of Vibrio are also known to proliferate in warm water, Gehman said.
Researchers and project partners hope the discovery will help guide management and recovery efforts for sea stars and the ecosystems affected by their decline.
