There’s hope for an iconic ocean predator that has mostly disappeared from California’s coastlines: the sunflower sea star (Pycnopodia helianthoides).

The population began to plummet in 2013, following an outbreak of sea star wasting disease, a deadly ailment that pocks starfish bodies with lesions and then dissolves their flesh. It affects roughly 20 species, and has killed a staggering 5.75 billion sea stars. But it’s been particularly catastrophic for sunflower sea stars. About 99% of the population along its southern range is gone, from Washington state to Mexico.

It’s the largest epidemic ever to occur in the ocean. Though the disease peaked between 2013 and 2017, localized outbreaks still occur, and the population hasn’t recovered.

Now, more than a decade after the epidemic began, the mysterious killer has finally been unmasked. Researchers at the University of British Columbia and the Hakai Institute in Canada just published their discovery, identifying a strain of the bacterium Vibrio pectenicida as a culprit.

Increasing sea surface temperatures in the Pacific Ocean, driven by anthropogenic climate change, are believed to have triggered or exacerbated the disease, according to the IUCN, the global wildlife conservation authority.

Sea stars are voracious predators that feed on sea urchins and other prey. In their absence, urchins have proliferated and laid waste to the kelp forests that historically lined the California coast, which are homes and nurseries to innumerable marine species, from fish and seabirds to otters and sea lions.

Now, a coalition of California aquariums, nonprofits, Indigenous groups and government experts is trying to bring this starfish back, and along with them, the ecosystem. But even with a groundswell of support from across the state, getting to this point took immense trial and error because little was known about these echinoderms.

A number of facilities are now raising sunflower sea stars, including the nonprofit Sunflower Star Laboratory in Monterey, California. It is currently raising  72 juveniles and is gearing up to begin controlled releases into the waters of Monterey Bay. It’s a key step in the journey to large-scale reintroductions.

Troubled stars

Sea stars are ancient creatures that first appeared on Earth around 480 million years ago, predateing the age of the dinosaurs by more than 200 million years. Over time, they evolved into nearly 2,000 unique species, forming a group called Asteroidea.

Sunflower sea stars are among the world’s largest, sometimes growing to the size of a bicycle tire and sprouting up to 24 arms. Their lifespan in the wild remains unknown, but they’ve survived in aquariums for decades. Most dwell at depths between sea level and 25 meters (82 feet), but they’ve been discovered as deep as 455 m (nearly 500 ft).

They’re adaptable, living in eelgrass meadows and kelp forests, as well as on mud, sand, shell fragments and rocks of all kinds. What matters most is food: They prey on clams, mussels, oysters and other bivalves, snails, and, most importantly, sea urchins.

Historically, they were found trawling for prey from Alaska’s Aleutian Islands down the Pacific coast to Baja California in Mexico. When sea star wasting disease hit in 2013, it essentially eradicated the creature from 2,700 kilometers (about 1,700 miles) of its southern range.

This gruesome affliction begins with splotchy lesions and proceeds to melt tissue. The stars contort and often lose their arms before succumbing within about two weeks of exposure.

In the laboratory, researchers have seen that warmer water temperatures speed up the progression and the mortality rates of the disease. Out in the sea, higher temperatures have been linked to outbreaks in specific regions.

Climate change may have additional impacts on an animal that’s already in critical condition. In California, the warmer the water, the fewer the sea stars, and 14° Celsius (57.2° Fahrenheit) seems to be a strong cutoff point.

A stellar whodunnit

For years, scientists have tried to figure out what caused this massive sea star die-off. Initially, toxic algae blooms were (wrongly) thought to be the culprit, then possibly “dead zones” that lacked oxygen. The sea star-associated densovirus was then blamed, and discounted.

Heat was involved but not thought to be the cause: In 2013, just as the mass die-off of sea stars began, a massive stretch of warm water dubbed “the Blob” engulfed the Eastern Pacific, extending hundreds of feet deep, maxing at 3.9°C (7°F) above normal.

Now, scientists say they believe they’ve finally cuffed the sunflower star’s killer. The researchers, led by Melanie Prentice and Alyssa-Lois Gehman at the University of British Columbia and the Hakai Institute, discovered extremely high levels of V. pectenicida in sick sea stars’ coelomic fluid, which is essentially sea star blood.

Then they grew it in the lab and injected it into healthy stars. Those animals fell ill with wasting disease symptoms. Their arms curled and fell off, and they eventually died. The new research was published in the journal Nature Ecology & Evolution.

The researchers say they’re convinced that they’ve solved this macabre mystery. What she saw happening in the lab mirrored what she saw in the sea, Gehman said. “This is consistent with what I see when I am diving and observing wasting in sunflower sea stars in the field,” she said. “It is unequivocal that V. pectinicida causes death and SSWD-like presentation.”

