Starfish have a superpower, one that seem almost magical—they can regenerate entire limbs. If one of their five appendages becomes stricken with an incurable disease, they just drop it off and grow a new one. No problem.

That is, until it became a problem—a very big problem. In 2013 and 2014, scientists started noticing that west coast starfish weren't their usual regenerative selves. Instead, they were just turning to mush. In some cases, disease expert Ian Hewson says, their limbs would fall off and walk around the ocean bottom like headless zombies.

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"They're almost indestructible, so this must have been really bad," Hewson told Popular Mechanics. "Usually, they would just lose a limb. This time, they seem to have hit the self destruct button."

The disease comes on quick, with sick starfish—or sea stars, as scientists call them—displaying symptoms starting with pockmarked lesions, then twisted and contorted arms, followed by an inability to cling to rocks. Finally, after only 48 hours since the onset of symptoms, they disintegrate into a slimy white sludge. Hewson, and his team of scientists, needed to find out what was going on.

Example of a wasted sea star, 2014. Cornell University

Crossing Off the Usual Suspects

Pete Raimondi, a researcher at the University of California Santa Cruz, sometimes goes scuba diving as part of his research. But when he took a plunge in 2013 to see this new disease's devastation first-hand, he'd never seen anything like it.

"With a sea star, if you cut an arm off, it can regenerate," Raimondi told Popular Mechanics. "In these cases, there was such rapid decay, arms would fall off and they weren't exactly alive, but they weren't exactly dead...it was super creepy, to be honest with you."

In nearly two years, what became known as sea star wasting disease would hit 15 to 20 species of starfish on the West Coast, from Alaska to Mexico, but two were hit particularly hard—the ochre and sunflower sea stars. The sunflower can grow to three feet wide, but these sea creatures are now very rare, Raimondi says. Since 2014, about 20 species of sea stars along the Pacific coast have seen population losses from 60 to 90 percent. It's the starfish equivalent of a real-life zombie horror film.

"We saw species disappear from reefs in just a week," Raimondi says. "They never went completely extinct, but they went from really abundant to extraordinarily rare."

"In these cases, there was such rapid decay, arms would fall off and they weren't exactly alive, but they weren't exactly dead..."

One of the first possible suspects considered was one familiar to many scientists, a force of human creation wreaking havoc on natural ecosystems around the world—climate change. Rising temperatures, whether on land or in the ocean, can change the ecosystem and cause cascading effects, stressing wildlife and making diseases more virulent.

But Hewson didn't buy it, particularly because studies showed mixed results. In one, starfish became sicker in warmer water, while in another, they became sicker in cooler water. In the wild the disease not only spread in cooler weather but intensified in some places during the winter months.

"All the work on warming is a bit conflicting. Yes, animals die faster in warmer temperatures or slower in colder temperatures, but they all die," he says. "And El Nino didn't kick in until 2014, so all this started and the big event happened while the waters were still cool."

Scientists needed to fish deeper for the answer.

An infected Sunflower sea star's arm separates from its body.

Finding a Killer

Hewson collects starfish. Live, frozen, magnets, stuffed animals, you name it. They decorate his laboratory at Cornell University where he keeps samples of 561 different species in a freezer he uses for research. It can be frustratingly delicate work, especially when two samples of Morning Sun Stars are now unusable because the shipment from Alaska sat in the hot sun in Memphis before arriving at his lab.

Hewson is an associate professor of microbiology, and his specialty is studying disease, particularly in tiny sea creatures like plankton. Particularly, Hewson wanted to explore different types of viruses using startling new advances in DNA sequencing. Whereas a decade ago scientists had to run each genome individually, now they sequence millions of genomes at once.

At first Hewson, had a solution without a problem—then starfish started dying in staggering numbers. By October of 2013, Hewson had received grants to study the crisis and west coast starfish samples started flowing into his lab from Mexico to Canada, up to 10 boxes a day.

Ian Hewson at his sea star lab at Cornell University. Caren Chesler

It's at Hewson's Ithaca lab where the detective work began. Using this new sequencing tech, Hewson took 14 healthy sea stars and compared their cells with those of 14 diseased ones. By December 2013, he and his team had figured out the most likely pathogen associated with the disease was a virus that was not abundant in the healthy animals.

Three months later, they went out West to collect samples in the field. They then injected the virus into a healthy starfish, and within a week or two, its arms twisted all about. It lost limbs it could not replace, and then began to deflate and waste away.

Healthy ochre sea star, left, next to a diseased one at Redwood National and State Parks. David Lohse/National Park Service

An Unexpected Return

This was one hell of a virus. Called sea star associated densovirus, or SSaDV, it could move into an aquarium and past its filtration system. Hewson and his teampublished a paper on it in November 2014, but the team still didn't know why the virus caused lesions to form and limbs to fall off and how it move from animal to animal. Hewson likened it to AIDS, where HIV remains dormant in some while in others it wreaks havoc.

But then, just as conveniently as the disease arose, it appeared to decline.

