Nearly five years after sea star wasting syndrome nearly wiped out the starfish population along the United States' West Coast, the starfish (also known as sea stars) is making a comeback.

In 2017, marine scientists in southern California discovered millions of starfish in tide pools after not finding any for years.

“It’s a huge difference," aquarist Darryl Deleske told The Orange County Register. "A couple of years ago, you wouldn’t find any. I dove all the way as far as Canada, specifically looking for sea stars, and found not a single one.”

Evolution helped save them

After scientists discovered that the starfish returned and were in fact thriving, they wanted to find out how they overcame the virus. They compared the DNA of starfish that died from the outbreak and ones that survived (or were born afterwards) and determined those from after the outbreak shared a gene that resisted the virus. There was even a 74-fold increase in starfish after the peak of the outbreak.

"When you’ve removed a whole bunch of them, you’ve shifted the whole genetic diversity of that population," researcher Chris Mah told The Guardian. "In other words, to put it in human terms, if you wiped out a huge chunk of the human species, you would change the genetic makeup of humans."

What caused the starfish to die?

In 2013, millions of starfish from Alaska down through California were struck by a mysterious, deadly disease that caused them to lose limbs or even completely dissolve into white gelatinous goo. The wasting syndrome struck at least 10 species of starfish and its exact cause was unknown, according to the University of Santa Cruz, which has been tracking the ecological destruction.

A similar wasting syndrome had been observed on the East Coast, where it was linked to a virus that follows a period of starfish overpopulation. On the West Coast, however, the only observed factor for the starfish wasting syndrome was warmer-than-usual waters (which could conceivably create a breeding ground for dangerous bacteria). The West Coast syndrome caused two previous mass starfish die-offs, the first in 1983-1984 and the second from 1997-1998. The 2013 die-off appears to have begun the previous summer and was already much greater in scope than any previous events.

Now, a team of scientists have confirmed that in fact warmer waters were the cause of the massive die-off across the entire West Coast in 2013 and earlier years. They analyzed data that scuba divers along the West Coast and confirmed that fewer sea stars were found alive in higher temperature waters.

"To think that warmer water temperature itself can cause animals to get disease quicker, or make them more susceptible, it's kind of a like a one-two punch," Joe Gaydos, the science director at the University of California, Davis' SeaDoc Society, told NPR. "It's a little nerve-wracking."

But the study didn't investigate why the unusually higher temperature water would make sea stars more vulnerable. One theory they proposed was the sea stars' immune systems might become weaker in warmer water.

Regardless of why the sea stars contracted the disease, the effects were dramatic. The starfish first developed lesions that quickly expanded. Starfish began to lose limbs and then completely dissolved. "They essentially melt in front of you," Pete Raimondi, chairman of UC Santa Cruz's Department of Ecology and Evolutionary Biology, told the Press Democrat. All of this occurs within just a few days or weeks of the first symptoms.

As many as 95 percent of starfish populations died off in some affected regions. Although many species are affected, the worst hit has been Pisaster ochraceus, a 20-inch-wide species commonly known as either the purple or ochre sea star. Raimondi told the Press Democrat that this particular sea star is a "keystone species" that eats mussels. Without the starfish to control mussel populations, mussels will multiply to the point of possibly crowding out other tidal species.

The school was monitoring sea star populations for some time, and its data is available as an interactive map or by individual species or location. Researchers asked citizen scientists to observe the starfish in their own areas and report any changes in population.

The Vancouver Aquarium tracked the die-off for several months. The video below demonstrates the devastating effect of the wasting syndrome on Hutt Island. In the first half of the video, the ocean floor is almost completely covered with starfish. In the second half — shot just a few months later — they have all disappeared.

Editor's Note: The story has been updated since it was originally published in November 2013.

Starfish make remarkable comeback in California years after deadly disease outbreak
The disease caused starfish to lose their limbs, dissolve and die.