Read: The great sea urchin crisis

Harvell and her colleagues considered a laundry list of possible causes, including storms, pollutants, and radiation from the Fukushima nuclear disaster. But the syndrome always looked like an infection, and in 2014 the team identified a possible culprit—a virus that it called sea-star-associated densovirus, or SSaDV. The 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 more than 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 a 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, the sunflower star 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 has 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 from 2013 to 2015, the sunflowers completely disappeared from the deep waters off of 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 past 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 had 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 the kelp forests fall, an entire ecosystem will fall too, including several commercially important species such as abalone, crab, and countless fish.