All good outbreaks run their course. A community perishes or perseveres. And purple sea stars (Pisaster ochraceus) off the Oregon coast have picked survival. They’re mounting an epic comeback, after a sea-star wasting disease decimated the marine animals in 2014.
It’s no secret that the condition triggered the biggest sea star die-off along the West Coast in recorded history.
What’s less known is that this steep decline in purple sea stars has been followed by an unprecedented flourish of younglings, according to new research from Oregon State University.
“It is remarkable,” said ecologist and evolutionary biologist Pete Raimondi of the University of California Santa Cruz, who wasn’t involved with the new study. “It’s hard to even appreciate that this massive replenishment event is happening so quickly after the loss of the adults.”
It’s unclear whether this is a true recovery. The disease lingers in pockets across the West Coast and has crept north into fresh territory. But the timing — and scale — of the Oregon comeback is surprising and hopefully suggests a positive forecast for this keystone species along the West Coast, experts say.
A youth revival
Sea star wasting disease was first spotted in spring 2013 near Vancouver and along southern and central California, but the Oregon coast was initially spared. However, the close proximity caused ocean ecologists like Oregon State’s Bruce Menge to take notice.
“Although we’re looking at these populations more or less continually, we started paying special attention to whether or not they showed signs of wasting disease,” said Menge, who led the new study published today in PLOS ONE. Signs are hard to miss among the sea stars. Their arms twist like contortionists, their suckers peel from rocks, and their bodies melt away.
But they also built separate experiments to track younger generations of sea stars. For instance, his team deployed “turfies” — squares of astroturf attached to tidal rocks. These serve as ideal landing zones for the sea stars’ microscopic larvae, known as settlers and recruits.
“We put these out for months, and then we bring them into the lab, sort out all the material and count the number of little sea stars,” Menge said. The team has conducted these surveys at 12 sites since April 2014, when the outbreak arrived in Oregon, offering a broad and long-term examination of the whole coastline.
Menge’s team found that sea star wasting cut the juvenile and adult population of purple sea stars by up to 80 percent by the end of 2014, but then the tides turned.
The number of young settlers surged during autumn 2014 and continued into the following year. By spring 2015 at some sites, baby sea star numbers increased up to 300 times relative to the previous year.
Oregon isn’t alone. Purple sea star younglings are thriving across the West Coast.
“The numbers were astonishing,” biologist Benjamin Miner, who works farther north at Western Washington University in Bellingham. “I’ve been walking around at low tides looking at sea stars for 20 years, and rarely do you find lots of juveniles.”
But soon after Washington’s big mortality event, Miner said little ones were everywhere.
“The findings are pretty similar to what other groups in California are seeing, which is a huge amount of recruitment of baby sea stars since this wasting disease,” marine biologist Carol Blanchette of the University of California Santa Barbara said.
The youth revival may be simply due to sea star reproduction and open real estate. A single female can produce millions of eggs each year. The resulting larvae float in ocean water for 45 to 60 days.
“A place that seems like it’s locally extinct can be repopulated very quickly, because if the babies are coming from somewhere else, not locally, then repopulation can occur really quickly,” Raimondi said.
But he says that sea star recovery isn’t a question of “will or won’t happen,” but rather, when and how long.
That’s because sea star wasting is still killing adults along the West Coast. If the disease catches hold in the younglings once they mature into adults, then the mass die-off may repeat itself. “We’re kind of just waiting to see as we move into late spring and early summer of this year whether we have fresh outbreaks,” Miner said.
Given the possibility of a sequel, researchers are desperately hunting for the triggers of the 2013 outbreak. In November 2014, research led by microbiologist Ian Hewson at Cornell University identified a virus that correlated with the presence of sea star wasting disease, but experts remain skeptical of whether it is one true cause.
“The [sea star associated] densovirus certainly is the best candidate, but I would argue that it’s not a great candidate, because we found it in lots of individuals that appear to be healthy,” Miner said of anecdotal results from his lab. “And I’ve reared those individuals up that we know are positive for the virus, and they don’t get sick. And I’ve also had individuals that get sick that I know are negative for the virus.”
Raimondi was one of the co-authors on the densovirus discovery paper, and even he has reservations about solely placing the blame on this microbe. The sea star associated virus isn’t new, he said. In fact, it’s been found in museum specimens that date back 70 years. Hewson’s team continues to examine the genetics of historical specimens (and to blog about it) to determine if the latest outbreak was caused by the virus suddenly mutating into a more potent form.
Meanwhile, a theory based on environmental stress is brewing.
“The fact that this has occurred along the whole west coast, from Alaska down into Baja California, suggests that it’s linked to a large-scale oceanographic change,” Menge said.
Many suggest a temperature trigger, given the height of the sea star die-off in 2014 correlated with the warmest water in decades off the northeast Pacific Ocean. Last week, Miner’s team reported that reducing temperatures can protect purple sea stars from the wasting syndrome. Plus, the far north regions of Alaska, where the waters were really cold, were the only place where sea stars seemed unaffected.
“But even now people are reporting signs of wasting up there,” Blanchette said. “So that does hint at this idea of a link between temperature and disease.”
However, this warming model doesn’t fit with Oregon’s outbreak. An upwelling event occurred during the height of Oregon’s epidemic, wherein deeper colder waters shot upward toward the coast.
“It doesn’t mean necessarily that temperature is not involved, but it’s likely a much more complicated story,” Menge said. His team is looking into the role of ocean acidification, which is impacting seas across the globe. Upwellings are regular events in the North Pacific that can carry acidic waters from the ocean floor into surface waters. Ocean acidification may have stressed the sea stars, making them more susceptible to the virus, Menge said.
A sunflower’s swan song
Deciphering these stressors is the key to solving this outbreak, in Raimondi’s opinion, and to predicting what may happen next. Purple sea stars are fierce predators, and without them, other organisms are multiplying without resistance and upsetting the balance of West Coast ecosystems.
Menge’s team has observed new blankets of gooseneck barnacles on the open spaces where sea stars used to be, and their report cites lower predation rates of mussels along the Oregon coast. He says that mussels will ultimately outcompete the barnacles.
Deeper subtidal waters off the coast of Washington are suffering from a separate invasion of urchins, according to Miner. These urchins would normally be checked by another species, the sunflower sea star (Pycnopodia helianthoides), but they haven’t recovered anywhere along the West Coast. When urchins run amok, they can consume enormous amounts of algae and mow down kelp forests. The forests serve as habitats for fish and support fisheries.
As far as anyone can tell, the sunflower sea star community is ecologically extinct along central California, Raimondi said. The animals aren’t totally gone, but their numbers are too low to corral urchins. Whereas scuba divers might have seen 100 or 1,000 in the past during a scuba dive, you might see one to 10 today, Raimondi said.
“If the kelp forest is diminished because of urchins, which may be enhanced because of Pycnopodia loss, you’d have a truly fundamental shift in a lot of things, including the fish community,” Raimondi said.
Raimondi doesn’t want to leave people thinking everything is doom and gloom. The increase in young purple sea stars shows how resilient these communities might be, he said. Plus, sunflower sea stars live in deep, murky water, so they’re hard to observe. It’s possible that their numbers dropped to this degree in prior outbreaks of wasting disease, but people didn’t notice.
“So it’s not to say that things are not going to recover. In my opinion, they will,” Raimondi said.