One scientist has raised a question, however. Ian Hewson, a marine ecologist at Cornell University, who has worked on SSWD, noted that the authors didn’t report data on lesions, a telltale sign of the disease.

While the absence of lesions is notable, “disease presentations can vary depending on the pathogen, environmental conditions and host response,” said Blake Ushijima, a microbiologist and marine disease expert at the University of North Carolina, Wilmington, who wasn’t involved in the study. Based on the new paper, he said he agrees that this bacterium is a cause of SSWD.

Gehman said her team did see lesions in some of the sunflower stars, but “chose not to use those as metrics because they were less consistent as signs of disease than arm-twisting and arm loss.”

Hewson likely has reason to be cautious: In 2014, he led a study incorrectly implicating a densovirus as a potential cause of the disease. He published a mea culpa last year exonerating the virus and reflecting on lessons learned from his experience.

Responding to the crisis

In response to the sunflower star’s startling demise, the Pycnopodia Recovery Working Group was formed in 2019, comprised of members from three countries and four U.S. states. This broad consortium includes experts from universities, nonprofit organizations, American Indian Tribes, Indigenous First Nations and government agencies.

Their goal: to develop a recovery strategy. Soon after the group formed, they realized the critical need to raise sunflower sea stars in captivity, which had never been done before. It was also crucial to identify what was killing them in the wild.

In 2021, the IUCN listed the sunflower sea star as critically endangered. Two years later, a review by the U.S. National Marine Fisheries Service proposed listing the imperiled star as a threatened species under the U.S. Endangered Species Act because it’s likely to become endangered within the foreseeable future throughout much or all of its range. A decision is still pending.

The Sunflower Star Laboratory coordinated with the working group to research and help launch a reliable way to raise these slow-growing creatures. “We were founded by local members of the Monterey Bay area community who were all watching the kelp disappear from in front of our eyes,” said Reuven Bank, chair of SSL’s board.

Early on, “nobody knew the sexes of the stars. Nobody knew how frequently they spawned,” said Ashley Kidd, SSL’s conservation project manager.

California aquariums that held them in their collections began close observation to identify the sex of their animals when they next spawned, producing either sperm or eggs. It turned out that Birch Aquarium in San Diego was the only facility with both sexes, home to one female and four males.

Researchers successfully synchronized the males and female to spawn by injecting the hormone 1-methyladenine into the stars’ arms. The team — experts from Birch Aquarium, the Aquarium of the Pacific, California Academy of Sciences, San Diego Zoo Wildlife Alliance, Sunflower Star Laboratory and other partners — managed to successfully fertilize sea star eggs.

To find the best way to produce sea star offspring in the future, they used sperm from the same male in three separate experiments: fresh; frozen at -80°C (-112°F) in a freezer; and cryopreserved at -196°C (-321°F). All were successful.

Raising the sunflower sea star ‘cupid cohort’

The fertilized eggs produced at Birch Aquarium — dubbed “the cupid cohort” because the spawning took place on Feb. 14 — were shipped to aquariums around the state as part of the Association of Zoos & Aquariums’ SAFE (Saving Animals From Extinction) program.

Moss Landing Marine Laboratories was among the destinations. There, Andrew Kim was working as an aquaculture research technician, and it became his duty to keep the larvae alive.

There was limited infrastructure, he remembers, “but it caused me to get creative.” He housed the larvae in beakers, tubs and other containers, fed them different kinds of algae, and tested different oxygen levels.

Kim learned that sunflower star larvae thrive under many different conditions as long as they’re well-fed. When he left Moss Landing to become lab manager at Sunflower Star Laboratory, his next challenge was overseeing the young stars’ journey to adulthood.

Starfish begin life as free-floating, gelatinous larvae, and only settle onto the seafloor, where they mature, if they detect that conditions are just right.

Kim added an alga, Calliarthron tuberculosum, into the water and found that it satisfied larvae, and they settled down. The lab is now home to 72 young sunflower stars. The largest, named Titan, measures 20 centimeters (8 inches) across.

As the stars have grown up, their unique personalities have started to emerge. “I have a couple of stars that every single time I feed them, they engulf the food,” said laboratory technician Mariana Jimenez, while others are “little babies, and they want love and attention.”

