I mentioned last week in my post about sea stars that beaches on Kodiak teem with an abundant variety of brightly colored sea stars. Sadly, though, sea stars are not as abundant here as they were a few years ago. I took a walk on the beach yesterday and was alarmed by how few sea stars I saw. Those I did see looked healthy, but the vast majority were wiped out by a deadly virus.
In June 2013, sea stars along the Pacific coast of the United States began dying in large numbers. Die-offs of sea stars have occurred before in the 1970s, 80s, and 90s but never of this magnitude. Within just three years, millions of sea stars from California to Alaska died from a disease called sea star wasting syndrome (SSWS). Sea stars with SSWS develop white lesions in the ectoderm quickly followed by decay of tissue surrounding the lesions which leads to fragmentation of the body and death. Biologists estimated 95% of some sea star populations were decimated by SSWS. While most species of sea stars were affected by SSWS, ochre stars (Pisaster ochraceus) and sunflower stars (Pycnopodia helianthoides) were especially hard hit.
The syndrome was first noticed in ochre stars in June 2013 along the coast of Washington state. In August 2013, divers reported a massive die-off of sunflower stars just north of Vancouver, British Columbia. In October and November 2013, large numbers of dead sea stars were noted in Monterrey, California, and by mid-December, SSWS had reached southern California. In the summer of 2014, the disease had spread to Mexico and parts of Oregon. SSWS was first reported in Alaska in Kachemak Bay in 2014, but it wasn’t until 2015 and 2016 that sea stars began dying in large numbers in Alaska.
Biologists are certain sea stars are dying from a virus, but when they isolated the virus, they realized this virus was present in preserved museum samples taken from as far back in the 1940s. They believe some other factor such as increased water temperature or a change in pH is stressing seas stars and allowing an otherwise dormant virus to rage through their populations. Researchers noted an increase in ocean water temperature preceded the outbreak of SSWS, and in areas where the water temperature rose the most, the disease was more widespread. To test the theory that increased water temperature played a big role in the breakout of the disease, scientists placed sea stars in aquarium tanks ranging in temperature from 54 degrees to 66 degrees Fahrenheit. The results were clear, the hotter the tank, the more quickly the sea stars succumbed to wasting.
The drastic reduction in sea star populations is evident on Kodiak Island, and biologists worry how the loss of sea stars will affect the intertidal community. Sea stars are considered a keystone species, important to maintaining diversity in the marine environment. Sea stars eat mussels and sea urchins whose numbers could now explode and decrease biodiversity in intertidal and subtidal communities.
Scientists consider the recent outbreak of SSWS the single largest, most geographically widespread disease ever recorded, and as ocean temperatures keep rising, they fear the outbreak of the disease will continue.
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