Finding out why sea stars are disappearing from the San Francisco coast

Six years ago, if you had poked your head under the water off the California coast, you might have seen vibrant populations of ochre and sunflower sea stars.

Finding out why sea stars are disappearing from the San Francisco coast

Six years ago, if you had poked your head under the water off the California coast, you might have seen vibrant populations of ochre and sunflower sea stars. Since then, those sea stars have melted away.

Scientists call the culprit “sea star wasting disease,” but many things about the epidemic, one of the largest ever seen in marine wildlife, are still unknown. This sharp decline has spurred scientists across the country to try to determine the cause and its cascading effects on intertidal ecosystems.

Our research lab didn’t start out planning to study an environmental disaster, but that’s where we’ve found ourselves.

At San Francisco State University’s Estuary and Ocean Science Center, we study a group of small six-rayed sea stars called Leptasterias. Because baby Leptasterias tend to stay close to their parents, in contrast to the young of other larger sea star species that travel far away, populations of of these sea stars may be especially quick to adapt to their particular environment over the course of generations. Our lab uses them to study this process of local adaptation, and specifically focuses on where different species occur and how that changes over time around the San Francisco Bay outflow.

Then, in 2013, wasting disease struck larger star species. Subsequently, in locations where we visited for years, numbers of Leptasterias stars decreased from hundreds to almost nothing. In repeated visits over the past few years, the little six-rayed stars have remained absent. However, nearby populations to the north and to the south of the San Francisco coast have managed to persist. Other scientists have noticed dramatic differences in the impact of wasting disease among sea star species and among different regions, too.

These differences created an opportunity to compare healthy, abundant populations to ones that were decimated. Making those comparisons, along with studying the sea stars in archives and museums that were collected before the epidemic, allow us to begin to figure out what the populations that succumbed to the disease have in common.

Our census of sea stars near the San Francisco Bay outflow in 2016, for instance, allowed us to see that those locally adapted Leptasterias populations had disappeared. Perhaps, the harsh warm and low-salinity environment and the low genetic diversity of these groups became their downfall, and compounded the effects of the disease. Just as informative, though, might be studying populations that have managed to survive. Here are a few of the ways we are tackling these questions.

Every year, the San Juan Island archipelago is affected by a large freshwater plume from the Fraser River in British Columbia. Because of climate change, scientists predict more frequent, heavy winter storms – this means larger volumes of freshwater pouring out along the northwest Pacific coast. This is particularly critical during times when the six-rayed star mothers are sheltering their vulnerable babies beneath their bodies in the rocky intertidal. Sea stars near these outflows could be at a greater risk of going extinct locally, similar to those in California.

In association with the University of Washington’s Friday Harbor Marine Lab in the San Juan Islands, Washington, Jeyna Perez, a SF State master’s student has been conducting genetic analyses and discovering patterns in the San Juan Islands that suggest the distribution of Leptasterias species there are related to the outflow. “These analyses show similarities with the patterns of genetic diversity our lab found around the San Francisco Bay,” she said.

Noah Jaffe, also a SF State master’s student, is focusing on a third region, between central California and Washington. The rugged rocky coastline of Oregon and far northern California hosts large populations of the six-rayed sea stars, even as these locations saw dramatic declines in larger star stars due to wasting disease. Unexpectedly, in Oregon, Jaffe found sea stars that had some of the same genetic types that had disappeared from the central California coast.

“This reassuring find brings new perspective to the loss around the San Francisco Bay outflow because these types of sea stars are persisting elsewhere,” he said.

Other graduate students in my lab are currently working to find other explanations for differences in Leptasterias survival. Some of us have joined forces with Dr. Roger Bland of the Physics Department at SF State to study differences in how Leptasterias attach to rocks and withstand strong wave forces. Others are looking at differences in their behaviors and body designs related to hunting or holding onto egg clutches. We are also working on finding out how much energy they use at different temperatures compared to real temperature profiles collected from electronic loggers in the rocky intertidal.

Despite the devastating impact of disease on sea star populations, there are many reports of revival along the coast. Ochre sea stars, for instance, are seemingly on the rebound. However, for low-dispersing, critical species such as Leptasterias, the effects of disease and environmental change could be both more severe and more permanent, as rebound is less likely for isolated populations.

We need to continue studying these species to understand what about these sea stars helps some to persist, while others die.

You can help, too. There are so many remarkable rocky habitats along our outer coast, and we find it hard to cover our important sites in the available low tides. We benefit from volunteers who come with us in the intertidal, or provide their own monitoring information to add to our data collection.

If you would like to know more about our research or participate in sea star citizen science, please email me at or visit

The article was written by Sarah Cohen, a professor of biology at San Francisco State University, and students Jeyna Perez and Noah Jaffe. It is one of an occasional series about the bay and sea around us through the eyes of researchers at SFState’s Estuary and Ocean Science Center.


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