Widespread Disease Turns Sea Stars Into Goo, Baffling Scientists

An ochre sea star on Pescadero Beach | Photo: Suzanne Black/Flickr/Creative Commons License

Sea Stars, popularly as starfish, are dying horribly along the Pacific Coast from Santa Barbara to Alaska, and scientists aren't sure why.

The disease striking several different species of sea stars involves whitish lesions that spread quickly, often causing arms to fall off. In the disease's late stages, which may take place just days after lesions appear, the affected animals turn to goo. The disease outbreak has been noted as far north as Sitka, Alaska; in California, documented sightings of sick starfish span from Bodega in Sonoma County to Goleta along the Santa Barbara coast.

This isn't the first time this wasting disease has been seen on the coast: an outbreak in the early 1980s knocked back California's ochre sea star population for several years. Previous study has linked a bacterium with the infection. But scientists aren't sure whether that bacterium causes the disease or just comes along for the ride. And the extent of this disease outbreak, along thousands of miles of coast, has them worried.

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The outbreak was first noticed in June along the Washington coast and has since been reported from dozens of different sites up and down the Pacific Coast. In addition to the mapped sightings, there are also reports of the disease along the Orange County coast and in the Gulf of California.

Previous outbreaks of wasting disease had been linked to warmer ocean water during El Niño events, but this year's epidemic seems to be happening in places where the water has been colder than usual as well.

At least 10 species have been found suffering from the illness: in California, the main victim so far seems to be the ochre sea star, Pisaster ochraceus, though sunflower sea stars (Pycnopodia helianthoides) seem to be falling prey to the malady in Monterey Bay. Other California species affected so far include Pisaster brevispinus, the giant sea star Pisaster giganteus, Dermasterias imbricata, the bat star Asterina miniata, the rainbow star Orthasterias koehleri, and Henricia, a.k.a slender sea stars.

So far wasting disease isn't uniform up and down the coast: there are large areas where sea stars have been seen thriving as recently as last week. There've been plenty of wasting disease cases noted in the Monterey Bay area, but sea stars in nearby intertidal areas appear healthy. Whatever is transmitting the disease, it appears to be able to travel for significant distances in seawater: ochre sea stars living in a saltwater aquarium with a piped in supply of seawater succumbed to the disease in late summer, and in this blog post U.C. Santa Cruz researcher Allison Gong describes losing 80 percent of her seawater table's collection of ochre sea stars." To put that in to context, this mortality rate is every bit as bad as some villages that were virtually wiped out by the medieval Black Death," Gong writes.

Losing ochre sea stars across much of their range threatens to disrupt tidal ecosystems. Pisaster ochraceus is considered a keystone species that maintains its local ecosystem. A voracious predator of mollusks, an ochre sea star can keep rocky tidepools and other environments open for other organisms to inhabit. Remove the Pisasters from a stretch of tidepools and the mollusks will take over, monopolizing the habitat to the detriment of other species.

Similarly, sunflower sea stars are keen to eat sea urchins and are thus very effective at keeping sea urchin populations in check. If the sea stars go, sea urchins may come to dominate more of the coastal environment.

Researchers are working to figure out just what's behind the dieoff. In the meantime, they're soliciting public sightings of wasting disease to help chart and track its spread, using the crowd-science app iNaturalist.

Christopher Mah, a starfish biologist with the Smithsonian Institution, suggests that a widespread outbreak of wasting disease could be devastating in an evolutionary sense as well as an ecological sense. "All of the asteriid starfishes in this area, including Pycnopodia (the sunflower starfish), Pisaster (the ochre star), Evasterias, Leptasterias, etc. on the Pacific coast of North America are endemic to the coast... Bottom line: You won't find these starfish species anywhere else in the world. These animals are an important part of the marine ecology of the Pacific coast of North America."

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About the Author

Chris Clarke is a natural history writer and environmental journalist currently at work on a book about the Joshua tree. He lives in Joshua Tree.
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