Scientists have isolated a virus they are pretty sure is causing the mysterious disease that has killed millions of sea stars on the Pacific Coast from Southern California to B.C. to Alaska by causing them to lose their limbs and eventually disintegrate into slime and piles of plates from its skeleton.
A study published Monday in the journal Proceedings of the National Academy of Sciences says a variety of densovirus is the likely cause of wasting syndrome among sea stars, also known as starfish. Varieties of densovirus are used as a biological control on cockroaches, and include the parvovirus that infects dogs.
Cornell University marine microbiologist Ian Hewson says they found larger amounts of the virus in sick sea stars than healthy ones, and the amount of virus increased as the disease progressed. Also, injecting material from sick sea stars that was filtered to concentrate virus-sized organisms caused healthy sea stars to get the disease.
Hewson said thousands of bacteria and viruses live in and on sea stars, but researchers suspected a virus was responsible for the disease because sea stars got sick in aquariums that drew water from the ocean, such as those in Vancouver and Seattle. The disease did not infect sea stars in museums that exposed the water to ultraviolet light, which kills viruses.
Hewson adds they don't know yet what triggered the outbreak of the virus, which can be found in plankton, sandy ocean bottoms, and sea urchins, and has been found in museum specimens of sea stars dating to 1942. He said It could be related to a population boom in one of the species heavily infected by the disease, a change in the virus, or changes in the environment. Some of the most heavily infected species are members of the same family, suggesting they may share a common vulnerability.
Past outbreaks of sea star wasting have been smaller and more confined in geographic area. The current one started in the summer of 2013 in Southern California and has since spread through Oregon, Washington, British Columbia and southern Alaska. It has infected 20 different species of sea stars, but primarily the five-legged ochre sea stars commonly seen in tide pools, and the sunflower sea stars that have up to 16 legs.
Change to species mix
Hewson said the disease was not likely to make any sea stars go extinct, but was likely to affect the mix of species in the intertidal regions of the ocean. Mussels, a favourite food of sea stars, are likely to become more abundant, for example.
'We had these areas which were once covered with sea stars and now they're covered with sea urchins.' - Ian Hewson, Cornell University
In some places, the deaths of hundreds of thousands, even millions, of sea stars has left an abundance of sea urchins, which would have served as meals for their voracious predators.
"We had these areas which were once covered with sea stars and now they're covered with sea urchins."
That in turn has impacted giant kelp, the dining choice for sea urchins.
The number of sea stars has slowly started to recover in Oregon, and will take several years to return to normal along the Pacific coast, Hewson said.
"We are so new to this outbreak even a year on that we haven't had a chance to observe any of the long-term impact."
Dr. Martin Haulena, a veterinarian at the Vancouver Aquarium, one of the aquariums that offered samples for the study, said sunflower sea stars were affected very quickly by the virus that caused such huge die-offs that may be part of the normal boom-and-bust cycle of nature.
"There's been supposition that urchins would take over where sea stars used to be," he said, adding long-term monitoring will be needed to determine the impact of the virus.
Climate change may play role
Curtis Suttle, a marine virus expert at the University of British Columbia, and Bruce Menge, professor of interactive biology at Oregon State University, were not part of the study. Both said the study, though it did not definitively identify a virus as the cause of the disease, was very persuasive.
They agreed that the increasing acidity of ocean waters associated with climate change could be a factor in triggering the outbreak, perhaps by making the sea stars more vulnerable to attack.
"If (viruses) get in through damaged areas (of the sea star), what causes the damaged area?" Menge asked. "If they don't get in though damaged areas, how do they get in?"
Warming ocean temperature appears to be less of a factor, because outbreaks in Oregon occurred in waters that have been colder than normal lately, Menge said.
Suttle said ocean acidification is affecting other marine invertebrates, such as commercial populations of scallops and oysters.