• 
  With sunflower sea star numbers so low, learning how to breed and raise them in captivity — and successful release them back into their native habitat on North America’s Pacific Coast is key to their recovery. Image courtesy of Ralph Pace/The Nature Conservancy.
• 
  Here, the stars were placed in the ocean for a few weeks; all survived and returned to the lab. Image courtesy of Ralph Pace/The Nature Conservancy.
• 
  Alyssa Gehman diving in Burke Channel. Image courtesy of Bennett Whitnell/Hakai Institute.
• 
  Vibrio pectenicida, the bacterium that was just identified as the sea star killer, was cultured in the Aquatic Microbiology and Virology Lab at the University of British Columbia in 2024. Researchers then injected the pathogen into healthy sea stars, and the ensuing rapid mortality was final proof that V. pectenicida causes SSWD. Image courtesy of Toby Hall/Hakai Institute.
• 
  Alyssa Gehman examines an adult sunflower sea star. Image courtesy of Kristina Blanchflower/Hakai Institute.
• 
  A sunflower sea star is shown in the late stages of a sea star wasting disease, which has killed billions of sea stars from over 20 different species, from Alaska to Mexico. Sunflower sea stars were especially hard-hit. Image courtesy of Grant Callegari/Hakai Institute.

One star, Bigby, has a reputation as an escape artist, using its tiny tube feet to crawl up and out of its container. “We’ve seen Bigby literally three-fourths of the way into another star’s tub,” Jimenez said.

But if all goes well, Bigby may someday be able to explore a wider world. SSL is working toward a small-scale release of stars to test what conditions they need to thrive. To that end, the lab has been in touch with California state regulators from the very beginning, Reuven Bank said.

“We’re addressing the path towards reintroduction from an interdisciplinary and holistic approach,” Bank added. “It’s a comprehensive plan with the ability to monitor them and the ability to produce them at scale.”

Sunflower stars have already been raised and released by researchers at the University of Washington’s Friday Harbor Laboratories. In August 2024, divers released 20 young stars off San Juan Island, ranging in age from 1 to 3 years old..

Looking towards recovery

It will take further research to understand all the factors that created and spread the sea star wasting epidemic, but for now, it occurs at much lower levels.

Looking to the future, the SAFE program published a 2024-2027 conservation plan for sunflower star recovery along North America’s west coast. It encompasses the Pycnopodia Recovery Working Group’s research and highlights seven main objectives: establishing recovery goals and pinpointing threats; monitoring remaining wild populations; expanding disease research and finding ways to fight it; expanding captive breeding and figuring out how to release stars; understanding the government permitting process and any possible biosecurity issues; and improving communication with the public.

There’s widespread support for sea star recovery. Even fishers who compete with sunflower stars for urchins want the many-armed giants back, Bank said.

“Purple and red urchin fisheries need healthy kelp forest to have healthy, fat urchins for them to harvest,” he said. “So many disparate groups within the community, who may even have been at odds on other conservation issues, are generally supportive of sunflower star recovery.”

Banner image: Healthy sunflower sea stars, in 2023. Image courtesy of Bennett Whitnell/Hakai Institute.

Prickly babies: A Jamaican nursery aims to restore sea urchins felled by disease

Citations:

Selgrath, J. C., Carlton, J. T., Pearse, J., Thomas, T., & Micheli, F. (2024). Setting deeper baselines: Kelp forest dynamics in California over multiple centuries. Regional Environmental Change, 24(3), 104. doi:10.1007/s10113-024-02260-1

Hodin, J., Pearson-Lund, A., Anteau, F. P., Kitaeff, P., & Cefalu, S. (2021). Progress toward complete life-cycle culturing of the endangered sunflower star, Pycnopodia helianthoides. The Biological Bulletin, 241(3), 243-258. doi:10.1086/716552

Prentice, M.B., Crandall, G.A., Chan, A.M., Davis, K. M., Hershberger, P. K., Finke, J. F., … Gehman, A. M. (2025). Vibrio pectenicida strain FHCF-3 is a causative agent of sea star wasting disease. Nature Ecology & Evolution. doi:10.1038/s41559-025-02797-2

Hewson, I., Button, J. B., Gudenkauf, B. M., Miner, B., Newton, A. L., Gaydos, J. K., … Harvell, C. D. (2014). Densovirus associated with sea-star wasting disease and mass mortality. Proceedings of the National Academy of Sciences, 111(48), 17278-17283. doi:10.1073/pnas.1416625111

Hewson, I., Johnson, M. R., & Reyes-Chavez, B. (2024). Lessons learned from the sea star wasting disease investigation. Annual Review of Marine Science, 17. doi:10.1146/annurev-marine-040623-082617

Credits

Sharon Guynup Editor

Topics

AnimalsBacteriaBiodiversityConservation SolutionsCoral ReefsDiseasesFishFisheriesInfectious Wildlife DiseaseMarine AnimalsMarine BiodiversityMarine ConservationMarine EcosystemsOceansResearchSaltwater FishNorth America

See Topics