Why Are All the Sea Stars Dying?

For the last year and a half, sea stars all along North America's Pacific coast - from Baja California all the way to Alaska - have been withering away into nothingness. Today, researchers announced they've found the culprit: it's a virus.

Sea star wasting disease (SSWD) has been known for several decades at least, but the current outbreak is so noteworthy both because of its geographical extent, and because it can attack some twenty species of asteroid sea stars - that is, those of the taxonomic class Asteroidea. First, the body becomes limp. Then it grows lesions, and eventually the star's arms detach and walk away, leaving the rest of the sea star to literally disintegrate.

Like any good detectives, the researchers began with the few clues they had. In their paper published today in PNAS, they write:

Some early patterns from the field supporting the hypothesis that SSWD is contagious. Within a region, SSWD has sometimes moved from site to site similar to an infectious disease. For example, the disease spread north to south in Southern California. All of the major aquaria on the North American Pacific Coast were affected by SSWD in Fall 2013, with mass mortality of captive asteroids. However, SSWD did not spread into aquaria that sterilize inflowing seawater with UV light but did spread into nontreated aquaria. Furthermore, the incursion of SSWD in aquaria with sand-filtered intake suggests a microscopic infectious stage, rather than a disease that spread only via contact with an infected host or vector. Overall, these patterns suggest a microscopic, water-borne, infectious disease agent, rather than environmental pollutants.

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To try to identify the cause of the disease, the researchers rounded up a bunch of wild sea stars that were not yet displaying any signs of SSWD. They took an individual that did have SSWD and extracted some of its tissue to create a substance that could be injected into live stars. Half of the healthy stars were given a boiled version of the substance, designed to kill any viruses that lurked within. The other half were given the substance derived from untreated tissue, viruses and all. And, as would be the case if the disease were caused by a virus, those who were given the treated substance remained healthy, while those given the untreated substance developed SSWD.

They've called the virus "sea star-associated densovirus" or SSaDV. Having identified it, the researchers were able to look for the virus in preserved sea stars in natural history museum collections. It turns out that the virus has been present along the Pacific coast of North America for more than seventy years and is among the first viruses known to infect echinoderms, the taxonomic phylum that includes sea stars.

One mystery appears to have been solved, but now there are more questions than ever: Do other organisms act as a reservoir for the virus? Just how does the infection work to kill the sea stars? And if the virus has been around for so long, why has it only recently become an epidemic?

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[PNAS]

Header image: The Margue/Wikimedia Commons

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