What Is Sea Star Wasting Disease? The Pacific Die-Off

What happens to a sea star with wasting disease?

The disease usually follows a recognizable pattern. A sick sea star may first stop eating and become sluggish, or look slightly deflated. Then small white lesions appear on the skin and spread, and the tissue around them starts to break down.

As the disease progresses, the animal loses its grip on the rock, because its water vascular system, the hydraulic system that powers its tube feet, can no longer hold pressure. The arms may twist, stretch apart from each other and detach, and shed arms can keep crawling for a while. Within a few days, the body often falls apart completely.

Some of these signs can also appear in sea stars that are simply stressed, for example after being stranded high on the shore in hot sun. Scientists therefore look for patterns: true wasting disease affects many animals at once in normal habitat, often side by side with others showing the same symptoms.

The 2013 outbreak: where and how big

In the summer of 2013, divers and scientists began noticing sick sea stars along the Pacific coast of North America. Early reports came from Howe Sound and other sites in British Columbia, from Washington and from California. Within months, wasting sea stars were reported from Alaska to the border of Mexico.

By late 2013, sunflower sea stars were dying in huge numbers around Vancouver, and ochre sea stars had vanished from some well-studied California shores. In Oregon, intertidal populations were hit in 2014. A smaller die-off also struck sea stars on the Atlantic coast of the United States, from New Jersey to Maine, in 2013.

Scientists could measure the losses so precisely because sea stars had been counted at many sites for years before the outbreak, both by research teams and by citizen scientists. Those records show the 2013 event was far larger than earlier outbreaks in 1972 on the US Atlantic coast and in 1978 in the Gulf of California.

Which sea stars were hit hardest?

More than 20 species of sea stars on the Pacific coast were affected, but some suffered far more than others.

The sunflower sea star, a giant with up to 24 arms and an arm span of up to about 1 m (3.3 ft), fared worst. Researchers estimate that more than 5 billion sea stars died in the outbreak, and the sunflower sea star declined by about 90 percent across its range. In 2020 the IUCN listed it as Critically Endangered. In 2023, NOAA Fisheries proposed listing it as threatened under the US Endangered Species Act.

The ochre sea star, the purple or orange star of Pacific tide pools, also lost a large share of its adults. Declines were proportionally greater in the south of its range.

What causes sea star wasting disease?

For more than a decade, the cause was a mystery. In 2014, researchers linked the outbreak to a virus called sea star-associated densovirus, but later studies in 2018 and 2020 found no link between the virus and the disease.

In 2025, a team of researchers from Canada and the United States published a study in the journal Nature Ecology & Evolution identifying a bacterium, Vibrio pectenicida strain FHCF-3, as a cause of wasting disease in sunflower sea stars. They found the bacterium in the body fluid of dying sea stars, grew it in the lab and injected it into healthy, captive-raised sunflower stars, which then developed wasting symptoms and died. A related strain of the same bacterium was already known to kill scallop larvae.

Important questions remain. It is not yet clear whether this bacterium also caused the deaths of ochre sea stars and other species, or whether it was behind the entire 2013 outbreak.

Water temperature clearly matters. Outbreaks have often been more severe in warmer water, and the record marine heat wave of 2014 to 2016, nicknamed “the Blob”, overlapped with the worst of the die-off. Yet temperature does not explain everything: at many sites the disease struck during cool conditions too.

Why it matters: urchins and kelp forests

Sunflower sea stars are major predators of sea urchins. When they disappeared, the purple sea urchin was left with fewer enemies, especially in northern California, where sea otters are absent.

Urchin numbers exploded and the animals ate their way through kelp forests already weakened by the marine heat wave. Along more than 350 km (220 mi) of northern California’s coast, more than 90 percent of the bull kelp canopy was lost, leaving bare rocky seabed covered in urchins. These areas are known as urchin barrens.

The loss of ochre sea stars matters too. It reduced predation on mussels, the very role that made the ochre sea star the original keystone species, and researchers are watching how rocky shore communities change as a result.

Are sea stars recovering?

Recovery differs from species to species. Ochre sea stars have bounced back in many places, with large numbers of young stars settling on the shore, especially in the north. Even so, adult numbers and the pressure they put on mussel beds have stayed below the levels seen before 2013 on many shores.

Sunflower sea stars have recovered far less. They survive in parts of Alaska and British Columbia, including some cool fjords, and in deeper water. Scientists and partner organizations, including the University of Washington, are now breeding sunflower sea stars in captivity. The 2025 discovery of the bacterium also opens the door to testing whether some sea stars are resistant, and to protecting animals before they are returned to the wild.

Is sea star wasting disease dangerous to people?

It is not known to affect people or pets. The disease has been recorded in sea stars and in some related echinoderms, including a few species of sea urchins and sea cucumbers.

The best way to help is simple. Leave sea stars where you find them, keep them in the water and avoid trampling tide pools. Long-term monitoring programs, such as the Multi-Agency Rocky Intertidal Network (MARINe), track sea stars along the coast, and some regional projects welcome observations and photos from visitors.