Scientists are homing in on a mysterious wasting disease that has killed billions of sea stars along the Pacific coast of North America since 2013. Sea star wasting disease can rapidly wipe out entire populations, leaving gooey puddles of tissue in its wake. A new study by University of Vermont researchers may unlock the pathways for infection by identifying early biomarkers of illness in wild sea stars.
"Sea star wasting disease has been an enigma in the field for a very long time," says lead author Andrew McCracken, a doctoral student in biology. "This puzzle is being put together much quicker now that we know where to look."
The study published today in the Proceedings of the Royal Society B offers clues to solve this infection that has devastated sea stars from Mexico to Alaska. Crucially, the UVM team detected early signs of immune and neurological disruption in sunflower sea stars even before showing physical signs of wasting.
"Sea star wasting disease is a really sad, gruesome disease," says McCracken. "The starfish melt and literally walk away from their limbs. I have seen them pull themselves apart."
The disease affects more than a dozen species, including sunflower sea stars, which have nearly been wiped out from it. These giant creatures no longer exist south of Washington state and are one of the primary predators of sea urchins, which can devour kelp forests.
Scientists have hunted for the cause of sea star wasting disease for over a decade. Only recently, an international team from the Haiki Institute , including McCracken, identified a strain of bacteria, Vibrio pectenicida, as a driver of sea star wasting disease—the first culprit pinpointed in over a decade.
For years, scientists including McCracken's advisor Melissa Pespeni, associate professor of biology at UVM, suspected the sea star microbiome would be a key to understanding wasting disease. Pespeni , a senior author of the new study, previously found shifts in the echinoderm's microbiome prior to visible signs of wasting. McCracken built on this work, testing for biological signs of disruption inside healthy and exposed starfish to uncover potential host-pathogen interactions.
"This is the closest look that we have ever had at an outbreak in the field happening and what the sea star is doing in response," Pespeni explains. "… Being able to integrate both what the host is doing and which microbes are present and at what abundance is super powerful to further understand the potential role of vibrio species in this wasting disease."
Key findings
Pespeni has studied sea star wasting disease nearly since the marine epidemic emerged. In 2016, a postdoctoral researcher in her lab collected tissue samples from living sea stars in regions of Alaska beyond reported outbreaks. Diver teams gathered tissue from sea stars not exhibiting signs of illness and from nearby populations experiencing wasting symptoms.
UVM researchers used this tissue to decipher which genes were activated in response to exposure to sea star wasting disease. They found sunflower sea stars exhibited an immune response along with shifts to the microbiome before lesions or other visible signs of wasting appeared. They also detected changes in the sea star's connective tissue.
Sea stars control their body through their catch-collogen system, McCracken explains. "When you pick up a starfish off of a rock and they go really rigid, that is them tightening through that system."
Sea stars with wasting disease lose tissue rigidity, develop lesions, and eventually disintegrate, he says. "We found some markers showing that early on, before you even see lesions, before they are losing arms, we are seeing differences in how they are regulating those systems."
By learning how the disease progresses and the various microbes involved, it could help scientists uncover resistant phenotypes.
"If we can [hone] in on those early interactions more, I think we can discover more about the mechanism itself," McCracken says. "We are in the early stages of figuring out how this works, which we need to know to be able to do anything about it."
While the team did find Vibrio pectenicida in samples, they could not directly link the species to the genomic changes in the starfish, he notes. "Vibrio pectenicida may either be the cause of the disease, the symptoms and everything, or it could be something that displaces bacteria in the microbiome, allowing for other pathogenic agents to come in."
Vibrio is a genus of bacteria, many of which are pathogenic—think cholera and forms of gastroenteritis—and proliferate in diseased tissue. As sea star wasting disease progresses, vibrio is among the microbes present, making it difficult to untangle which players are truly problematic. The UVM study, however, showed vibrio present early in infected individuals.
"[Vibrio] is always the sinister character there in the dark room, if temperatures are warm, or oxygen conditions are low, it's going to be there," Pespeni says, "but in this case, we are seeing it in exposed animals that don't have signs of wasting … which lends support to vibrio being a potential cause and worth further investigation. We also found that its abundance is correlated with changes in sea star genes that we know are responding to disease."
And because vibrio is not a new bacterium in marine environments, a major question remains: why did it start triggering mass die-offs in sea stars?
Next steps for investigation
Starfish move by pumping water through their bodies, making them particularly vulnerable to whatever is floating around in the ocean. They are also sensitive to changes in temperature. Some of the only sites where healthy populations of sunflower sea stars still exist are in northern fjords, which tend to have colder waters and more fresh water supplies, McCracken notes.
He hopes the UVM team's recent findings can inform conservation groups monitoring the disease and working to restore sea stars to their native waters. Efforts often involve rearing star fish and transplanting juveniles back into the wild. Some species, such as ochre sea stars, have even rebounded.
"One of the important things from this research is once you see it, it could be too late," McCracken cautions. "If you were monitoring a space and you see an actively wasting sea star, picking up another seemingly healthy one nearby and trying to transport it out of there, they are probably already infected."
He wonders if future research could be designing molecular markers that detect vibrio or other agents of concern in water samples before transplanting young sea stars.
With mass die‑offs now subsiding, Pespeni sees a critical window to investigate whether surviving sea stars carry signs of natural resistance.
"It's possible that the sea stars that survived have traits that made them more resistant to disease," she says. "If that's the case, understanding what distinguishes them could be key to revealing which populations or species are most at risk during future outbreaks."
