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Virus Calculated as Culprit Killing Sea Stars
[Reprinted from USGS Science Features: Top Story, November 17, 2014.]
Apart from SpongeBob’s pal Patrick, it’s hard to think of sea stars as living creatures. They’re sold in beachcombing shops as rock-hard, dried specimens for home décor, and artists use their symmetric shapes as motifs for ocean-inspired patterns. But sea stars are actually soft-bodied, colorful, mobile animals that hunt and feed on the ocean bottom, and since 2013, a mysterious disease has been killing them in droves along the U.S. and Canadian Pacific Coast—causing their arms to fall off and spilling their innards (https://www.youtube.com/watch?v=SOgOe3pH5Qc), and wasting away their bodies as if a heat gun were melting them from the inside.
The “Sea Star Wasting Disease” behind this deadly epidemic has been intensely studied by marine scientists in recent months, and now, a prime suspect has finally been identified as a probable cause of the disease.
Researchers from Cornell University, the U.S. Geological Survey (USGS), and other institutions managed to isolate and analyze the genome of a previously unidentified virus in the sick sea stars. They published their findings on this “sea star associated densovirus (SSaDV)” in the December 2, 2014, issue of the Proceedings of the National Academy of Sciences (“Densovirus associated with sea-star wasting disease and mass mortality”). (Read the Cornell University press release, “Densovirus named top suspect in devastating sea star wasting disease.”)
Led by Cornell University microbiologist Ian Hewson with Cornell disease ecologist Drew Harvell, the study team comprises researchers from a who’s who of science institutions: Western Washington University, Wildlife Conservation Society, University of California-Davis, California Science Center, Los Angeles County Museum of Natural History, Monterey Bay Aquarium, National Park Service, University of South Florida, USGS, University of California-Santa Cruz, Seattle Aquarium, University of Washington, University of Connecticut, and Vancouver Aquarium.
The sea star mortality was so sudden and unprecedented that biologists were unprepared and short on resources, says study coauthor Kevin Lafferty, a marine ecologist and parasite specialist with the USGS Western Ecological Research Center. “Lots of people were scrambling to learn what was going on. Ian started having interesting findings with densovirus, so he and Drew helped pull together a team of interested scientists to donate time to the effort.” The USGS Western Fisheries Research Center also provided aquarium space at their Marrowstone Field Station in the course of the research.
Clues to an Outbreak
Lafferty is based at the University of California-Santa Barbara’s Institute of Marine Sciences (Parasite Ecology Group) as adjunct faculty (http://www.msi.ucsb.edu/people/faculty/kevin-lafferty). He used to see sea stars all the time when diving in his local kelp forest. “Now I can’t find any,” he says. “That level of destruction close to home piqued my interest in the cause of the disease. It became personal.”
Sea star die-offs have been observed in past decades, but none were at this geographic scale. Since June 2013, Sea Star Wasting Disease cases have been reported from Baja California in Mexico, southern California, northern California, Oregon, and all the way to southern Alaska. Worse, as many as 20 sea star species have been affected.
“Because it’s happening underwater, this devastation may be difficult for many people to picture,” says Lafferty. “But imagine if all the songbirds from Alaska to Mexico started falling out of the sky, dropping their wings, and disintegrating into a pile of feathers. You’d wonder if you were in an Old Testament-style plague.”
Scientists raced to find a cause, ranging from pollution to storm surge damage. But when Sea Star Wasting Disease began to hit an aquarium that used natural seawater from the ocean, new clues emerged.
The disease did not spread into tanks where incoming seawater was treated with ultraviolet light, but it did spread into tanks with untreated water—suggesting that a living pathogen was in play instead of a chemical pollutant. Furthermore, trapping mechanisms like sand filters did not halt the spread of the disease from tank to tank, suggesting that the pathogen was microscopic and transferrable by water, rather than only via direct contact between infected sea stars.
Minced Tissue and Crunched Numbers
With that in mind, Cornell researchers devised a laboratory experiment using sunflower sea stars (Pycnopodia helianthoides) that showed symptoms of Sea Star Wasting Disease, removing tissue samples from these sick animals. The tissue was minced and blended in seawater, then passed through super-fine filters designed to remove bacteria but allow smaller materials—including viruses—to pass through.
This filtered blend was then injected into healthy sea stars, which all began to show wasting symptoms after two weeks. Researchers then took tissue from this set of newly diseased sea stars to create a second blend, injecting this into yet another group of healthy sea stars. Sure enough, these sea stars also showed wasting symptoms after two weeks.
Meanwhile, another group of healthy sea stars was subjected to the same experiment, but they were injected with a boiled version of the blend intended to destroy any biologically active material. None of these sea stars developed wasting symptoms, and the experiments proved that some sort of virus-sized, biologically active entity could trigger wasting symptoms in one sea star, and continue to infect others.
“We went back to these sick and healthy sea stars to filter and extract virus particles from their tissue, and sequenced the genomes of these virus particles to uncover the types of viruses present,” says Hewson, the Cornell microbiologist and the study’s senior author. “The sick animals had higher prevalence of one viral group, the densoviruses. We were able to assemble a near-complete viral genome from the sick sea stars—a densovirus genome new to scientific record—and gave it a name: ‘sea star associated densovirus,’ or SSaDV.”
But was SSaDV also related to wasting symptoms in sea stars in the wild? Drew Harvell, the marine disease specialist at Cornell, had been making field observations of the wasting disease and collected samples for the viral genome analysis. She rallied researchers at different institutions to gather tissue from more than 300 sick and healthy sea stars from the wild, across 14 different species. Kevin Lafferty at USGS then performed statistical analyses on the results. “We confirmed that wild sea stars were more likely to be diseased if they carried a high viral load of SSaDV,” says Lafferty.
If SSaDV is indeed the culprit behind the current Sea Star Wasting Disease epidemic, that would leave researchers with even more mysteries. During the study, the team also analyzed preserved sea stars from past decades for viral traces, and found evidence of SSaDV or similar viruses in museum specimens dating back to the 1940s.
But if SSaDV has been in the wild all this time, and appears to be a virus that can be freely transmitted in the water column and in sediments, why is it all of sudden a problem now?
“There’s a lot to untangle,” according to Lafferty, whose USGS research often examines the role of disease organisms in natural food webs (for example, see “The Nature of Things - Invasion of the Brain Snatchers”) and their usefulness as indicators of ecosystem function (“Tackling Devastating Human Parasite—Schistosomiasis”). “Parasites and pathogens are part of the natural food web, and just like wolves or sharks or any other organism, they can be affected by changes in the environment or their prey. So something has changed recently in Pacific waters—perhaps booms in sea star numbers, some stressor to sea star immune function, or some other environmental disturbance—to have set the stage for the current sea star wasting epidemic.”
Late-Breaking News—Worries about Sea Urchins
In some areas along California’s central coast, sea urchins are carpeting the seafloor as sea stars, their predators, disappear. In some southern California areas, however, the urchins themselves are losing their spines and dying, a wasting event that scientists do not yet understand. A recent article in National Geographic (“As Sea Stars Die, New Worries About Urchins”) describes the complexities of how “effects of the sea star die-off seem to be reverberating along the California coast, altering the prey and predator relationships of urchins, sea otters, kelp, and even human anglers.”
How You Can Help
As the investigation into Sea Star Wasting Disease continues, the public can still contribute citizen science data by reporting new cases of the disease that they observe while diving or exploring tidepools. Look for instructions at the UC Santa Cruz website at http://www.seastarwasting.org.
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