“I’ve never seen a decline of this magnitude of a species so important,” Drew Harvell, the lead author of the study, published in the journal Science Advances, that documented the sunflower sea star’s retreat into possible extinction off California and Oregon.

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If the study had a purpose, she said, it was to call attention to the sea star’s demise so that federal officials would take action to list it as endangered and work to save it, possibly with a breeding program using sunflower stars that are surviving in parts of Washington, Alaska and Canada.

“It’s big news and cause for major management action,” Harvell said. “We felt there wasn’t enough attention.”

Harvell and her team of researchers traced the sea star’s decline using diver surveys of shallow waters. Between 2006 and 2014, divers saw “anywhere from two to 100 stars during their dives,” the study said.

After that, they saw total devastation. In at least 60 percent of surveys in Washington and Canada, no sunflower sea stars were seen. In California and Oregon, 100 percent of surveys recorded the unthinkable: zero sightings.

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The coastal Pacific is the only place sunflower sea stars are known to exist. It’s been three years since divers have spotted one below Washington.

Their disappearance could not have come at a worse time.

Sunflower sea stars started dying off around the time that the population of their favorite prey, purple sea urchins, exploded. The voracious urchins feed on vegetation that is key to the ecosystem in that area of the Pacific — bull kelp forests that support young fish, snails, crabs, birds and a range of other animals.

Over the last few years, California’s fifth largest fishery, red sea urchins harvested for sushi, has cratered. A once teeming population of decorative snails called abalone, which drew tourists and professional divers, has also crashed. Both animals rely on kelp to survive, and their combined loss has cost the state tens of millions of dollars, according to economists.

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Scientists are wondering if the freak warming anomaly, disease and their adverse effects are sign of things to come.

“What happens before anything else is you get this coincidence between ocean warming and the outbreak of the disease that took out the sea star,” said Mark Carr, a professor of ecology and evolutionary biological for the University of California at Santa Cruz. “We’ve been trying to understand why. We have never seen this kind of disease outbreak in sea stars, and we’ve been trying to understand how the hell it happened."

“We know we’re going to see more warming in the future because of global climate change,” Carr said, “so who knows if we will see more of these purple urchin outbreaks. You do wonder whether we’re transitioning into a new environmental dynamic that we just haven’t seen in the past.”

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Wednesday’s study focuses on the demise of the sunflower sea star, a top predator in the tidal waters in California and Oregon that was as common as a robin, researchers said. Like wolves that control deer populations and bats that control crop-eating moths, they kept purple urchins under control for 600 miles.

With the sea stars on the prowl, purple urchins cowered in cracks and crevices of coral and waited for broken kelp leaves to float their way. “They scared the heck out of them. They caused them to stampede trying to get out of their way,” Harvell said.

But with sea stars virtually annihilated, the urchins spread out across the ocean floor by the tens of thousands and gobbled the forests. The affected areas have a grim nickname: urchin barrens.

The sunflower sea star is a member of a species once known as starfish. Marine biologists changed the name because, among other things, they’re not fish. Also, the sunflower isn’t star shaped. It looks more like the sun.

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It’s an eye-popping sight, a technicolored creature that can grow as big as an SUV hubcap. Individuals often live together on coral in large pastel communities, and they move across the ocean floor using dozens of squiggly arms.

“They are pretty incredible. They can get to be, like, three feet across, so giant,” said Cynthia Catton, an environmental scientist for the California Fish and Wildlife Service. She went diving with a friend from New Hampshire who saw one for the first time. “I think her reaction was telling. They’re just this alien. They’ve got 20-plus arms moving independently, gliding across the reef. It’s fascinating to watch.”

The way sunflower sea stars wasted from disease is as grisly as they are beautiful. They melted and dissolved as if in a horror film. And they were trapped. In the beginning, when the disease was first detected, it was thought that warmer tidal waters were a killing field. But the study showed no evidence that deeper and colder depths are a refuge.

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Scientists are more concerned about the bleak outlook for bull kelp forests is more worrisome because it holds up an ecosystem. When it disappeared, so did other animals and aspects of California’s economy.

“California has very reduced kelp cover,” Steve Lonhardt, a marine ecologist for the National Oceanic and Atmospheric Administration’s Monterey Bay National Marine Sanctuary. “Bull kelp is missing from San Francisco Bay to north of Point Arena, a tremendous stretch of coast. Areas where there was kelp forest is dominated by urchins. People along those coasts are extremely alarmed because the kelp supports the red urchin fishery.”

A kelp forest is habitat “for literally hundreds of thousands of marine organisms,” Lonhardt said. “So when you lose that . . . imagine all the birds and insects that rely on trees. Kelp is delicious. Everything in the marine system loves to eat kelp."

To lose kelp is to lose a diet and nutrients, the cornerstone of the food web. Herons can walk in the middle of the ocean on a thick and healthy kelp forest and hunt for juvenile fish. Otters wrap themselves in it to take naps.