‘A giant leap forward,’ UCSC expert says of breakthrough in mass starfish die-off

A discovery this week of what could be causing mass death among sea stars has been making waves in the scientific community, marking “a giant leap forward” in understanding local ocean ecosystems, said Peter Raimondi, who has tracked the mysterious illness for more than a decade as a professor of marine biology and statistics at UC Santa Cruz.

On Monday, researchers from the University of British Columbia and the University of Washington published a groundbreaking paper in the journal Nature Ecology & Evolution identifying a pathogenic bacterium, Vibrio pectenicida, as the likely cause of a disease that has killed off large numbers of sunflower sea stars in the Pacific Ocean across the western coast of North America.

Multiple species of sea stars, commonly referred to as starfish, have seen their populations decimated over the past 12 years by a marine epidemic known as sea star wasting disease. However, until this week, researchers had been puzzled as to why. 

The disease begins with the animals developing lesions or wounds on their bodies, which then progress into a massive bacterial infection. 

Wasting disease typically starts with a behavior that resembles arm-twisting movements in the sea star. Soon after, the arms detach from the body of the star, leading to its death. The creatures can die in as few as three days after the illness first appears. 

Raimondi, who was not involved in the research that discovered the bacterium, said the wasting disease spread rapidly across sea star populations. After about eight weeks, it had infected multiple species.

Sunflower stars were particularly affected and have since nearly vanished from tide pools in Northern California and Oregon, the southern part of their range. 

In 2023, NOAA Fisheries proposed listing the sunflower sea star as threatened under the Endangered Species Act after its population fell by 90% from 2013 to 2017.

Raimondi first noticed the disease during a routine dive in Monterey Bay while conducting ocean research with a class in the fall of 2013.   

“In the second week [of class], we started seeing arms crawling along the bottom, and these dismembered sea stars, [in the beginning] all one type, the sunflower sea star,” said Raimondi, noting a clear characteristic of wasting disease. 

UCSC researchers at the Center for Integrated Spatial Research quickly launched into action, creating an interactive map and data display to help researchers use the data collected by the Multi-Agency Rocky Intertidal Network in a new way to understand the patterns of the disease. 

While the bacterial discovery excites Raimondi, he emphasizes that key questions about the marine ecosystem remain. 

Raimondi still wants to know what makes sea stars sensitive to the disease. He wonders if the bacteria causing wasting disease is uniquely lethal to the creatures, or if the sea stars that are dying are already suffering from other unknown factors, and the bacterial infection is worsening the conditions for an already sick animal.   

About 15 different species of sea stars were affected by the disease, including the ochre sea star, which is commonly found in tide pools and is one of the more recognizable species. They were also starting to die from what was believed to be a wasting disease. 

“Now, what’s still unclear is whether it’s the same [bacteria] for all the other sea stars,” said Raimondi.

The health of sea stars is essential to the kelp forests along the Pacific Ocean. Without substantial sea star populations, uncontrolled sea urchin populations largely decimate kelp forests. 

Sea otters, known for keeping urchin populations in check, often can’t do the job alone, particularly after the kelp forests have disappeared.

Otters tend to avoid the areas where kelp forests have already disappeared, said Raimondi. Without kelp, sea urchins don’t produce a gonad when there is a lack of food. Gonads are the animal’s reproductive organ, where they store nutrients, and are the only real edible tissue in a sea urchin — “The otter soon learned that there’s no point eating them there, and they go elsewhere,” Raimondi said.

Another mystery that Raimondi finds interesting is why the kelp forests around Santa Cruz have never been heavily affected by urchins, despite the widespread loss of sea stars.

Kelp forests north of San Francisco have been devastated because urchins have eaten all of the kelp without sea stars to keep the populations under control. The kelp forests in Monterey were also severely affected at the same time, but have since begun to recover. 

Raimondi said researchers don’t have much information on why Santa Cruz’s kelp forests have remained somewhat unscathed. He pointed out that fewer studies have been conducted along the Santa Cruz County coast up to Año Nuevo State Park, simply because the water conditions, being particularly rough, are not as favorable for research dives. 

Water temperature, potentially driven by the ongoing effects of climate change, could play a role in both accelerating the spread of wasting disease and the decline of kelp forests. 

When waters are warmer, kelp forests grow more slowly due to a lack of nutrients, bacteria tend to proliferate more easily, and urchins eat a lot more. “So you’ve got this, really, this almost demonic intervention, like all the things that are bad for kelp happened at the same time,” said Raimondi. 

However, he noted that Santa Cruz’s cold ocean waters are beneficial for the long-term survival of kelp forests.

Cooler winds from February to July bring in cold, nutrient-rich water, said Raimondi, which prevents the water here from getting too warm, even during heat waves. However, he added, it’s possible urchins simply didn’t go rogue and eat as much kelp in Santa Cruz as they did off the coast of Monterey. 

As scientists look to reintroduce sunflower sea stars along the Pacific coast, for Raimondi, it’s essential to investigate whether the disease remains prevalent in the area. 

“Are you going to put out a whole bunch of sea stars and they’re just going to die because the disease is still there?” he asked. 

As restoration efforts continue, Raimondi believes that strides in research, such as the study published this week, will help find solutions to some of the ocean’s mysteries.

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