Scientists may have pinpointed the cause of sea star die-offs

Saturday, December 6, 2014, 1:40 PM - New research says the culprit behind large sea star die-offs on the North American west coast may be a virus.

The paper, published in the Proceedings of the National Academy of Sciences, says the evidence shows the disease is transmissable to healthy sea stars, and that the likely cuase is a virus-sized microorganism known as a densovirus.

Specifically, the organism, which the scientists identify as a sea star-associated densovirus (SSaDV), was found in greater abundance in diseased sea stars than in healthy ones.

But the find doesn't mean sea stars' problems are at an end, according to Cornell University's Ian Hewson, who led the research.

"Once a virus like this is in the ecosystem, inoculating or otherwise trying to shield the sea stars from disease is virtually impossible," Hewson told The Weather Network. 

At the same time, Hewson says the infection itself is self-limiting. Viruses, he says, decay rapidly outside of a host, and once a population thins out enough, the virus will be starved of new hosts, and seem to disappear.

"It's not gone, but rather becomes less lethal," Hewson says. "No virus has ever wiped out its host population. Rather, viruses play extremely important roles in nature by increasing biodiversity by allowing less dominant organisms to compete for available food [and] space."

The virus didn't appear overnight. It was discovered in sea star samples in 1942, and may have been around for longer. Hewson says the disease may seem to be more prominent just now because sea star populations were higher in the Pacific Northwest than a decade prior, making transmission and mutation easier and faster.

But Hewson says environmental factors may have boosted the densovirus' chances.

"For example, we know that the oceans are changing because of temperature rise - and are becoming more acidic because they absorb more carbon dioxide from the atmosphere. However, we do not have data at this stage to link either of these to the disease," he says. 

"We do have contrary data - for example the disease spread into new areas and intensified when waters cooled, suggesting the disease does not have a universal relationship with temperature, if it exists at all."

Canadian connection

Hewson said he became involved after hearing reports that sea stars at the Vancouver Aquarium were sickening and dying at the same time as sea stars in nearby ocean waters.

The water in the aquarium comes directly from Howe Inlet, meaning the disease's cause made it past the larger-particle filtration system.

He also noticed that sea stars at the Seattle Aquarium, which lived in water treated with UV light, did not become sick.

In the course of metagenomic testing, they ruled out a bacterial cause and identified their particular densovirus, SSaDV, as the only one more common in the tissues of sick sea stars than in healthy ones.

By taking the virus from infected sea stars and injecting it into healthy ones, they found that it could indeed cause the wasting disease.

"Hence, combining these results we can say the disease is caused by a virus and that SSaDV is our most likely candidate for that virus," Hewson says.

Still, Hewson and his team note they can only say SSaDV is the "best candidate" for the culprit.

"Definitively proving that a virus is the cause of any disease usually involves testing the effects of that virus on stem cell cultures of the animal it infects, something which is not possible for sea stars since there are no cultures of any marine invertebrate available," he said, adding other factors prevent nailing down SSaDV as the be-and-end-all cause.

"We would love to conduct more research, however funding is extraordinarily tight," he said. "There are many unresolved questions we have about this disease and SSaDV that demand further investigation."

Dr. Jeff Marliave, the Vancouver Aquarium's vice president, marine science, who was one of the study's authors, says the sea star die-off was one of the best documented in history, given the extensive coverage of the event, along with what he says was unprecedented cooperation between scientists, divers, veterinarians and other disciplines.

"They were melting, melting in front of our eyes. It wasn't happening in the aquarium at the time, it was happening in the wild, and it spread very quickly," he says.

He says the phenomenon began appearing mostly around larger coastal metropolises, where he said certain kinds of sea stars were thriving after a sharp population uptick over the last decade.

"We're quite interested in the possibility that this is nature's leveller for this type of marine animal that very naturally has huge population explosions," Marliave says. "They happen to do very well around raw human sewage, probably one of the reasons it happened around urban areas."

He said although the denosvirus was found in captive sea stars, whose numbers dropped drastically, they weren't hit as dramatically as the wild populations, which were much more numerous and densely packed.

"The ones in the field were on top of each other, like, they were ridiculously overcrowded," he said. "Like a plague of locusts, they were eating everything on the sea bed. It was really out of whack." 

He says green sea urchins, which are preyed upon sunflower sea stars when they first settle, have seen a rebound in their numbers since the die-off began, although he says the seabed likely won't return to the exact point it was before sea star numbers rose a decade ago.

"I think we're going to find this densovirus associated with other die offs. Now that we know, people are going to start looking more closely."