I see my first sunflower sea star in a plastic container barely large enough to hold a sandwich. It’s eating lunch. I am in a garage-like government laboratory on the curled tip of Washington state’s Marrowstone Island, not far from a dark and drippy cedar forest at the northern edge of Puget Sound. Tubes pump icy ocean water into dozens of containers, each of which serves as a makeshift condominium for a single baby sea star. Around me, scientists are popping off lids and documenting which container walls each animal is touching. “Near side, left side, bottom,” one murmurs as another jots notes.
The sunflower stars (Pycnopodia helianthoides) are being kept in isolation to avoid transmitting germs. They are, in a sense, social distancing, for the same reason we did in the early days of COVID-19: Some might carry a pathogen that could kill the others. That’s why researchers are logging arm positions. They are tracking which stars have slowed their movements. They want to know which might soon be dead.
I’m here because this team believes it has unravelled a monumental mystery. In 2013, sea stars began dying along North America’s West Coast, victims of a plague known as sea star wasting disease. The condition first surfaced in the Pacific Northwest, where scientists and scuba divers noticed sea star arms flexing and corkscrewing in tight spirals before tearing off and shimmying away like dancers in some macabre ballet. Grisly lesions, white and splotchy, spread like gangrene across their surfaces. Internal organs seeped from open wounds. Bodies would bloat and then deflate, the muscles dissolving in soupy puddles that left behind flattened sheaths of flesh. These, too, eventually decayed until all that remained were ghostly silhouettes, a sickly residue on the seafloor.
Within months, the disease had spread thousands of kilometres in each direction, from Baja California to the Gulf of Alaska. The body count quickly reached the millions. After a few years, it topped several billion. At least 26 species were affected. And although the carnage has since slowed—in part because there are fewer of the invertebrates left to kill—it’s ongoing. Yet for more than a decade, scientists haven’t been able to figure out what’s causing it.
“There’s a lot we don’t know about the ocean,” Alyssa Gehman tells me, as she and her two colleagues make their rounds among the sea star condominiums. Gehman, who grew up in Seattle and is now a disease ecologist with the University of British Columbia and the B.C.-based Hakai Institute, has been an exuberant sea star fan since their kaleidoscopic variety mesmerized her on a middle-school kayak trip. She learned to dive as a teenager and later studied juvenile six-rayed stars. She and her sister even have matching sea star tattoos.
In recent years, Gehman has joined other scientists trying to figure out why so many sea stars are dying. Early on, some suspected it might have been caused by toxins from harmful algal blooms. Others worried it could be the result of oxygen loss at sea. At one point, scientists thought they’d solved the mystery, identifying a virus that seemed a likely assassin. But efforts to confirm that finding came up short, prompting researchers to abandon their conclusion.
Now, after four years of work and endless rounds of RNA sequencing, Gehman and her team think they’ve found the real culprit: the causative agent, in science speak, behind perhaps the worst marine die-off in modern history. But given the false leads of the past, Gehman will settle for nothing less than certainty. That’s why she’s testing, and testing again. And that’s why I’m here to watch.
She nudges me closer as a colleague cracks open another container. Inside, a 16-armed sunflower star (not all have five), the colour of a faded basketball clings to one wall. This, like the others, is a healthy baby sunflower star, the species hit hardest by wasting. They range in colour from sunbeam to lavender and can move the length of a king-sized bed in two minutes flat. To me, though, this particular specimen already looks dead.
Then it suddenly comes to life. Its arms peel off the container walls and fold like a swimmer in a flip turn. It floats belly up, its tiny tube feet wriggling, the mouth at its centre agape. Despite lacking a brain or even a head, this creature somehow seems to grasp that the removal of the container’s lid means Gehman’s team is about to feed it. Anticipation of the pending smorgasbord revs the sea star’s nervous system as much as opening the kibble drawer excites my dog. I find it charming; Gehman goes even further. “We love these animals,” she says.
And sea stars’ ecological importance supersedes their capacity to delight. They’re so essential that the term “keystone species” was first coined about them; research shows that removing sea stars from intertidal food webs lets other invertebrates take over, which often mow down kelp forests or driving out species such as chitons, limpets, anemones, and barnacles. Sea stars generate such outsized influence, in other words, that their collapse is reshaping coastal waters in severe and disturbing ways.
Scientists know the only way to reverse the damage is to rescue and restore wild populations. But “we can’t save them if we don’t know why they’re dying,” Gehman tells me. She plops a meal of mussels into the container of the hungry sunflower star. Then she methodically moves on to the next step in her mission to track down whatever is killing the creatures she adores.
