Sea urchins have no brains or hearts. But put them in the proximity of the unmistakable sunflower sea star—with its array of up to two dozen arms—and somewhere in their pin cushion-like body, they sense trouble. That’s the main finding in a new scientific study by ecologists and undergraduates at the University of California, Santa Cruz, who wanted to see if this particular type of sea star would deter urchins from eating kelp.
For this study, published on July 9 in the journal Proceedings of the Royal Society B, UC Santa Cruz students who completed the university’s highly regarded scientific diving training donned SCUBA gear and placed caged sunflower sea stars (Pycnopodia helianthoides) on the sea floor a few miles east of Sitka, Alaska, where resident urchins have turned once-thriving kelp beds into barrens.
The course, BIOE 159 Marine Ecology Field Quarter, is an immersive and immensely rewarding experience—offering students not only the opportunity to do true aquatic research, but also have their work published and give them a sense of what’s possible career-wise. In past years, students in the course have spent the quarter in the Gulf of California in Mexico and Moorea in French Polynesia.
“I feel very grateful to have had the privilege of working on this study alongside my peers. Participating in the entire process, from diving to scientific writing, was exciting and impactful as an undergraduate student,” said the study’s lead author, Rae Mancuso. “I hope the findings from this field experiment contribute in some way to the restoration of our all-important kelp forests.”
Importance of kelp
Collectively, these underwater forests act as nurseries for thousands of marine species, including commercially important ones like abalone and rockfish. Beyond their ecological value, kelp forests contribute an estimated $500 billion annually to the global economy, serving as a key ingredient in products ranging from toothpaste and pharmaceuticals to salad dressings.
Then about a decade ago, kelp forests in some large regions of California and Oregon were lost at roughly the same time that sunflower sea stars went locally extinct—largely due to an outbreak of a devastating wasting disease in 2013. Many of the affected areas have not seen recovery of either Pycnopodia or kelp, prompting interest in how to restore the forests, as well as questions about the role of sunflower sea stars in the loss of kelp forests and their potential use in recovery.
“We show that the sea stars create a ‘landscape of fear’ among red sea urchins in degraded urchins barrens that reduces grazing on kelp,” said the study’s senior author, 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.”
How they did it
Mancuso said Kroeker and other faculty leading the field course generously helped his classmates develop the research project that the paper was based on. The students then placed pairs of cages made of plastic pipe and covered with fine mesh at each of three different locations where degraded urchin barrens existed. Kelp blades were fastened to lines tied to all the cages as bait, and with them spaced about 60 to 100 feet apart, one cage was kept empty as the experiment’s control condition, while a sunflower sea star was placed in the other.
After just 24 hours, the results were in: Red urchins stayed an average of about 6 feet away from the kelp tethered to the cages with sea stars in them. This stood in stark contrast to the behavior of green sea urchins also in the area, which weren’t deterred at all and ate the fastened kelp.
Despite the mixed results, the study found that the sea stars clearly deterred one type of urchin, and for that reason, Pycnopodia conservation should be considered alongside other approaches to kelp-forest recovery. The authors suggested that an increase in the presence of sunflower sea stars, either natural or artificial, could have a beneficial effect on kelp forests by deterring urchin herbivory without requiring divers to manually and continually remove urchins.
Purple urchin eaters?
The authors also hypothesized that free-roaming sea stars may keep urchins further away from kelp forests, and that additional research is needed to test whether the presence of Pycnopodia would have a similar effect on purple sea urchins—the most notorious kelp deforester in the region.
“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.”
Other authors from UC Santa Cruz on the paper, “Sunflower sea star chemical cues locally reduce kelp consumption by eliciting a flee response in red sea urchins,” include Mancuso’s former classmates Rosie Campbell and Nathan Hunter, and Pete Raimondi, professor of ecology and evolutionary biology. Sarah Gravem at Oregon State University and Aaron Galloway at the University of Oregon also contributed to the study.
This work was supported by a grant from the Nature Conservancy and the National Science Foundation.
Reference: Mancuso RT, Gravem SA, Campbell RS, et al. Sunflower sea star chemical cues locally reduce kelp consumption by eliciting a flee response in red sea urchins. Proc R Soc B. 2025. doi: 10.1098/rspb.2025.0949
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