Starfish strikes in sea urchin fear to save kelp forest

The brainless multi-army predator may be a powerful ally in the underwater war to save North American kelp forests.

New research led by undergraduate students at UC Santa Cruz shows that sunflower starfish (pycnopodia helianthoides) triggers the strong fear reaction of the Red Sea Urchin, causing them to escape and stop eating kelp. This finding illuminates how the return of these long-lost predators can help reverse the collapse of critical marine habitats that support fisheries, biodiversity and the global economy.

Create a “landscape of fear”

Kelp Forest is one of the most productive and most valuable ecosystems on the planet. They masked young abalone and rocks, protecting the coastline, and global economic activity is estimated at $500 billion per year. But in places like California and Oregon, these forests have disappeared over the past decade. The main culprit? Overgrazing of sea urchin – especially after a mysterious 2013 disease, sunflower-seeded starfish that once brought urchin populations to control.

Now, researchers have shown that these starfish don’t need to eat sea urchins to affect them. Just being nearby is enough. “We demonstrate that starfish creates ‘fear landscapes’ in degraded gallbladder arrays, thus reducing grazing of kelp,” said Kristy Kroeker, senior author of the study, Kristy Kroeker, professor of ecology and evolutionary biology at UC Santa Cruz.

How the experiment works

To test the influence of starfish, student researchers conducted field experiments near Sitka, Alaska, where sea urchin beds were already covered by sea. Team with bound kelp leaves placed in cage sunflower seeds starfish on the seabed. Empty cages are used as controls. After just 24 hours, Red Sea Urchin avoided about 6 feet of starfish dyed kelp, despite their hunger and food availability.

• Red sea urchin cage with sunflower seeds starfish
• Green sea urchins will not be affected, continue to eat seaweed
• Kelp grazing is reduced by more than 70% within 25 cm of predator
• The fear effect extends to a radius of 8.5 m2

“My well-educated speculation will also prevent purple urchins from grazing, but that’s a question of how long and how long,” Crocker added.

The power of the predator does not kill people

This indirect predator effect, known as the non-consumption effect, is increasingly considered a powerful force in ecosystem recovery. Instead of eating prey, predators change their behavior. In this case, the chemical clues of the sunflower starfish are released into the water, emitting red naughty children. Starfish are confined in a mesh cage without physical contact with sea urchins.

Lead writer Rae Mancuso, an undergraduate at the University of California, Santa Cruz, helped lead the dive team, calling the experience transformative. “I’m very happy to be able to work on this research with my peers,” Mancuso said. “I hope these findings… have contributed in some way to the restoration of our most important kelp forest.”

Impact on recovery

The authors of the study argue that reintroduction of sunflower-seeded starfish may be a viable, passive approach that can prevent urchins from grazing – providing an alternative to manual removal of urchins for labor-intensive and expensive exercises.

More research is needed to test whether free-roaming starfish will effectively block purple sea urchins, and whether the fear effect will last long enough to allow kelp to recover. But these discoveries open the door to new, more natural interventions to restore underwater ecosystems.

“We have seen ecologically meaningful behavioral shifts in hunger, gonad-deficient sea urchins, and they still choose to avoid kelp near starfish,” the author wrote. That may just be the edge of the kelp forests needing recovery.

Diary and citations

Published in Magazine on July 9, 2025 Royal Society B: Proceedings of Biological Sciences

doi: 10.1098/rspb.2025.0949

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