.Sea Star Die-off Baffles Scientists

1452NEWS2Stars of Santa Cruz ocean go missing

Tide pools along Santa Cruz beaches are rich with marine life. Peer into the pools and you’ll see dark olive crabs darting between crevices and clambering over purple urchins and gelatinous anemones to find protection.

But one creature familiar to the same tide pools, the starfish, may be becoming less common. The multi-legged invertebrates, known to scientists as “sea stars,” are disappearing up and down the West Coast, thanks to a disease known simply as “sea star wasting syndrome” which has recently swept across Pacific shores. Leaving the withered remains of millions of sea stars in its wake, the disease may spread to the next generation of sea stars that is taking the place of the first wave of dead stars. While the disease is still largely a mystery, scientists just identified a candidate virus that could provide answers.

Scientists from across the country just released strong evidence that the disease is caused by a microbe known as densovirus. Their study was published Dec. 2 in the scientific journal Proceedings of the National Academy of Sciences.

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“We are absolutely confident that a virus-sized organism is responsible for the disease,” says Hewson, who led the study that uncovered the candidate virus. When his team transplanted the densovirus from infected to healthy stars, the same symptoms emerged. They even looked at preserved stars from 72-year-old museum collections and found traces of the virus, suggesting that it had lain dormant all this time.

Hewson found multiple viruses dwelling within the tissues of afflicted stars. But densovirus was the only one capable of infecting healthy animals (in essence, using them as hosts), and he regards it as the most likely cause of the disease.

Anyone who happens to come across a star stricken with wasting disease should avoid eating while watching the sickness unfold (or plan on skipping their next meal). Gruesome symptoms appear quickly, beginning with lesions that crop up around their bodies. The whole animal may take on a deflated appearance. In some cases, the star’s internal organs begin to dribble forth from its open wounds. Then its arms start falling off. In just a few days, the once-efficient predator deliquesces into disorganized pulp. (Remember when Major Toht’s face melts off in Raiders of the Lost Ark?)

But wasting syndrome is not new. Stars in Santa Cruz began showing signs of disease last year.

“All UCSC tide pool monitoring sites in the Monterey Bay area have been greatly impacted,” says UCSC researcher Melissa Redfield. The university’s wasting syndrome webpage features maps showing where the disease has spread, with several dots peppered around Natural Bridges and on the wharf, too.

Large die-offs have happened before, with some events affecting as few as one species of sea star. But what’s happening today is more severe.

“The current event is far more geographically widespread and affects a much wider diversity of sea stars than in previous years,” says biological oceanographer Ian Hewson of Cornell University, who studies the cause behind the mysterious disease. “The current event, which has occurred in 20 species of sea stars and counting, from Anchorage [Alaska] to Baja California, is by far the largest marine disease event ever seen.”

The first cases of this most recent outbreak were recorded on the Washington coast in June 2013. From there, the disease spread in both directions. It hit Vancouver, British Columbia two months later. Sea stars in Monterey began dying off by October.

But this outbreak is different, and there may be other causes behind its pervasiveness, explains UCSC researcher Melissa Miner. Because so many sea star species are afflicted and the disease’s onset has been so sudden, there’s still plenty left to investigate.

“The discovery of the virus that is associated with sick and sometimes healthy-appearing stars is a good first step,” writes Miner via email, “but it by no means tells the complete story.” She suggests that links between environmental factors and the outbreak may exist, but the relationship is not yet clear.

Miner and her colleagues have hypothesized that a number of factors may inhibit a sea star’s ability to fight the virus, just as stress may reduce any person’s ability to fight sickness.

“At this point we know the virus is associated with sick sea stars,” adds Miner, “but it has also been found in stars that superficially appear healthy. We don’t understand the cause of the disease, but suspect that other factors are involved (e.g. temperature, pollution, pH) that could be decreasing sea stars’ ability to fight the virus.”

Citizen scientists are pitching in, too, as volunteer high school students and divers scour the coast to share their observations with researchers.

“The information provided by recreational divers, snorkelers and folks in the intertidal has been useful in figuring out which species are being affected and where,” says Hewson.

The data will tell researchers how marine communities respond to the die-off, which will inform future studies. Citizens and scientists alike are already observing juvenile sea stars moving into the vacant real estate.

“In a few areas,” writes Miner, “we’re seeing large numbers of juvenile stars that, if they survive, would mitigate the recent loss of adults.” These young stars are popping up in Santa Cruz and Big Sur, but they, too, show signs of disease.

Subsequent generations may grow resistant, but only time will tell if they can adapt. With a potential virus identified and so many eyes in the water, scientists may be able to predict how new stars will fare.

“Sea stars will probably not be wiped out entirely by this disease,” says Hewson, “though they will become rarer. Hopefully their populations will increase slowly as resistance to the virus amongst the sea star population grows, or pathogens become less lethal.”

The sea star die-off could have ramifications throughout the tide pool, too.

Some star species bear a lot of weight on their spiny, calcified shoulders. As ecological linchpins in marine food webs, certain sea stars keep mussel or urchin populations in check. When they disappear from the environment, those animals can grow wildly abundant, sometimes throwing the balance into biological disarray.

When one animal bears greater influence on its ecosystem than others, ecologists call that organism a “keystone species”—a phrase that actually comes from studying sea stars. Zoologist Robert T. Paine coined the concept in 1969 after studying the Ochre sea star, Pisaster ochraceus, one of the many species afflicted by wasting syndrome. Paine dedicated 25 years to removing the Ochre star from a tidal plain on Tatoosh Island, Washington. When the Ochre stars were gone, mussels dominated the ecosystem. Not all sea stars are keystone species, though. Some disappearances may lead to only mild changes in the ecosystem composition.

Photo: Researchers across the U.S., including at UCSC, are researching possible causes of death for sea stars. A mysterious virus looks to be a likely culprit.


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