Sea urchins don’t understand predators the way we do. They have no brains, no hearts – just raw instinct. When a sunflower sea star approaches, they don’t need complex reasoning to sense danger – they simply steer clear.
This is the conclusion of a new study that examined how sunflower sea stars affect sea urchin behavior. The research was conducted by ecologists and undergraduates at the University of California, Santa Cruz.
The team set out to understand whether these massive, many-armed sea stars could help protect kelp forests by simply being present.
Sea stars create a landscape of fear
The team from UC Santa Cruz conducted the experiment off the coast of Sitka, Alaska, where local sea urchins have wiped out once-thriving kelp forests, turning them into underwater deserts.
The students, trained in scientific diving through the university’s BIOE 159 Marine Ecology Field Quarter, placed cages on the ocean floor. Some held sunflower sea stars (Pycnopodia helianthoides). Others were left empty.
Kelp blades were tied near each cage to act as bait. The question was simple: Would sea urchins avoid the kelp if a sunflower sea star was nearby?
After just 24 hours, the answer was clear. Red sea urchins kept their distance – about six feet, on average – from cages containing the sea stars. In contrast, green sea urchins, also present in the area, showed no hesitation and munched through the kelp.
“We show that the sea stars create a ‘landscape of fear’ among red sea urchins in degraded urchin barrens that reduces grazing on kelp,” said Kristy Kroeker, professor of ecology and evolutionary biology at UC Santa Cruz.
“These are very hungry urchins that are dissuaded enough by the scent of a sea star to deter grazing on kelp forests, which is promising for thinking about their role in kelp-forest recovery.”
Why kelp forests matter
Kelp forests aren’t just scenery. They provide critical shelter and breeding grounds for thousands of marine species, including commercially valuable ones like abalone and rockfish.
They’re also a financial powerhouse, contributing an estimated $500 billion each year to the global economy. Kelp is used in everything from pharmaceuticals and cosmetics to food products like salad dressings and toothpaste.
But around 2013, something changed. A wasting disease swept through populations of sunflower sea stars, pushing them to local extinction along much of the west coast.
At the same time, huge sections of kelp forest disappeared, especially in California and Oregon. Since then, neither the sea stars nor the kelp have made a full comeback.
That’s raised big questions. Are sea stars a key part of keeping urchin populations in check? Could bringing them back help kelp forests recover?
Students led the research
This study didn’t come from a high-budget lab. It came from a hands-on field course that takes students into real-world research environments.
In past years, the course has brought students to the Gulf of California in Mexico and to Moorea in French Polynesia. This time, they ended up in the chilly waters of Alaska.
“I feel very grateful to have had the privilege of working on this study alongside my peers,” said Rae Mancuso, the study’s lead author.
“Participating in the entire process, from diving to scientific writing, was exciting and impactful as an undergraduate student. I hope the findings from this field experiment contribute in some way to the restoration of our all-important kelp forests.”
The students worked with faculty to design the experiment and set it up across three locations in urchin-dominated zones. They built pairs of cages, spaced about 60 to 100 feet apart, and monitored the urchin activity closely.
Sea stars scaring off sea urchins
Although the sea stars only deterred red sea urchins – not the green ones – researchers say it’s a promising lead. Simply putting sea stars in the area (rather than trying to remove urchins by hand) could help restore balance to the ecosystem.
Even better, this strategy might be less labor-intensive than current methods, which often require divers to manually cull urchin populations.
The researchers are also curious whether sunflower sea stars might scare off purple sea urchins, the worst kelp destroyers in California. That part is still unclear.
“My educated guess is that they will deter purple urchin grazing as well, but it’s a question of how much and for how long,” Kroeker said.
“There are many unknowns that need to be addressed and many steps that need to be taken between our results and the reintroduction of Pycnopodia for kelp-forest recovery.”
The full study was published in the journal Proceedings of the Royal Society B Biological Sciences.
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