Sea stars emerged nearly a quarter-billion years before the dinosaurs, when much of the complex life on Earth lived at sea. They came to thrive in every ocean, from the soft, warm sands of the U.S. Atlantic Coast to the ice-cloaked waters around Antarctica to the depths of the Mariana Trench. (Since they don’t have fins or gills and aren’t fish, some experts balk at calling them starfish, though many welcome the term colloquially.) Most sea stars can’t outrun the fish, mammals, birds, or crabs that eat them, so they protect themselves with armour, prickly spines, neurotoxins, or slime. They are, however, most famous for another defence mechanism: re-growing arms after losing them. Some can even regenerate whole bodies from a single, severed limb.
Sea stars have captivated people through the ages. The Aztecs used their dried bodies in altars, offering them to the war god, Huītzilōpōchtli, while to ancient Egyptians, a sea star represented the goddess Isis. They’ve been memorialized in everything from Bronze Age pottery to children’s cartoons like SpongeBob SquarePants.
Today, about 2,000 species populate the planet, in a seemingly endless array of shapes and colours. Most have five arms, but some have six, or seven, or 11, while others may grow 40 or more. There are blue ones with long, thin appendages, and glossy, sparkly, fat-winged red ones. Some have stripes, splotches, or bands, and one grows arms that look as if a baker dotted them with tiny Hershey’s kisses. It’s called a chocolate chip sea star.
Astropecten articulatas, the royal star, has a brilliant purple center trimmed in blaze orange. The mounded, pentagon-shaped puffball Culcita novaeguinea looks more like a pincushion than a star, while Iconaster longimanus appears to have been pieced together by an artist working with ornate panes of stained glass. The paddle-spined star rarely gets larger than a pinkie fingernail. But sunflower stars grow as large as truck tires—presuming, of course, that they live long enough.
One of the earliest indications that something was wrong came on September 6, 2013, when marine scientist Drew Harvell first heard about a mass death of sunflower stars. A colleague had just returned from scuba diving near Vancouver, B.C., and posted a video accompanied by a horrifying description: “The bottom… was absolutely littered with arms, oral discs, tube feet, [and] gonads,” the colleague wrote, according to Harvell’s book Ocean Outbreak: Confronting the Rising Tide of Marine Disease. “It was kind of creepy.”
Harvell, a professor of ecology and evolutionary biology with Cornell University who is based in Washington state, was intrigued, though not yet deeply worried. But then in December, while attending a meeting in California, she experienced a moment where “something bad turns big,” as she later wrote. Scientists in Monterey Bay had spied thousands of dead stars from at least 10 species. In Seattle, more were piling up below piers. Harvell hopped a flight back to Seattle and headed for Puget Sound that very evening. Donning a headlamp and slipping over rocks, she saw the scale of the disaster firsthand. “It was spectacular how quickly that spread,” Harvell tells me on a wet afternoon last fall when I meet her at the University of Washington’s Friday Harbor Labs on San Juan Island.
Harvell knew that mass die-offs in nature, even among sea stars, are not uncommon. For years, as an expert in marine diseases, she’d used this UW facility to track outbreaks of other illnesses. This epidemic, though, wasn’t restricted to a limited geographic region; it extended almost the length of the continent. It wasn’t limited to one or two species, but affected more than two dozen. And it was clear that whatever unknown pathogen was killing sea stars had a co-conspirator: heat.
In 2013, around the time Harvell was alerted to the die-offs, an enormous patch of warm seawater had begun to form across a swath of the eastern Pacific Ocean, at first in the Bering Sea and the Gulf of Alaska. The Blob, as it came to be called, initially spread hundreds of kilometres across and hundreds of meters deep. In time, this overheated pool expanded to cover more than 1,600 kilometres (1,000 miles) in every direction. It stretched 90 meters (300 feet) down, held seawater that was as much as 3.9 degrees Celsius (seven degrees Fahrenheit) hotter than normal, and upended an entire universe.
Warm-water species like ocean sunfish appeared off Alaska. Nazca boobies more common to Central America started showing up in California’s Farallon Islands. Millions of Pacific Northwest seabirds died because their ocean food was suddenly nowhere to be found. Dozens of whales washed up dead, too. And then sea stars collapsed.
“This thing was like nothing we’d ever seen,” Harvell says. “Nobody ever expected something of this magnitude.”
Sunflower stars were hit particularly hard, with 91 percent of the species killed across its range, from Mexico to Alaska. The International Union for the Conservation of Nature added sunflower stars to its red list, classifying them as critically endangered. Most troubling of all: Unlike smaller sea star die-offs in the 1970s, 1980s, and 1990s, the population did not rebound. More than a decade after they began to die, sunflower stars are still dying. And the consequences of their absence are still reverberating.
