A Starfish-Killing Disease Is Remaking the Oceans

The voracious sunflower starfish was once as common as a robin, but a new disease has almost wiped it out—with wide-ranging consequences.  

“There were arms everywhere,” Drew Harvell recalls. “It looked like a blast zone.”

It was 2013, eight days before Christmas. Harvell and her colleagues were walking along Seattle’s Alki Beach, sweeping their headlamps over wet gravel, exposed by a receding tide. Wherever they looked, they saw dead and dying sea stars. Some had disintegrated into white mush. Others were still alive, their bodies riddled with sores and their arms twisting at grotesque angles. Yet others seemed to be pulling themselves apart. “There were arms separating from sea stars, arms walking off by themselves,” says Harvell, an ecologist at Cornell University who studies marine diseases. “That was my first experience of the magnitude of it.”

Similar omens had been accumulating all fall. Harvell had received emails about a mysterious disease outbreak afflicting sea stars in British Columbia. She had read blog posts about “a huge mortality event” that was littering the sea floor with disintegrating arms. She had heard reports that even captive sea stars in Vancouver Aquarium were dying. This unprecedented phenomenon, known as sea star wasting disease (SSWD), ultimately affected more than 20 species. Similar die-offs had occurred before, but never at this scale. All along the western coast of North America, from Alaska to Mexico, the stars were blinking out.

Harvell and her colleagues considered a laundry list of possible causes, including storms, pollutants, and radiation from the Fukushima disaster. But the syndrome always looked like an infection, and in 2014 the team identified a possible culprit—a virus that they called sea star-associated densovirus, or SSaDV. That virus doesn’t cause SSWD in every affected species, though, so there’s still a lot of uncertainty about the syndrome’s cause (or causes). Its impact, however, is undeniable.

In a new analysis, Harvell collated data from over 10,000 surveys, carried out by trained citizen scientists diving off the Pacific coast. Their observations showed that SSWD has brought one especially susceptible species—the mighty sunflower star—to near-total ruin.

The sunflower star is the starfish equivalent of a Tyrannosaurus—a huge, voracious, unmistakable alpha-predator. With a three-foot diameter, up to 26 arms, and hundreds of tubular feet, it runs down clams, sea urchins, and snails at a top speed of six inches per second. “This thing was as common as a robin,” Harvell says. “You would go on a dive and always see sunflower stars.” But since 2013, it has largely vanished from most of its former 2,000-mile range; only in Alaska do appreciable populations still remain. In just a few years, an emerging disease has caused the continental-scale collapse of a once-common species, and started to remake the underwater world.

“Some people have said that maybe they migrated to deeper water, and they’re down there somewhere,” Harvell says. But data from the National Oceanic and Atmospheric Administration have snuffed out that hope. They show that between 2013 and 2015, the sunflowers completely disappeared from the deep waters off California and Oregon, and declined by 99.2 percent near Washington. In 2016, NOAA researchers couldn’t find a single individual in almost 700 trawls. This last summer, they saw just one. “That shocked everyone, including me,” Harvell says.

The loss of any species is a tragedy. But the loss of the sunflower is especially devastating because it’s a keystone predator—a creature that has a disproportionately large influence on the world. The legendary ecologist Bob Paine coined the keystone concept in 1963, after yanking starfish from a Washington beach, and hurling them into the sea. A year later, the mussels that the starfish would have eaten had overrun the shoreline, displaced the creatures that formerly lived there, and remodeled the landscape.

SSWD is effectively carrying out the same experiment, but on an epic scale. In the absence of the sunflowers, the sea urchins they hunt are running amok, eating their way through the Pacific’s kelp forests. Kelp is a tagliatelle-like seaweed whose meter-tall fronds shelter vast communities of marine life. If they fall, an entire ecosystem will fall too, including several commercially important species like abalones, crabs, and countless fish.

Such changes have already begun, and especially in places where that other major predator of sea urchins—the sea otter—has also declined. Once-lush worlds of green and yellow foliage are now “urchin barrens”—desolate domains of purple spines and chewed stumps. “Kelp forests along the west coast have been hit hard, and are likely to diminish further as these sunflower star predators become extremely rare,” says Carol Blanchette from the University of California, Santa Barbara.

“Things are currently not looking great,” says Melissa Miner from the University of California, Santa Cruz. “But from talking to researchers and divers, my understanding is that [the sunflowers] are still present throughout their entire range; you just need to look harder for them now. My hunch is that this is a fast-growing species, which might have the potential to recover quickly if whatever is causing SSWD subsides. I think there is hope.”

But disease is only part of the story. Harvell’s team found that the sunflower’s decline coincided with abnormally strong heatwaves, and the higher temperatures rose above their usual levels, the more likely the stars were to disappear. Harvell suspects that warm waters could have either boosted the growth of whatever microbe is behind SSWD, or stressed the sunflowers, making them more susceptible to infections. “The warming didn’t necessarily trigger the outbreak, but I think it increased the impact of the disease,” Harvell says.

This may be the norm in the future. In Harvell’s upcoming book, Ocean Outbreak, she documents several cases in which infections have wreaked havoc on corals, abalone, salmon, and other marine creatures. In some cases, the changing climate has worsened these contagions.

Land-living animals face the same double whammy. While the sea stars were disintegrating, on the other side of the world, two-thirds of the world’s population of saiga—a bulbous-nosed Asian antelope—dropped dead. They died without warning, in a few days, over an area the size of Florida. And it seems that they were killed by a normally harmless nasal bacterium that, thanks to an unprecedented spell of heat and humidity, infiltrated their bloodstreams and poisoned them. Climate change plus contagion equals mass mortality: It’s a chilling equation for a changing world.