Why are all these starfish suddenly turning into goo?

Since June of 2013, a large number of starfish belonging to least 20 different species have been melting - yes, melting - on the shores of the Northeast Pacific. Now, scientists say they have pinpointed the virus behind these die-offs.

A study published this week in the journal Proceedings of the National Academy of Sciences says a transmissible densovirus – a taxonomic group that includes the smallest-known virus particles – is to blame.

The sea-star "wasting disease," as it is known, is not new. Marine ecologists have been using the term to describe die-offs like this one for more than thirty years. But last year's outbreak underscored the need to identify the poorly-understood sickness because of its extensive spread geographically. Millions of starfish from Baja California, Mexico up to Southern Alaska have died from the disease, say scientists.

The effects are not pretty: infected sea stars first become lethargic, then typically grow lesions, and lose the elasticity and autonomy of their limbs. From the human observer, these signs of the sickness make the starfish appear as though it is tearing itself apart.

The research team, led by microbial ecologist Ian Hewson, looked at more than 300 starfish from more than a dozen different species that showed signs of disintegration. About half of the sea-stars the team investigated had been residing in public aquariums while the other half were taken from their natural habitats in intertidal or subtidal zones. The researchers passed tissues from the infected specimens through filters that wouldn't allow bacteria to go through but would allow virus particles, then inoculated healthy sea stars with the virus-sized material.

The starfish showed signs of the disease as soon as ten days after the material was introduced into their bodies, indicating that a virus-sized microorganism was responsible for the mass mortality. And the densovirus that scientists thought might be the culprit was found more abundantly in the ailing sea-stars than in the healthy ones.

It also turned out that the sea-star wasting disease has been around for longer than scientists may have thought. The team looked at museum specimens that had been preserved in ethanol and found evidence of the densovirus in sea stars from as early as 1942.

"At this point we have no idea why something that seems have been present [in the past] has all of a sudden become so lethal," co-author of the study Peter Raimondi, a marine ecologist at the University of California, Santa Cruz, told Nature.

Even if the densovirus is indeed the wasting disease renamed, scientists still have the weighty task ahead of them of figuring out why this outbreak has been so devastating and how, if possible, to control the virus.

For now, the wasting disease continues to spread, threatening the marine ecosystems that are kept in balance by sea-stars, which keep bivalve populations down and, consequently, algae present.

"We don't seem to be at the end of this yet," said Raimondi.

[Editor's note: An earlier version misstated the year in which scientists began observing the die-offs in the Northwest Pacific.]