Nowhere has the human and ecological toll of all this death been clearer than in Northern California. Without their many-armed predators, purple sea urchins (Strongylocentrotus purpuratus)—one of sunflower stars’ key foods—have taken over the ocean bottom along the coast. Their population jumped 60-fold in a single year, so displacing the red sea urchins (Mesocentrotus franciscanus) prized by sushi eaters that a commercial fishery for red urchin was declared a federal disaster in 2019. Livelihoods were shattered.
“Christmas this year was really hard,” says Grant Downie, a commercial diver in Fort Bragg, California, who once made up to $150,000 a year harvesting red urchin.
The changes Downie has seen on the seafloor are nearly as troubling as those in his bank account. Voracious purple urchins, a species for which there is no market, have mowed down lush bull kelp (Nereocystis luetkeana); along a 350-kilometre (217-mile) stretch of coast in Sonoma and Mendocino counties, 90 percent of kelp died. That’s left no food for red abalone, causing a US $44 million recreational abalone dive fishery to close. Baby rockfish, which typically hide within long, waving kelp strands, have also dwindled, as have many other species. Much of the region now resembles underwater clearcuts—a patchwork of overgrazed seafloor that scientists call “urchin barrens.”
“This ecosystem is predicated on balance—we need a certain combination of species to make it function,” says Tristin Anoush McHugh, kelp project director for The Nature Conservancy’s California Oceans Program. “The loss of sunflower stars transformed it.”
John Muir once mused, “when we try to pick out anything by itself, we find it hitched to everything else in the Universe.” In some places, the weave on that tapestry is particularly tight.
Without intervention, Northern California’s crisis will almost certainly worsen. Down south, kelp forests hold lobster and a fish called sheephead that eat purple urchins. But north of San Francisco, few animals aside from sunflower stars keep purple urchins in check. Even after purple urchins eat all available food, they don’t quickly die. They can go dormant for years, living in a zombie-like state.
Several years ago, The Nature Conservancy and others began paying divers, including Downie, to help restore kelp forests by harvesting purple urchin. The conservation nonprofit is also trying to find ways to sell those urchins for food or shell crafts. But although Downie likes the work, restoration efforts cover only a fraction of the vast underwater barrens, and the market for purple urchin remains minuscule. In the long run, the best hope for ecosystems and economies is restoring wild sunflower star populations.
To raise sunflower stars in captivity and release them to the Pacific, however, scientists must first learn how to keep stars from dying. And for that, they must know what’s killing them in the first place.
Back in the lab on Marrowstone, Gehman and her team prep 42 baby stars for some of their final experiments. They split the stars, still in plasticware, into two groups on separate tables, preserving half to serve as a control group. They ready syringes and microscope slides.
When marine animals die in localized groups, Gehman tells me, scientists tend to look to pollution or weather anomalies: a heat wave, an oil spill, even sewage overflow—something that alters ocean chemistry or introduces new and harmful microbes. If “you’re just seeing a bunch of dead things in one site, then you’re going to say, ‘Did something big just happen recently?’”
But when die-offs are enormous and widespread, suspicion moves to contagions. Just as COVID-19 jumps quickly from person to person, diseases can move swiftly at sea. The difference: We understand the human body well enough to know which microorganisms don’t belong. For sea stars and millions of other marine creatures, “when it comes to what parasites, what pathogens, what bacteria, what viruses are common… we just really don’t know,” Gehman says.
In 2020, after prior attempts failed, The Nature Conservancy decided to finance one more effort to determine the cause of death. With additional support from the Hakai Institute, the organization agreed to pay for Gehman’s research. Soon after, she received her first shipment of healthy stars from Puget Sound. Her team slowly exposed them to a slew of contaminants: the tanks of sick and dying stars; seawater housing ill stars; ground-up flesh from dead stars; sick stars’ internal fluids. In each case, the healthy stars died, usually within a week. When the team heated ground-up tissue and bodily fluids from sick stars to kill pathogens before injecting the mixture into healthy stars, none got sick. That confirmed that a microorganism, and not heat alone, was to blame. But what kind of organism?
One way to narrow down the identity of a disease agent is to determine its size. Bacteria are single-celled animals, but viruses can be 10 to 100 times smaller. So Gehman’s team ran coelomic fluid—the closest thing to blood in a sea star—from sick animals through a plastic device that works like a coffee filter. The device filtered out larger microorganisms. When Gehman then injected that filtered fluid into healthy stars and they did not get sick, it suggested that the germ causing sea star wasting disease was too big to be a virus.
