Nature News

The Smallest Stars Have Gone Out

More than 20 species of sea star suffered in a disease outbreak that started in 2013. But in the Bay Area, one small star hasn't returned.

February 11, 2020

Parts of this story appeared previously in the Tiburon Ark newspaper.

Once there were thousands, a galaxy of tiny stars strewn over the rocky beaches of West Marin and the San Mateo coast. But within only a few years, Leptasterias pusilla, or the six-rayed sea star, vanished from Bay Area coastal beaches.

Scientists suspect that genetics and disease played a role. But there’s an additional mystery. Sea stars of all kinds perished up and down the West Coast from a still not-fully understood marine disease outbreak starting in 2013, but some of them have now recovered, at least in places. Leptasterias pusilla in the Bay Area, though, have not come back. A team of San Francisco State University biologists who’ve studied Leptasterias here for years conclude in a November 2019 paper in the journal PLOS ONE that the disappearance of the tiny stars might signal important changes in the Bay environment.

“There’s something about the sites in Marin and San Francisco,” says SFSU biology professor Sarah Cohen, who co-authored the paper. “They’re not healthy in the way we would want them to be.”

Leptasterias sea stars are tiny, only an inch or two across, but they are voracious carnivores. They eat other invertebrates like mussels, snails and barnacles, keeping these other intertidal species in check.

In 2010, the stars were abundant on the beaches and rocky tidelands of Marin from Point Bonita to Slide Ranch, and south of San Francisco as well. Six years later, researchers scoured the beaches around the San Francisco Bay outflow, including Rodeo, Muir, Slide Ranch, Point Bonita and Duxbury Reef. They found not one Leptasterias pusilla.

“There were thousands at Muir Beach alone,” Cohen says. “They were at Slide Ranch, Point Bonita. They just disappeared.”

Leptasterias habitat
A photo diagram shows Leptasterias habitat on the coast — and hints at the stresses they survive. (Diagram by Ashley Smith Loveland, San Francisco State, courtesy Sarah Cohen)

Leptasterias are a good “local reporter” species, Cohen says, meaning the health of the species is a good indicator of the health of their environment. That’s because the little stars are homebodies. They do not send their offspring wafting out into the ocean to colonize other beaches the way some other stars do. Parents “brood” their offspring until the baby sea stars are developed enough to walk away from their mothers. Families spend their lives together in the same colonies. They’re particularly vulnerable to changes in water temperature and salinity, so studying the health of Leptasterias stars tells us a lot about what’s happening at a particular intertidal site. Localized colonies have low genetic diversity, too, which might make them more vulnerable to disease.

“This is so important because climate change and environmental variation occur differently in local ways,” Cohen says. “If we want to understand and develop the ability to predict which changes are significant to which animals we need local reporters.”

As many as 20 species of West Coast sea star have been ravaged in recent years by what scientists call sea star wasting syndrome, which is perhaps the largest outbreak of marine disease ever observed. Beachcombers noticed bigger stars like the purple and orange Pisaster ochraceus dying first. But when Cohen’s team looked closer they found that the tiny Leptasterias, which are far less visible, had died off in some places altogether.

That’s where the mystery comes in. While Leptasterias were affected by sea star wasting syndrome everywhere on the West Coast, it’s only in the Bay Area where they were completely wiped out. They’re still found in abundance further north along the border with Oregon, Cohen said. South of the Bay Area, the stars also remained, although in smaller numbers than in 2010.

The scientists asked if something about the coastline near the outflow of San Francisco Bay could be uniquely unhealthy for six-rayed stars.

“There are tons in Oregon,” Cohen says. “The (wasting disease) is visible but the species is not knocked back like here. This regional disappearance makes you wonder.”

Leptasterias on a rock
Leptasterias can be tough to spot. (Photo courtesy Sarah Cohen)

Sea stars living close to San Francisco Bay already have it tough. While sea stars living in most intertidal zones can count on stable conditions, those living close to Bay outflows have to endure variations in water temperature and salinity along with urban pollutants.

“It’s an extremely stressful place to live if you’re a sea star,” says Noah Jaffe, a graduate student in marine biology at SFSU and lead author on the PLOS ONE paper.

In the past 10 years, these natural variations have become more pronounced due to a warming climate. Winters alternating from extremely wet to extremely dry have caused Bay outflow to swing from very salty to almost fresh water at times. Sea stars can’t cope with fresh water very well because they can’t osmoregulate, or, in other words, they can’t regulate the amount of salt in their cells, Jaffe says. And they absorb water into their bodies through their skin, making them particularly susceptible to changes in water temperature and salinity.

Leptasterias pusilla
Leptasterias pusilla in a tidepool at Pigeon Point, showing some of the color variation that makes the species so difficult to identify. (Photo by Sarah Cohen)

In years of intense drought, like the one California experienced between 2011 and 2017, toxins in the Bay can become concentrated, Cohen says. When the tide flushes that water to the ocean, animals living near the Bay outflow get a higher dose of those pollutants.

Marine animals like sea stars also had to cope with the infamous “Blob” of warm water that developed in the eastern Pacific Ocean between 2013 and 2015. Dive surveys have shown a link between warmer waters and the virulence of sea star wasting syndrome in the species Pycnopodia helianthoides, according to a study published last year in the journal Science Advances.

Any one or all of these changes might have weakened the stars, making them susceptible to the wasting syndrome, the scientists say.

“They might have been just at the level of what they could tolerate and then this pathogen came along and pushed them over the edge,” Jaffe says.

leptasterias pusilla
Leptasterias pusilla stars in a tidepool in 2016. (Photo by Mai Ly Cohen Barschall, courtesy Sarah Cohen)

A lack of genetic diversity might also have played a part, although it is difficult to disentangle from the environmental factors, Jaffe says. Because Leptasterias stars remain in their specific local areas, often isolated from other populations, they tend to evolve local adaptations. Jaffe’s investigations discovered that stars found near the Bay Area represented a unique genetic sub-grouping of Leptasterias pusilla that was found nowhere else on the coast. That sub group is now extinct, he fears.

The team’s sampling ended in 2016, though Cohen and her lab continue monitoring sites and searching beaches, hoping the Leptasterias star is making a comeback. So far there have been no sightings. She has put out a call for volunteers willing to sample beaches in remote parts of Marin County and is happy to train them to take a statistically valid samples, she says.

Jaffe says he hopes the research will encourage closer scrutiny of inter-tidal environments, establishing a baseline for what species exist there now before climate change radically alters it.

“There is so much we don’t know, and it does seem that global warming is likely to reduce this biodiversity,” he says.

About the Author

Gretchen Lang has won numerous awards for her environmental reporting in the Bay Area. A longtime foreign correspondent, she returned to the Bay Area in 2010 and now works for a small community weekly newspaper in Tiburon.

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