Just five years after a wasting disease swept California starfish populations, killing many of the creatures, scientists have discovered a microevolution in one species of sea star that helped it survive.
The microevolution shows how the sea stars rapidly responded to the onslaught of wasting disease with a genetic shift, according to a UC Merced study published last week in Proceedings of the National Academy of Sciences.
And it gave scientists a rare opportunity to study and document how a species might change in a short period of time.
Lauren Schiebelhut, the lead author of the study, was a graduate student at UC Merced in 2012, when she started collecting DNA samples from ochre sea stars, or Pisaster ochraceus, as part of a project on their genetic structure that also examined how juvenile ochre sea stars move around in the open ocean before returning to normal habitats, she said.
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About a year later, 81 percent of the population was dead — taken out by sea star wasting disease, which turns the normally rigid starfish into gooey blobs, according to a news release from UC Merced. It was one of the largest mass mortality events recorded in a keystone marine species, according to the study.
Because of the samples taken in 2012, Schiebelhut and her colleagues had an unusual snapshot of what the population looked like before the disease took hold — giving them the opportunity to document evolutionary change in action.
"It became an opportunity to track how the gene pool of Pisaster is changing through time," said Schiebelhut, now a postdoctoral fellow in UC Merced professor Michael Dawson's lab. "We’re still going out there annually and collecting samples to see how they’re changing through time." Dawson is also the study's co-author.
Schiebelhut said that for this study, researchers compared two snapshots of time: the baseline population before they were ravaged by disease, and the time period after the wasting disease hit hard in 2013.
Researchers sampled adult starfish who survived the disease, as well as juvenile sea stars who returned from the open ocean to their normal habitat during the height of the disease.
Scientists were looking for a "parallel shift," meaning that they were looking for a similar change between all original adults and surviving adults, as well as between original adults and the returning juveniles, or recruits.
"We were able to rule out that it was just random genetic change and it was most likely natural selection because we saw the same shift in the adults and the recruits," she said.
Schiebelhut and her team took samples of the starfish from 16 sites, from just south of Monterey Bay up to Point Arena in Mendocino County. They also sampled starfish in Cayucos, Shell Beach and San Luis Obispo — but less frequently.
The original point of the sampling, back in 2012, was to find juvenile sea stars that drifted far away from their original homes. But as wasting disease took root, the sample sites helped scientists figure out that the sea star populations were declining — and they gave scientists consistent places to monitor.
Sea star wasting disease affected about 20 different species of starfish and nearly extinguished the sunflower sea star, which was once common along California's Central Coast.
Though the disease's peak was in mid-2013 through early 2014, Schiebelhut said they still see starfish with wasting disease — and as recently as last month.
"We're not quite out of the woods yet," she said.
She added that though there aren't many studies on how an extreme event would change the ochre sea stars' gene pool, the starfish do appear to be bouncing back.
"They seem to be recovering, they seem equipped to have a response and to persist," she said. "I'm hesitant to extrapolate that to other species. There's a lot of factors that go into whether a species will persist or not."
Schiebelhut and Dawson are expanding their study to include other species of sea stars to see whether they had a similar response to the disease, according to a news release from UC Merced.
Schiebelhut said they're collaborating with scientists at Cornell University, the University of Georgia and UC Santa Cruz to work different angles of understanding the disease. The cause of the disease, as well as why some sea stars are more susceptible to it than others, is still unknown.
"The concern is that as these events become more frequent through time, the species will reach a point where they can't respond as effectively," she said. "It's harder to respond when you have a one-two punch versus a period of relative stability."
Schiebelhut added that anyone can help scientists keep track of starfish, which live primarily in intertidal zones on the coast "in an area that people can go and see themselves."
People can log their observations of wasting disease in sea stars on a site operated by UC Santa Cruz, she said.
"Making these regular observations can be very important," she said. "That can be useful to scientists and useful to research."