More than a decade after sea stars began mysteriously dying along North America's Pacific coast, scientists have identified a microscopic parasite as the likely cause behind sea star wasting disease (SSWD). The discovery resolves a long-standing ecological puzzle that has impacted over 20 species and transformed marine ecosystems.

Sudden decline

Beginning in 2013, populations of sea stars—especially the once-abundant sunflower star (Pycnopodia helianthoides)—experienced rapid and widespread die-offs. Infected animals developed lesions, lost limbs, and disintegrated within days. At the time, the cause was unknown, with hypotheses ranging from warming seas to viral infections.

The parasite Hannebuoy marinum

A team led by scientists from Cornell University and the University of Washington has now identified a protist from the newly classified genus Hannebuoy as the likely culprit. The parasite invades sea star tissues and triggers the severe physical deterioration associated with SSWD. Lab tests showed that exposure to Hannebuoy marinum caused healthy sea stars to develop symptoms and die—fulfilling Koch’s postulates for disease causation.

Ecosystem impact

The sunflower star is now listed as critically endangered. Its loss has allowed sea urchin populations to explode, which in turn has decimated kelp forests—crucial habitats for numerous species. The cascading effects have altered marine biodiversity and ecosystem stability across large areas of the northeast Pacific.

Scientists are now exploring how the parasite spreads and whether environmental conditions such as temperature or stress play a role in triggering outbreaks. There is hope that the discovery could lead to conservation strategies or management approaches for future sea star recovery.