In 2013, a viral disease that turns sea stars to goo struck with unprecedented ferocity along the Pacific coast of Oregon and California. The wasting disease first twists the arms of sea stars, then withers them and eventually causes the animals to disintegrate completely. The epidemic killed millions, so it came as a surprise when scientists recently counted an unprecedented number of juveniles off the Pacific coast.
A team of researchers from Oregon State University led by marine biologist Bruce Menge has been tracking purple sea stars (Pisaster ochraceus) for years. During the height of the epidemic in 2014, the number of larval sea stars on rocks was similar to or a bit lower than that of previous years. “But a few months later, the number of juveniles was off the charts—higher than we’d ever seen—as much as 300 times normal," he says in a statement from the university. “It wasn’t a case of high settlement, or more sea stars being born. They just had an extraordinary survival rate into the juvenile stage. Whether they can make it into adulthood and replenish the population without succumbing to sea star wasting disease is the big question."
The sea star wasting epidemic is one of the most extensive disease events ever recorded in a marine species, the researchers report in the journal PLOS One. Sea stars are important predators in these marine ecosystems, keeping other animals in check. Their loss could potentially upset local food webs. The disease even affects sea stars' cousins, sea urchins.
More available food, thanks to the death of adult sea stars, might be the biggest factor behind the recent boom, Menge and his colleagues write. The adult population had been decimated by as much as 84 percent, paving the way for this new generation.
"It's remarkable," ecologist and evolutionary biologist Pete Raimondi at the University of California, Santa Cruz, tells Nsikan Akpan at PBS Newshour. "It's hard to even appreciate that this massive replenishment event is happening so quickly after the loss of adults."
Sea star females can release millions of eggs a year, and the larvae float in ocean currents for 45 to 60 days before settling down, Akpan reports. That reproductive strategy means that sea stars are capable of a quick rebound even in areas where it seems they've been wiped out. Sea star recovery, then isn't a question of "will or won't happen," but when and how long it will take to happen. A slow recovery would mean greater upset to the food web balance.
Researchers still don't know why the virus exploded into such a big epidemic in the first place. Previously, some thought that warming waters might have made the echinoderms more susceptible to the disease, but Menge and others have noted that the virus spreads during colder periods of the year, reports Will Houston for the Eureka Times-Standard. If there is a climate or environmental cue influencing the disease outbreak, it isn't obvious.
That uncertainty makes the researchers cautious about claiming that the population boom is a true recovery. Just because numbers look good now, doesn't mean that this new crop of young sea stars won't succumb to the disease when they get older. For now, Menge and other researchers will continue to watch and wait to see if sea stars are out of danger.