White lesions on a sea star's arms are the first sign of sea star wasting syndrome. (Credit: Melissa Miner)
Scientists working to solve the mystery of Sea Star Wasting Disease—and to learn more about the possible keystone species Pisaster ochraceus, the ochre sea star—are reaping the benefits of long-term monitoring of the species along the West Coast. Dr. Melissa Miner, a UC Santa Cruz researcher in the Department of Ecology and Evolutionary Biology, spoke with EM about her two decades of work with the Multi-Agency Rocky Intertidal Network and her recent efforts surrounding the ochre sea star.
In 2013, people began to notice that sea stars all along the West Coast were in the grip of a strange disease. The stars exhibited lesions and quickly succumbed, deteriorating and wasting away. This hobbled sea star populations in record-breaking numbers, and the iconic ochre sea star was hit hard. This was Sea Star Wasting Syndrome (SSWS), and while scientists like Miner and her team from UC Santa Cruz are working to discover its causes and assess how much the species has declined, thanks to their ongoing monitoring efforts in the area, they aren’t flying blind.
Some scientists believe that SSWS is caused by a virus, but as yet there is no conclusive evidence about the root cause of the disease. And while this has been an unprecedented wave of mortality for the ochre sea star, scientists have observed these kinds of disease outbreaks before. In fact, that’s one reason why they’ve been observing the intertidal region for more than three decades.
“I work on a long-term monitoring project, the Multi-Agency Rocky Intertidal Network (MARINe); the ochre star, Pisaster ochraceus, is one of the many species that we monitor up and down the coast,” explains Miner. “This project has been going on for over 30 years, and I’ve been involved for over 20 of them. This was sort of fortuitous, in a way, that we had all this long-term data for this particular species that was hit so hard—not fortuitous for the stars, of course. But in terms of having good information about what normal looks like for these populations up and down the coast, we have really good information for the post disease period which allows us to determine the impacts of the disease and predict recovery rates.”
Like other non-food species, humans know little about sea stars, despite our appreciation of their beauty.“It’s one of the iconic intertidal species,” remarks Miner. “But it’s not a species that people eat, so in terms of a lot of the basic information, such as how fast they grow, survivorship of the juveniles, all the stuff that we know a lot about for species like lobster and things that people eat, we just don’t know a lot about them.”
Had MARINe not been monitoring sea stars along the coast, scientists would know far less about them as they work to unravel the mysteries of SSWS. The ability to produce this kind of long-term data is the strength of this type of monitoring project.
“It’s unfortunate that it’s usually the devastating things that reinforce the need for this type of data, but it is really hard to secure long-term funding for these sorts of things,” details Miner. “Ecologists typically get a pot of money that lasts for a year, three years—five years is considered really good—and then that project is done. So to have something like this that’s been sustained for such a long time is pretty hard to do.”
“Our tracking map shows where people are seeing signs of disease, and also where people have gone out to the coast and not seen any sign of disease,” Miner describes. “These are really quick observational data, so you can just go out to the coast with your family and check out the sea stars if it happens to be low tide. We have a form that people can use to submit those observations, which are really helpful for researchers who are looking at these possible factors that might correlate with disease, such as water temperature and pollutants. We only get out to our sites once or twice a year, but this also gets us data from areas that we would never go to.”
When the SSWS outbreak began in 2013, sea stars with lesions on them appeared as far south as Mexico, and as far north as Alaska. Miner and her team observed intertidal areas dotted with dead and dying stars. They began to study how different areas were affected, and why.
“In terms of the disease, nothing really has panned out to be the smoking gun yet,” Miner states. “It’s definitely suspicious that so many sea stars died during the same period of time. The timing of when the disease hit hardest was different in different areas along the coast. I think that there’s got to be some commonality there, and there may be other factors that exacerbated the problem.”
Although many stars have died, the disease isn’t always fatal. Stars that have recovered have provided scientists with some interesting behaviors to study.
“It’s pretty gruesome, but they can sometimes survive,” Miner explains. “Sea stars are really good at dropping appendages so that they can regrow them. They do it all the time if, say, a predator tries to eat them, so what we’re seeing a lot of now are these individuals that have dropped arms. We’ll see stars with three normal-sized arms and two small arms, or sometimes they’ll grow back in interesting patterns, where they’ve got six or eight arms instead of the normal five because they don’t grow back quite right. But they definitely survive; there’s a high mortality rate for sure, but it’s not necessarily a death sentence if they get the disease.”
This implies that in some cases, at least, stars “know” they are sick in time to save themselves, but scientists aren’t sure how this happens, or why it doesn’t happen in time to save the stars that die.
