A decade-long mystery has finally been solved: Scientists have identified the bacterial culprit responsible for the mass die-off of over 5 billion sea stars along the Pacific coast of North America, a catastrophic event that reshaped marine ecosystems from Mexico to Alaska.

According to a new study published in Nature Ecology and Evolution, researchers have pinpointed Vibrio pectenicida, a marine bacterium, as the cause of sea star wasting disease, a condition that has decimated more than 20 sea star species since the outbreak began in 2013.

The most severely affected species, the sunflower sea star, saw its population plummet by nearly 90% within five years, effectively disappearing from large stretches of the Pacific coast.

“It’s really quite gruesome,” said Alyssa Gehman, a marine disease ecologist at the Hakai Institute. “Infected sea stars develop lesions and begin disintegrating, losing limbs as the disease progresses.”

Decade of misdiagnosis

For years, scientists suspected viruses were to blame, including a densovirus later found to be benign. The breakthrough came when researchers shifted their focus from decaying sea star tissue to the coelomic fluid, the internal body cavity where Vibrio pectenicida thrives undetected.

This discovery reshapes the scientific understanding of the epidemic and opens the door to new interventions. Strategies under consideration include selective breeding for immunity and restoration efforts in key coastal regions.

The consequences of the die-off extend far beyond the sea stars themselves. These predators play a vital role in marine food webs. In their absence, sea urchin populations have exploded, decimating kelp forests, a cornerstone of coastal ecosystems. In Northern California, scientists estimate that 95% of kelp forests have vanished due to unchecked urchin grazing.

The study’s authors emphasize that reversing the damage will require urgent conservation strategies and ecosystem-level restoration efforts, especially as marine environments face additional stress from climate change.

The long-overdue identification of the bacterial cause provides hope for one of the most dramatic wildlife collapses in recent memory, and a path forward to restore balance beneath the waves.