"For the third year in a row, we're seeing very large numbers of tiny sea stars appearing in the shore in many of the sites," Bruce Menge, a marine ecologist from Oregon State University, told Popular Mechanics. Menge's team found an unprecedented surge of young starfish, mostly ochre sea stars, at nine study sites in Oregon—about 300 times as many as in the year before.

A diver from the Oregon Coast Aquarium observes a health sea star. Oregon Coast Aquarium Flickr

While this revival is welcome news, it's just as spooky as the disappearance. What may be happening, Menge says, is that having fewer adults around means more babies can thrive. With the adults gone, the babies can now eat and grow rapidly, possibly leading to a population boom—though it's too early to tell.

"There were more babies in one year than in the previous 20 years combined, in some places," Raimondi says. "We'll just see how that translates into recovery. If the disease is still present, they may die."

Hewson says he isn't surprised. There are always oscillations in the populations of echinoderms and other insects over time, he says, booms and busts that are the dance between predators and prey. It happens with water fleas and algae, wolves and rabbits. Sometimes a species will bounce back, sometimes it won't, like the Heliaster starfish, which was virtually eliminated by a disease in the 1970s, or tropical urchins in the Caribbean, which were wiped out by a pathogen in the early 1980s.

Of course, the very nature of a virus means it rarely kills off its host population entirely. That would be self-defeating. The virus needs its host to grow and replicate, like a vampire. "Viruses do come and go, particularly those that are highly lethal, but they tend to be self-limiting," Hewson says. "When the host population becomes rare enough, the virus starts to become more latent."

Starfish, of course, aren't a species easily eradicated, mostly because they can be almost indestructible when they're healthy. You can cut them up into three pieces and get three sea stars. If you cut an oyster or a mussel in half, both pieces die. "They're incredibly good at regenerating," says Megan Dethier, a research professor at the University of Washington. "We can look to them for the potential for regeneration in more complex organisms."

Caren Chesler

From a biological standpoint, starfish have an unusual anatomy. Instead of blood coursing through their veins, they have water. They take in that water through a little pore and pump it around their bodies as a means of propulsion. No other animal does that, Hewson says.

In the name of research, scientists have thrown everything they can at starfish to test their limits. They put them in horrible water, with salinity levels much greater or much less than what is in the ocean. They've subjected starfish to all kinds of physical disturbances, kept them in the dark for long periods, vivisected them, and taken out internal organs. Still, they don't die.

"We can take bacteria that we culture from the environment, in an incredibly high density, until its milky white, and they still don't die," Hewson says. "They're fascinating animals, the whole group of echinoderms, their resilience, their development, their evolution."

Which makes this mass-killing virus all the more ominous.

Caren Chesler

The Search Continues

There are two tanks of sea stars in Hewson's lab at Cornell. Most cling to the sides as if someone threw them at the wall of the tank and they just stuck wherever they landed. You can see their little suction cup tube feet that fill the underside of their limbs.

Hewson sticks his hand in and pulls out a dainty melon-colored sea star that has a slate gray underbelly and long thin legs. He then takes out a larger sea star with limbs as thick as hot dogs but looks more like a salted pretzel. The limbs curl and straighten, reminding you that this alien-like creature is a real living thing.

Despite the unanticipated population boom, Hewson continued to study how the disease outbreaks occur, how they spread, and most importantly, how they can be mitigated in the future. It's time well spent. Starfish have been around since the dinosaurs, so a virus good at killing a creature more resilient than humans is a virus worth studying.

In more recent experiments, Hewson is trying to elicit the symptoms of the disease by injecting it into healthy animals in the lab and seeing how they respond. He is also looking at the genetic makeup of animals that have survived and comparing them to the makeup of animals present in the environment prior to 2013, cross referencing bags upon bags of frozen starfish in varying stages of decay.

The virus needs its host to grow and replicate, like a vampire.

But Hewson is also trying to determine whether SSaDV can be transmitted from adults to larvae. To do this, he is using a new technology called RNAScope, which allows investigators to look at thin sections of tissue—anywhere from 0.0003 - 0.0007 inches thick—to see which cells the virus has infected at particular stages. The researchers are also hunting for the virus in the sperm or eggs of the starfish, and they are using the new DNA sequencing technology to compare the viral sequence from the sick sea stars to viral genomes from sea stars in museums and around the world.

Hewson's also narrowed down how the virus is transmitted. Because it decays rapidly in water, it's unlikely to travel much of a distance from one infected animal to another. It's either being transmitted among starfish already living one on top of the other, or by another animal, who can carry the virus but is not affected by it, like a tick transmitting Lyme disease.

And it's that kind of analogy reminding us that devastating disease isn't something exclusive to starfish, and Hewson's research could help inform how human diseases could spread. When you consider ebola, severe acute respiratory syndrome (SARS), and avian influenza (bird flu), tomorrow's victims may be us. The more we understand about disease—any disease—the more knowledge we have to mitigate or completely avoid the next one.

So Hewson and his colleagues must stay ever vigilant. Because like something out of a horror movie, it's possible the virus is still hanging out in the population, waiting for the right time to re-emerge. Once again, the virus will feast on this easy prey and have its moment of deadly glory.