Scientists Finally Know What Killed Billions of Sea Stars

This article is a guest post written by Mariella B. L. Careaga ([email protected]).

Source article: Prentice, M. B., Crandall, G. A., Chan, A. M., Davis, K. M., Hershberger, P. K., Finke, J. F., Hodin, J., McCracken, A., Kellogg, C. T. E., Clemente-Carvalho, R. B. G., Prentice, C., Zhong, K. X., Harvell, C. D., Suttle, C. A., & Gehman, A. M. (2025). Vibrio pectenicida strain FHCF-3 is a causative agent of sea star wasting disease. Nature ecology & evolution, 9(9), 1739–1751. https://doi.org/10.1038/s41559-025-02797-2

Featured Image Caption: Sea star wasting disease has affected over 20 sea star species along the Pacific coast of North America. Image Source: blmcalifornia, Public domain, via Wikimedia Commons

A Mysterious Disease

Beginning in 2013, billions of sea stars along the west coast of North America started to die from a mysterious disease that caused their arms to twist and fall off and their tissues to melt. The disease, known as sea star wasting disease (SSWD), was responsible for one of the largest die-offs of marine animals in recent years.

Sea stars are keystone species in coastal ecosystems, where they act as natural predators of bottom-dwelling invertebrates like clams and mussels, keeping their numbers in check. Since the 2013 mass die-off, SSWD has affected more than 20 species of sea stars, with some species, such as the sunflower sea star (Pycnopodia helianthoides), experiencing more than 90% population decline.¹

Early attempts to identify the cause of SSWD pointed to a virus as the most promising candidate; however, inconsistencies in the findings led scientists to reconsider this idea, leaving the culprit behind SSWD lurking in the shadows.²

In Search of SSWD Culprit

In a recent study published in Nature Ecology & Evolution, an international team of researchers sought to identify the agent causing SSWD by combining a set of exposure experiments, genetic data and field observations.

The team focused on the sunflower sea star, a species that was once common along the west coast of North America and has been greatly affected by SSWD. The researchers monitored wild-caught or captive-bred sea stars for 2 weeks for visible signs of disease, such as arm twisting and limb shedding, before classifying the sea stars as healthy and including them in the experiments.

To induce SSWD in the laboratory settings, they exposed healthy sea stars to small pieces of tissue or to sea star “blood” (called coelomic fluid) collected from diseased sea stars. More than 90% of the sea stars exposed to the disease developed signs of SSWD and died, while those in the control groups — stars that were either exposed to heat-treated or filtrated versions of the diseased tissue or coelomic fluid — did not.

Since heat-treating or filtering the diseased samples halted the spread of SSWD, the researchers hypothesized that a cellular microbe was behind it. To identify the pathogen, they collected coelomic fluid from both diseased and healthy sea stars and performed a metatranscriptomic analysis, which allowed the researchers to analyze the complete set of all RNA molecules expressed by the microbes found in the sea stars’ “blood”. A type of bacteria called Vibrio pectenicida (V. pectenicida) was abundant in samples from diseased sea stars compared to those from healthy sea stars. A closer look at the bacterial community using 16S ribosomal RNA sequencing, a method that allows scientists to identify more carefully the bacteria present in a sample, confirmed the previous results, indicating that high levels of V. pectenicida were associated with SSWD. 

SSWD Mysterious Causative Agent Unmasked

While the previous findings revealed a strong link between V. pectenicida and SSWD, a causal relationship was still missing. To determine whether the bacterium could cause the disease, the researchers isolated the bacteria from the “blood” of diseased sea stars and injected it into healthy stars. All sea stars exposed to the live bacteria showed signs of SSWD and died, while those exposed to heat-treated (dead) bacteria survived.

Next, the team went a step further to confirm that V. pectenicida was the pathogen causing SSWD. They visited the fjords of British Columbia, Canada, and collected “blood” from sea stars living in locations deemed affected or not affected by SSWD. They found that the V. pectenicida was more frequently identify in sea stars from affected sites than in those from apparently unaffected areas. What’s more, when they reanalyzed data from sunflower sea stars affected by the disease in Alaska in 2016, they found that V. pectenicida was more prevalent in the diseased sea stars than in the healthy ones.

While the results strongly implicate V. pectenicida as the cause of SSWD, many questions remain unanswered. Researchers still do not know what mechanisms the bacteria use to cause the disease or how it spreads in the wild.

By improving our understanding of SSWD, the study findings pave the way for better responses against the disease, helping us to more effectively protect coastal marine ecosystems.   

Secondary sources:

1. San Diego Zoo Wildlife Alliance. Breakthrough for critically endangered sea star recovery: cryogenically preserved specimens produce offspring. https://sandiegozoowildlifealliance.org/pr/SunflowerSeaStar

2. Hewson I, Johnson MR, Reyes-Chavez B. Lessons Learned from the Sea Star Wasting Disease Investigation. Ann Rev Mar Sci. 2025;17(1):257-279. doi:10.1146/annurev-marine-040623-082617

Edited by Julia Bebout