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Decades-old Virus Behind Devastating Sea Star Epidemic

ByTim De ChantNOVA NextNOVA Next

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For more than a year, divers and scientists have been reporting a mysterious syndrome that is depopulating the Pacific Ocean of sea stars. Once abundant, the echinoderms have been slowly wasting away. Affected sea stars die horrifically—they develop lesions and lose arms until they eventually disintegrate, leaving behind nothing more than their calcium-based skeletons.

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sea star healthy
A healthy sea star in Bodega Bay, California

Now, oceanographers think they have found the culprit, a densovirus. While scientists haven’t been able to grow the virus in lab tissues—a key step in the formal identification of what’s behind an epidemic—the weight of evidence suggests the virus is indeed the cause. Here’s Erik Stokstad, reporting for the journal Science:

Scientists sent hundreds of tissue samples to Ian Hewson, a microbial oceanographer at Cornell University. When he sequenced the DNA in the samples, he discovered that a densovirus was more common in the sick stars than in ones that looked healthy. (Densoviruses are known to infect insects, crustaceans, and some sea urchins.) Additional evidence came from experiments conducted by marine ecologist Drew Harvell of Cornell and other researchers, who took tissue from sick sea stars, filtered out everything larger than viruses, and injected the tissue into apparently healthy sea stars. They developed symptoms—and, concurrently, the amount of densovirus in their bodies increased. Other sea stars injected with sterilized tissue did not develop symptoms of the wasting disorder.

Densovirus has been found in museum samples as far back as the 1920s, but only recently has it been causing massive die-offs. Scientists offer a few possibilities as to why the virus has only recently reached epidemic proportions.

sea star wasting disease
A sea star with a missing limb, a symptom of sea star wasting syndrome

For one, the virus may have mutated, becoming more contagious or deadly in the process. Also, sea star populations have been booming in the recent past. Epidemics are a hallmark of overpopulation. Competition for food can stress individuals, leaving them more susceptible to illness. Furthermore, population booms can lead to higher densities of seas stars, making viral transmission much easier.

Ocean acidification may also contributing to sea stars’ stress. The oceans have been absorbing billions of tons of carbon dioxide emitted from the burning of fossil fuels. That has been lowering the pH of seawater. Since sea stars and other marine organisms spend their lives submerged in seawater, any change can have drastic consequences.

Whether the ultimate cause of the epidemic is a viral mutation, overpopulation, ocean acidification, or some combination, scientists are watching sea star populations to see what happens next. As sea star numbers nosedive, other organisms have been flourishing, sending affected marine ecosystems into a state of flux. What will the ocean look like when things settle down? No one really knows.