Cause of death for billions of sea stars finally identified

More than a decade after a mysterious marine epidemic began wiping out sea stars, scientists have finally identified the cause.

Published in Nature Ecology & Evolution, the new study reveals that the devastating disease – known as sea star wasting disease (SSWD) – is caused by a strain of Vibrio pectenicida.

This marks a major breakthrough in understanding the largest documented marine epidemic in the wild.

Unmasking the killer

Sea star wasting disease has killed billions of sea stars along the Pacific coast, affecting over 20 species from Alaska to Mexico.

Researchers from the Hakai Institute, the University of British Columbia (UBC), and the University of Washington set out to investigate, working with partners including The Nature Conservancy and the U.S. Geological Survey.

After four years of testing, the experts found that the culprit was V. pectenicida strain FHCF-3. Once inside the sea star, this strain causes lesions, arm loss, and tissue breakdown. The sea star can die in as little as two weeks.

“When we looked at the coelomic fluid between exposed and healthy sea stars, there was basically one thing different: Vibrio,” said Alyssa Gehman, senior author and marine disease ecologist at the Hakai Institute and UBC. “We all had chills. We thought, That’s it. We have it. That’s what causes wasting.”

Sea star loss disrupts ocean balance

The loss of sea stars, especially sunflower sea stars (Pycnopodia helianthoides), has reshaped coastal ecosystems. Over 90 percent of these giant sea stars are gone.

“When we lose billions of sea stars, that really shifts the ecological dynamics,” said Melanie Prentice, first author and evolutionary ecologist at Hakai 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.”

Gehman explained that after the sunflower sea stars disappeared, sea urchin populations exploded. Without predators to keep them in check, the urchins quickly devoured vast areas of kelp forest.

“Until this event, we didn’t really grasp the importance of the link between sunflower sea stars and kelp. We knew sunflower sea stars ate urchins. We didn’t know how much they mattered until they all died,” said Gehman.

Why kelp forests matter

Kelp forests are vital marine habitats. They support fisheries, attract tourists, shelter wildlife, and hold cultural value for coastal First Nations and tribal communities. They also store carbon and protect coasts from storms.

“Understanding what led to the loss of the sunflower sea star is a key step in recovering this species and all the benefits that kelp forest ecosystems provide,” noted Jono Wilson of The Nature Conservancy’s California chapter.

The research shows the importance of knowing what you’re fighting. According to Prentice, it is incredibly hard to work on solutions without knowing what the pathogen is.”

“Getting this information makes management and recovery possible. Now all these doors are opened up to us, so we can start doing a lot more work to understand the drivers of disease and the roles played by different species,” said Prentice.

Lab tests confirm the bacteria

The research team didn’t stop with detection. Amy M. Chan, a marine microbiologist at UBC, cultured the bacteria from infected sea stars. They then injected it into healthy ones. The rapid deaths confirmed V. pectenicida FHCF-3 as the cause of SSWD.

The team now wants to explore why this bacterium became such a threat. One possible answer is warmer oceans. Many Vibrio strains grow faster in warm water, and SSWD outbreaks often follow temperature spikes.

“Those patterns of Vibrio in general suggest that we really should look down that road to see how temperature dependence matters,” said Gehman.

Recovery for sea stars

Now that scientists know the cause, they are ready to act. They aim to test for the disease, track outbreaks, and breed resistant sea stars. They are also exploring reintroduction into the wild.

“Now that we found the causative agent of disease, it makes me more hopeful that we might actually be able to do something for sunflower sea stars,” said Gehman.

“We can be really targeted in how we work with them, and I think that’s going to help us move a lot faster and to try to tackle sea star wasting disease.”

According to Prentice, there is a parallel with the human experience of going through the COVID-19 pandemic, and how much COVID tests really changed the way we interacted.

“Now that we know what causes sea star wasting disease, we can develop a test to screen samples, and that will tell us which animals we can move around and what areas are best for reintroduction.”

The future for sea stars

“This finding opens up exciting avenues to pursue and expands the network of researchers able to develop solutions for recovery of the species,” said Wilson.

“We are now actively pursuing studies looking at genetic associations with disease resistance, captive breeding of the animals, and experimental outplanting to understand the most effective strategies and locations to reintroduce sunflower sea stars into the wild.”

The death toll has been staggering. But with this breakthrough, the tide may turn. The future of sunflower sea stars (and the kelp forests they protect) could depend on what comes next.

The study is published in the journal Nature Ecology & Evolution.

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