“I think that’s a big part of the problem; there’s just so much basic research that hasn’t been done on these animals, so there’s a lot that we just don’t know—for example, about how they respond to disease,” remarks Miner.
For now, the massive mortality trend has passed, and recovery has begun in some places. The stars are not, however, totally out of the woods.
“I wouldn’t say that the event is over at this point; we’re still seeing low levels of diseased animals at most of our sites although it seems to have lessened in impact,” clarifies Miner. “It’s not completely gone, but the situation is much more hopeful than it was a couple of years ago when huge numbers of animals were dying off. If things continue like that where it’s just a small number of individuals that are impacted, hopefully we’ll get to the place where we’re not seeing any evidence of disease. Places north of the Santa Cruz/Monterey area are looking pretty good, from northern California up through Washington and British Columbia.”
The signs that are giving the researchers hope include good levels of recruitment and juveniles that survive over time.
“The stars have babies that float around in the water for a period of a couple of weeks, and then they can recruit to new areas as tiny little the larvae settle out and grow up into the stars that we see on the rocks,” Miner details. “But that takes a long time, to get to the big stars that we think of. That’s one of the things that we tried to look at in this recent paper: whether there was a higher mortality rate between that juvenile period and when they start to be reproductive.”
Scientists already know from their long-term monitoring of the ochre sea star that even during normal large recruitment events in the absence of disease, juvenile mortality rates are fairly high. Compared to those normal high rates, Miner and her team are observing juvenile mortality rates during this post disease period that are even higher. Southern California appears to be hardest hit by SSWS.
“We are seeing individuals survive to the adult stage in at least a portion of those sites in that northern region of their range,” remarks Miner. “But in Southern California, anywhere south of Point Conception near Santa Barbara, things are not looking good for recovery. We’re seeing almost no signs of juveniles in the intertidal zone. Without those those new individuals coming in, there’s no way that those populations are going to recover.”Southern California has been stricken with disease events in the past, so the intertidal zones in the area were already at risk.
“Every five to 10 years or so we see a similar type of event where large numbers of stars die showing the same sorts of symptoms that we saw during this event, with the lesions and the tissue degradation leading to death,” Miner describes. “So populations are already depressed by those those earlier events, and then this was just almost the nail in the coffin.”
Still, the ochre sea star is persisting in other areas.
“I wouldn’t say at this point that anybody’s worried about stars going extinct,” clarifies Miner. “You can still find stars and all the areas that we could find them before; they’re just much, much more rare than they were in the past.”
Now Miner and her team are also focusing on what’s next for the stars and their work.
“Monitoring the whole area will continue as long as we continue to receive funding for it, so that’s one aspect, continuing to monitor these populations for recovery, both in terms of number of stars and the size distribution,” states Miner. “In order for a population to be considered recovered, we want to see a distribution of animals of all different sizes that resembles what it looked like prior to the disease; then you know you’ve got enough individuals that are reproductive and can sustain the population. They also make different ecological contributions, those larger individuals, than a bunch of small individuals, as you can imagine; for example, they eat different things. So having thousands of animals that are an inch big is really different from having 500 really big stars.”
The team is also interested in documenting changes in the community overall. One such questions surrounds the idea of whether Pisaster ochraceus is a keystone predator. This hypothesis has been tested with mixed results. This issue matters, because the loss of a keystone species, if that should happen here, which dominates the system can drastically change how it functions. Miner isn’t sure that the ochre star is a keystone species everywhere.
“It appears that their role varies based on other physical factors,” she explains. “A site that has really high sand influence or some other disturbance, freeing up space for other organisms to settle, might look really different in five years after we’ve seen recovery of the stars.”
Miner expects to observe a range of changes from place to place along the West Coast; she does not expect to see the same post-disease impact everywhere there are stars.
“I think the interesting thing for us will be to look at those across this whole stretch of coast from Alaska to Southern California and start to look for patterns in terms of sites that we did see big changes and sites where we didn’t,” remarks Miner. “And that can help us start to tease apart which factors might be really critical in shaping community structure.”
If stars aren’t always serving the same function from place to place, it may mean that this flexibility helps them persist and survive.
“We may have to rethink the idea that they are a dominant force everywhere,” adds Miner. “I think there are people that feel that if you lose sea stars the communities are going to change. And although I think we will see that in some areas, I think we won’t see changes that are as obvious in other areas.”
One thing that this most recent SSWS event has already proved is that long-term monitoring yields valuable data—as does information contributed by citizen scientists like those who use the MARINe website to share their observations of sea stars.
“I think that’s one of the really interesting things about this event is that it’s not just people on the coast who are interested in it,” muses Miner. “We’ve gotten interest from artists and authors and people from all over the place. I guess starfish are one of the things that everybody cares about everywhere, whether you live on the coast or not.”