Death of 5 Billion Starfish Baffled Scientists—Until Now

Researchers have traced the devastating loss of more than 5 billion sea stars—known colloquially as starfish—along the Pacific coast of North America over the past decade to a bacterial culprit.

The findings, published in Nature Ecology and Evolution, help explain an ecological crisis that saw sunflower sea star populations plunge by up to 90 percent from Alaska to Mexico since 2013, reshaping vital marine ecosystems in the U.S. and Canada.

Why It Matters

The mass die-off, driven by “sea star wasting disease,” has had devastating ripple effects across Pacific coastal ecosystems, including those off the U.S.

These predators, especially the sunflower sea star, help regulate sea urchin populations. Their disappearance led to unchecked sea urchin growth, which in turn destroyed extensive kelp forests—habitats called the “rainforests of the ocean” due to their biodiversity and importance for marine mammals, fish, and invertebrates. In Northern California alone, kelp coverage dropped by 95 percent within a decade as a result.

What To Know

In 2013, sea stars from Alaska to Mexico started exhibiting symptoms of “wasting syndrome”—including twisted limbs, lesions, and disintegration—that gradually devastated more than 20 species, with the sunflower sea star most affected.

“It’s really quite gruesome,” said marine disease ecologist Alyssa Gehman from the Hakai Institute in British Columbia, Canada. The disease was so severe it often caused the sea stars’ arms to detach entirely, added Gehman, who helped pinpoint its cause.

Scientists had previously suspected a virus, but years of research showed that the densovirus found in dying sea stars was not the cause. Focus shifted after researchers analyzed the sea stars’ coelomic fluid—their internal body fluid—and identified Vibrio pectenicida, a bacteria also known to infect shellfish, as the real culprit.

In the lab, fluid from sick sea stars was injected into healthy counterparts, causing the wasting symptoms to recur. When the team heat-treated this fluid, killing the microbes, healthy sea stars did not develop disease—definitively pointing to a microbial origin. The bacteria was isolated and pure cultures were shown to cause the illness.

The findings, achieved after more than a decade of setbacks, open the door for targeted conservation strategies. Marine microbiologist Rebecca Vega Thurber of UC Santa Barbara, who was not involved in the study, called it the solution to a “long-standing question about a very serious disease in the ocean.”

What People Are Saying

Blake Ushijima, a microbiologist at University of North Carolina, Wilmington, praised the team’s detective work, saying: “It’s incredibly difficult to trace the source of so many environmental diseases, especially underwater.”

Drew Harvell, a researcher at Cornell University and University of Washington and study co-author, told The Washington Post: “It’s personally incredibly fulfilling to me to have such a solid answer after all this time.”

Jason Hodin, a senior research scientist at the University of Washington and study co-author, said: “The lack of understanding what the disease is has really been a pretty major impediment to being able to move forward with all the kinds of restoration strategies that we’d like to be able to do.”

What Happens Next

Scientists still aim to clarify how the bacterium spreads in the wild, whether it is native or introduced, and what influences—such as warming ocean temperatures—might fuel future outbreaks.

This article contains reporting by The Associated Press