Finding the actual culprit would still prove cumbersome, though. After sampling stars before and after exposure, as well as stars that never got sick, Gehman’s team then set about painstakingly sequencing RNA from each microbial community, using sophisticated new technology that allows scientists to take a piece of clothing, a fingernail, or even a fleck of skin and use relatively cheap genetic methods to evaluate all bacteria in a sample. Gehman’s researchers also benefited from zeroing in on the stars’ bodily fluids rather than just using samples from sea star flesh.
One day in late 2023, an hour before a scheduled Zoom meeting, the team stumbled on their answer. Gehman’s colleague Melanie Prentice, also with the Hakai Institute and UBC, had begun reviewing the sequencing analysis. Scrolling through data on her laptop, an unmistakable pattern jumped out. Of the hundreds of microorganisms in the samples, enormous quantities of bacteria from the genus Vibrio seemed present in sick stars.
The pattern was so strong, Prentice tells me, that she assumed she’d bungled the analysis. “I’ve definitely done something wrong here,” she recalls thinking. She went back to the beginning. But the more the team tried disproving its discovery, the clearer it became: They found high concentrations of one particular species of Vibrio in 100 per cent of sick stars. They found none of that species in the healthy ones. They tested the remains of stars that had died in Alaska in 2016 and B.C. in 2023 and found the same type of Vibrio there, as well. The team cultured colonies of the bacteria and exposed healthy stars. They all died, too.
Now, in August 2025, after another year-and-a-half of work confirming and reviewing their findings, Gehman and her team have published a peer-reviewed study in the journal Nature Ecology & Evolution identifying Vibrio pectenicida, a saltwater-loving bacterium that works its way into sea star fluids, as the likely “dominant pathogen responsible for sea star wasting disease.” Gehman’s team has tracked down the killer.
“It’s great work,” says Colleen Burge, a research associate with the University of California, Davis Bodega Marine Lab who also supervises the California Department of Fish and Wildlife’s shellfish health laboratory and had worked on previous attempts to find the pathogen behind sea star wasting disease. “It’s extremely difficult to take a sick animal, isolate something, and introduce it to a healthy animal. We’re not working with lab rats or some clean population. Sometimes it just takes time.”
Rebecca Vega Thurber, director of the Marine Science Institute at the University of California, Santa Barbara, says Gehman’s results make intuitive sense. The genus Vibrio occurs naturally in the sea and includes more than 100 species, including the bacteria that cause cholera and one type of shellfish poisoning, both of which can be deadly to humans. “When I heard it was a Vibrio, I literally did a facepalm and thought, ‘Of course it is,’” Thurber says, groaning. “They are the most notorious disease-causing agents in the ocean.”
Vibrio are also known to be more pathogenic and spread faster as temperatures climb. And scientists know that stars residing in cold water have been slower to show signs of sickness and slower to die. It’s no wonder that scientists have even taken to calling the Vibrio genus a “microbial barometer” of climate change.
Still, many unknowns remain. Did the marine heatwave make the bacterium more virulent? Did it weaken sea stars’ immune systems? And if climate change and warming oceans presage similar outbreaks, can Gehman and her team’s work help better prepare marine scientists for future die-offs?
“We think that if these guys can get past this pandemic, [past] their sensitivity to this disease, they have a pretty decent chance of potentially persisting in the wild,” Jason Hodin tells me from his lab at Friday Harbor, a few hundred meters from where I’d met Harvell. Hodin, a senior research scientist at UW, is standing in the rain outside a small building up a hill from a faded dock. He’s showing off a maroon-tinged sunflower star named Martha. It’s the size of a grizzly bear’s head, “and it’s not done growing,” he says.
At roughly age 12—no one knows for certain just how long sunflower stars live—Martha is the parent of several lab-grown sea stars. The creature is male (it was named before anyone knew its sex, which you can’t identify until a sea star spawns and broadcasts eggs or releases sperm) and resting in a row of tanks with several others. Martha’s offspring were raised using fresh sperm, but Hodin has bred many other test-tube stars by thawing sunflower star sperm that had been deep-frozen by colleagues in California. Hodin calls those animals his “starcicles.” They have names like Gelato, Snowcone, and Slushie, and “they’re doing great,” he says.
Hodin is fascinated by creatures who change form—“I’m even wearing my shirt,” he says, pointing to his T-shirt depicting a folk music festival celebrating metamorphosis. So when The Nature Conservancy asked him to try breeding sunflower stars, Hodin was excited. Sunflower stars go through a series of developmental stages—from eggs to poppy seed-sized larvae that float around for months to juvenile stars that settle on the sea floor and add more arms as they grow. And while plenty of sunflower stars have lived in aquariums, no one before Hodin had figured out how to consistently grow one from egg to adult. “Less than a handful of [species of] sea stars have ever been raised this way,” he says.
