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Virus Calculated as Culprit Killing Sea Stars

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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.

Sea stars are suffering from a wasting disease epidemic in which they lose limbs and literally disintegrate in a matter of days
Above: Unlike their smiling cartoon brethren on television, real-life sea stars are suffering from a wasting disease epidemic in which they lose limbs and literally disintegrate in a matter of days. USGS photograph by Kevin Lafferty. [larger version]

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.”)

Healthy sea stars are vibrantly colored, active predators
Above: In contrast to pale, sick specimens, healthy sea stars are vibrantly colored, active predators, like this giant sea star (Pisaster giganteus) capturing a cowrie snail. USGS photograph by Kevin Lafferty. [larger version]

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.

Kevin Lafferty
Above: Kevin Lafferty [no larger version available]

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.

Sea urchins crawl upon the disintegrated corpse of a sea star
Above: Sea urchins crawl upon the disintegrated corpse of a sea star that succumbed to Sea Star Wasting Disease—a smattering of unrecognizable white matter. USGS photograph by Kevin Lafferty. [larger version]

“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.

The sunflower sea star can be found in a variety of colors in the wild
Above: The sunflower sea star (Pycnopodium helianthoides) can be found in a variety of colors in the wild, like this healthy specimen with a lavender hue. USGS photograph by Kevin Lafferty. [larger version]

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.

A simplified diagram of the Sea Star Wasting Disease experiment devised at Cornell University
Above: A simplified diagram of the Sea Star Wasting Disease experiment devised at Cornell University, used to prove that a virus-sized, biologically active entity was triggering wasting symptoms in sea stars. Graphic by Ben Young Landis, USGS, based on the paper by Hewson and others, 2014. [larger version]

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.

Screenshot from a video showing a healthy sea star succumb to Sea Star Wasting Disease
Above: Screenshot from a video by Robert Beck of Brigham Young University and Benjamin Miner of Western Washington University that shows a healthy sea star gradually succumb to the mysterious symptoms of the Sea Star Wasting Disease—including an arm walking away from its deteriorating body. Video courtesy of Western Washington University. [larger version]

Many-Armed Mystery

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.”

Tidepool scenes of vibrantly colored sea stars could become a rarity as the Sea Star Wasting Disease spreads
Above: Tidepool scenes of vibrantly colored sea stars could become a rarity as the Sea Star Wasting Disease spreads. When asked recently to take photos of sea stars along his home coast of Santa Barbara, California, USGS ecologist Kevin Lafferty responded: “There are no sea stars now to photograph.” USGS photograph of Pisaster ochraceus sea stars in San Juan Islands, Washington, by Kevin Lafferty. [larger version]

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.

Related Sound Waves Stories
USGS Scientist Recognized for Research Achievements in Parasitology
April 2009

Related Websites
Science Features: Top Story
Sea Star Wasting Syndrome
Seastar Wasting Disease
Densovirus associated with sea-star wasting disease and mass mortality
National Academy of Sciences
Densovirus named top suspect in devastating sea star wasting disease
Cornell University
Hewson Lab at Cornell University
Cornell University
Welcome to the Harvell Lab Website!
Cornell University
Kevin Lafferty
Western Ecological Research Center
Marrowstone Field Station
Parasite Ecology Group
Kevin Lafferty
Sea Star Wasting Syndrome Map
Pacific Rocky Intertidal Monitoring
The Nature of Things - Invasion of the Brain Snatchers
Tackling Devastating Human Parasite—Schistosomiasis
As Sea Stars Die, New Worries About Urchins
National Geographic

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in this issue:

Virus Calculated as Culprit Killing Sea Stars

Scientific Portrait of the Largest Dam Removal in U.S. History

California Seafloor Mapping Program Reaches Milestone

Future Wave and Wind Effects on Pacific Islands

California’s Sea Otter Numbers Holding Steady

New USGS Research Vessel in the Great Lakes

Spotlight on Sandy
Five New USGS Oceanographic Datasets Published Online

Explore Coastal and Seafloor Images along U.S. Coasts

Getting Out of Harm’s Way—Evacuation from Tsunamis

USGS at the 2014 St. Petersburg Science Festival in Florida

Tribal GIS Training in the Northeast U.S.

Undamming Washington’s Elwha River—Public Lecture

Geologist Brian Atwater Receives Communications Award

Frozen Heat—New International Report on Methane Hydrates

Jan. / Feb. Publications

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