What's killing all the sea stars?

Living on Earth
Dying sea star

On Orcas Island, off the coast of Washington State, thousands of sea stars cluster in the nooks and crannies of this rocky patch of coastline. But that may not be the case for much longer.

After decimating populations up and down the west coat, Sea Star Wasting Syndrome has arrived here. It's a peculiar, gruesome malady that makes the sea stars rip themselves apart.

Scientists think the cooler waters and swift currents surrounding Orcas Island, which is part of the the San Juan Island chain, may have been protecting this population. But now the sun is out, the water is warming and the sea stars are getting sick.

“It’s a lot worse than it was last week, which is what we expected — but it still doesn’t help to see the future when you see it,” says Drew Harvell, a marine epidemiologist with Cornell University and the University of Washington.

Harvell has been studying marine disease for decades. She says this is the largest documented marine epidemic in human history. She points at a handful of orange and purple stars nestled into the rocks nearby — one’s arm is hanging on by a single gnarled string of flesh.

“It kind of just looks dried out," she says. "Wasted, thin, deflated — sea stars are not supposed to look like that."

When Harvell’s team first visited this site, one out of every ten stars was sick. Now almost half of the sea stars here are showing signs of the wasting syndrome. Scientists have confirmed that there is an infectious agent to blame, but they’re not sure if it is a bacteria or a virus. It appears to be transmitted through the water or physical contact.

“Most of the stars in our waters probably have already been exposed [to the disease]‚ because if we bring them into the lab and expose them to temperature stress, they get sick,” Harvell explains.

Warm water stresses out sea stars — it's much like when humans get overtired while fighting a cold, which helps the sickness takes hold. The hypothesis is that warmer waters may not cause the disease, but they probably exacerbate it.

That hypothesis also seems to hold true further down the coast.

Carol Blanchette, a research biologist at UC Santa Barbara who has been following the disease outbreak there, says sea stars have been “pretty much wiped out at most sites” on the southern coast. The disease has progressed rapidly at a time when the winter waters have been warmer than usual.

But Blanchette is also exploring a new hypothesis: she thinks the disease may be connected to what sea stars are eating. UC Santa Barbara has an aquarium right on the ocean where tanks are filled with pumped-in seawater. The aquarium’s sea stars got sick at the same time as the nearby wild population did.

But then scientists noticed something interesting. In one tank, sea stars were fed mussels harvested from the shore near the aquarium. In another tank, the sea stars were fed frozen squid. The stars that ate the wild mussels got sick; the stars that ate the squid didn’t.

As "filter feeders," mussels could be concentrating the pathogen in their flesh — and sea stars eat a lot of mussels.

“I don’t want to make too much of this,” Blanchette says, “because it’s a very small sample size. But it is interesting that there may be a link to acquiring the disease through the food that they’re eating.”

Sea stars are an apex predator — a keystone species. This type of creature helps maintain the natural balance in a given ecosystem. What happens if they all die off?

“Losing a predator like that to the system is bound to have some pretty serious ecological consequences,” Blanchette says. “We really don’t know exactly what the system is going to look like in [their] absence ... but we’re quite certain that it’s going to have an impact.”

Scientists at Cornell hope to identify the pathogen behind Sea Star Wasting Syndrome in the coming weeks, and their findings will be submitted to the journal Science. The sooner, the better — sea stars have survived mass wasting syndromes in the past, but no one has ever seen a die-off as big as this one. 

This story is based on a report from PRI's environmental news magazine, Living on Earth. Ashley Ahearn reports for the public media collaborative, EarthFix.

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