The task was not without complications. When most people think about metamorphosis, their minds go to caterpillars turning into butterflies, or tadpoles to frogs, he says. “You can see that process with the naked eye.” But the only way to watch what happens with sunflower stars is through a microscope. “We literally knew nothing about those early stages: where they live, what they eat, what they need to grow.”
Hodin’s success came through trial and error. He learned, for example, that when they’re young and sharing a tank, stars eat one another. “When they’re at this size and younger, especially younger, they’re cannibalistic,” he says, grinning. “I kind of describe it like the way my brother and I were—well mostly it was his fault.” After a while, the creatures seem to start liking each other so much that they wind up draped all over one another. Hodin also figured out that juveniles need to start hunting almost immediately. So he started raising juvenile urchins for his juvenile stars to chase and eat.
He also gleaned valuable information about how sea stars feed throughout their life cycles. Adult sea stars can eat an urchin every day, but when they’re babies they may eat 10 baby urchins in the same span. That suggests it may take far fewer stars than once thought to restore balance to underwater ecosystems. Inside the lab, he shows me photos of stars he’s released into nearby eelgrass beds. He and his collaborators have been tracking them to see how they survive and grow. He found that although they disperse, they usually come back to the same piling where they were set free. Hodin assumes they possess some type of imprinting ability. “They presumably have some kind of map in their head about the environment around them,” he tells me.
Ashley Kidd, conservation manager at the Sunflower Star Lab in Monterey Bay, is one of several colleagues now trying to replicate Hodin’s techniques. “Jason will never toot his own horn,” Kidd tells me later. “But he cracked the code. His work was seminal.” It’s a giant step forward in helping sunflower stars recover. But a long list of questions remains before actual restoration can begin to repair the damage of losing nearly 6 billion sunflower stars. “Nobody has a crystal ball,” Kidd says. “But we’re breaking it down into digestible pieces that are being worked on by lots of people.”
Researchers in Oregon, for example, are trying to figure out exactly how many urchins in how big a radius an individual sunflower star might consume. That could tell them how many millions of baby stars need to be raised to restore balance to West Coast ecosystems, how many urchins need to be fed in captivity, and at what age they can be released to the wild to maximize survivability. And of course, researchers are exploring whether stars can be selectively bred to reduce or eliminate their susceptibility to V. pectenicida—or if it’s possible in some way to inoculate them against it.
Although much remains to be done, Jono Wilson, director of ocean science at The Nature Conservancy in California, says glimmers of a path forward are becoming clear. “The way I see it in my head is there are probably six facilities up and down the coast in the next five to 10 years growing millions of sea stars,” he says.
A few weeks after I first meet Gehman at the lab, we reconnect amid sun-bleached driftwood in West Seattle’s Lincoln Park, a yawning stretch of pebbly shore across Puget Sound from the broken-toothed skyline of the Olympic Mountains. After a while, Gehman—wearing a T-shirt advertising a Japanese parasite museum, a trucker’s cap silhouetted with evergreens, and her ever-present Xtratuf boots—splashes into the surf and begins turning over rocks.
Gehman has been visiting this park since she was a teenager. At 16, she worked as a beach naturalist here, helping visitors spot knobby sea cucumbers, moonglow anemones, and thick-horned nudibranchs. The biggest draw by far were sunflower stars. Not only was their size astonishing, but they were also ferocious predators that sent marine life scurrying. That entertained tourists and offered teachable moments. “And,” Gehman says, “they were everywhere.”
Back then, at low tide on any 400-meter stretch, she’d see several adult sunflower stars. On a 40-minute scuba dive she might see 15. Wandering the water’s edge with me, however, she doesn’t spot a single one—not even after we walk for nearly an hour.
Still, in small pockets near shore and out in deeper waters, millions of sunflower stars remain. It’s a fraction of their former abundance, but Gehman has seen them on dives; they’re the source of the healthy animals that she, Hodin, and others use for experiments and breeding stock. She even discovered populations of healthy stars living in the chilly fjords off British Columbia’s central coast. There, as well as in other locations, such as off Puget Sound’s Whidbey Island, V. pectenicida somehow hasn’t taken hold—or at least hasn’t yet turned entire ecosystems upside down.
These refuges provide glimmers of hope. If all goes well, some day in the future, Gehman might retrace the steps she and I walked and see what her younger self once did: a coastal environment teeming with brilliant, hungry bursts of enormous waggling arms; an entire universe at the edge of the continent, connected once again to everything else.
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