In search of the starfish killer: the quest to save the original keystone species

In October 2013, after hearing of the Vancouver die-offs, Seattle aquarium staff veterinarian Lesanna Lahner sent divers to examine local sea star populations . The results were grim: about 40 percent of the sea stars under the aquarium’s pier were sick. By November, the three species that usually lived under the pier were gone. Then, the stars in the display tanks started dying. She tried treating the animals with antibiotics, in the hopes they had a bacterial infection; they wasted away all the same.

"We’ve lost hundreds," Lahner says. "We lost the majority of sea stars at the Seattle Aquarium."

In sea stars, dropping an arm is a common response to injury, but in this disease, it appears to be linked to the lesions. Lahner began euthanizing the sickest animals — though some do recover from small lesions, once a sea star has lost several arms and parts of its gut are eviscerating, there’s no hope left. Lahner assumes the sea stars are suffering, although starfish don’t have a centralized nervous system and no "brain" per se. A nerve ring — around the mouth — in the body of the sea star connects to the radial nerves, which run down each arm.

The deaths in Seattle provided clues. The aquarium doesn’t filter water from the ocean, in order to try to maintain a bacterial environment that’s as close to natural as possible. But the system could also give a pathogen a way into the tanks.

Researchers including Lahner, Miner, Fradkin, Raimondi, and Ian Hewson, a specialist in ocean viruses at Cornell— a group Hewson refers to as "the sea star mafia"—began to collaborate in October 2013 to find a cause for the deaths. Scientists, including Lahner, began sending Hewson samples of starfish; more than 460 sea stars, some sick and some healthy. "We kept Fed-Ex in business with dead stinking packages of sea stars," he said. Researchers in his lab sliced off bits of the animals’ arms and put them in a blender with some water — then used a technique called PCR to amplify the bits of DNA floating around in the blender. By doing that, they could start figuring out what was living on or in the starfish. They describe their findings in a paper published November 17th in the Proceedings of the National Academy of Sciences.

Testing ruled out possible culprits: fungi, protozoans, and several species of bacteria. Using material from the blender that had been filtered to exclude all but virus-sized particles, they began inoculating healthy sea stars. The healthy stars that were inoculated began to display symptoms of sea star wasting disease; if the material was boiled before the sea stars were exposed, they didn’t get sick. Genome technology suggested that a densovirus, a type of virus, was to blame.

But the virus may be only part of the story. Not only were some of the sick sea stars testing negative for the virus, some of the healthy ones were testing positive. And here’s another weird thing: the lesions, the first symptom of the disease, didn’t show higher levels of the virus — it didn’t seem to be causing them.

"These ecosystems are dirtier and more complicated than terrestrial ones," Raimondi says. Even figuring out that a pathogen might have been involved was difficult.

When scientists looked at the lesions, they didn’t find heavier loads of the virus, Hewson says. What they did see was that the bacteria there are the same that occur in healthy sea stars, there’s just more of them. Starfish are covered in millions of bacteria; usually they live harmlessly on the animals’ skin. There are some clues the virus may be affecting the sea stars’ immune system, by affecting a type of cell in the animal that ordinarily engulfs and consumes bacteria, Hewson says. That may create conditions where the bacteria that are ordinarily benign can compromise the sea stars.

But the same virus destroying the sea stars now was also present in samples of healthy sea stars held in museum collections from 1942 — meaning it’s been around for at least 72 years. And in those stars, at least, it didn’t seem to cause illness. Something has clearly shifted, Hewson says.

"Obviously, the oceans are changing, and in places becoming warmer and more acidic," Hewson says. Observations suggest that daytime low tides during periods with lots of sunlight seem to make it easier for sea star wasting disease to spread. That might mean the virus spreads better in warm water. It may also explain the lesions, if they're caused by bacteria. Bacteria grow better in the heat, too.

The virus probably isn’t the whole story, both Lahner and Raimondi say. Lahner took sea stars that were disintegrating at 54 degrees Fahrenheit and cooled their water to 50 degrees. "They all went from falling apart, having their viscera hanging out, to pretty healthy in a day," she says. "I came back, and they were like, ready for the cover of Vogue. They were perfect." So temperature may play a role in the animals’ ability to recover as well.

The north Pacific basin, on average, has warmed by about half a degree centigrade from 1955 to 2013, according to data from the National Oceanic and Atmospheric Administration. "That’s kind of a scary number," says Andy Allegra, a NOAA data specialist. "It’s very large."

A time lapse video of an affected star (Vancouver Aquarium)

And that’s not all. The oceans absorb carbon dioxide — about half of what’s released into the atmosphere. When the carbon dioxide interacts with the water, it makes the water more acidic, especially near the surface. Ocean water over the past 300 million years has been slightly basic. Data on pH levels hasn’t been recorded as dutifully as temperature, but a long-running station sampling the Pacific, published in 2009, found "a significant decreasing trend" in pH over the course of 20 years, meaning it’s becoming more acidic. Acidification has been shown to slow sea stars’ growth, suggesting it stresses the animals.

Lahner borrowed some mathematicians to analyze the data around two die-offs in and underneath the Seattle Aquarium. There were 20 variables, but only two showed a strong association: pH and dissolved oxygen content. Lahner and others are trying to tackle the question of acidity now. "I totally think climate change is involved," Lahner says. "I just don’t have evidence yet."

It’s also true that there had been a boom in the starfish population before the die-off, Hewson says. In Vancouver, before the die-off, the starfish were so overpopulated that local divers were in danger of creating underwater avalanches by toppling piles of them. An abundance of sea star hosts makes it easier for a virus to spread, since the animals are more likely to come in contact with each other. The farther the virus spreads, the more opportunities it has to mutate and become more virulent.

There is one piece of good news: viruses do not generally wipe out their entire host population, Hewson says. To do so would be counterproductive. Of course, that assumes the virus is the cause of the die-off, and no one’s proved that yet. The scientists seem to be shaking out into three camps, Raimondi says: the ones who think sea star wasting is environmental, those who think it’s caused by a pathogen, and a third group, who thinks —as he does — that it’s both.

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