These urchin-eating sea stars might be helping us reduce carbon levels

The 24-armed sunflower sea star is not a picky eater, which may makes it crucial to restoring kelp forests.
Purple sea urchins clumped together in an urchin barren.
Sea urchins can absolutely destroy highly productive kelp forests if given the opportunity. Sonia Kowsar / Pexels

There’s a case to be made that the world’s greatest forests are not terrestrial. That’s in large part due to kelp. Like their less watery counterparts, kelp forests play an important role in carbon cycling across the planet, converting carbon dioxide into oxygen through photosynthesis and sequestering the carbon beneath the ocean’s surface. 

Kelp forests are located in shallow coastal waters across the globe, including in the northeast and all along the Pacific coast in the United States. Despite taking up only a tiny fraction of the ocean, they’re incredibly diverse. Charles Darwin marveled at just how many species are present in kelp forests in his diary while aboard the HMS Beagle. However, they are incredibly fragile ecosystems. Once disrupted, it’s very difficult for the forests to recover.  

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With the presence of purple sea urchins off the coasts of the western United States, the destruction of kelp forests has become much faster. But new research from Oregon State University published today in the journal Proceedings of the Royal Society B shows that the sunflower sea star, a 24-armed behemoth of a sea star living in kelp forests on the west coast may be a major asset to preserving those important ecosystems, namely by fighting off pesky sea urchins.

Sea urchins are a natural part of the ecosystem, and act as scavengers, feeding on dead kelp and other detritus that falls to the ocean floor. However, when there’s not enough food for them to go around, past research has found that they’ll begin feasting on living kelp. This disrupts the ecosystem, and if not left in check, leads to the formation of an urchin barren, with no kelp to be seen and urchins packed tightly along the ocean floor. Once a barren forms, the rebirth of a kelp forest is all but impossible. Any new kelp growth will promptly be devoured by the urchins, which are able to survive with little food and will live for at least 20 years. 

Marine biologists long ago realized that the predators of sea urchins are part of the problem. Sea otters, considered one of the keystone species of the ecosystem, have been hunted to endangered status. Other predators, like the sunflower sea star, would have to pick up some of the slack. Unfortunately, a sea star wasting disease has decimated the population in the last decade, leaving the population critically endangered. 

This study examined just how effective the sunflower sea star is as a predator of sea urchins by raising well-fed and starving sea urchins in a lab setting. After about six weeks of collecting and raising urchins, the researchers let 24 sea stars free to feed. The sea stars consumed an average of 0.68 urchins a day, and when the urchins were starving, like they are in nutrient-poor urchin barrens, sea stars ate even more. That is a major difference between the sea stars and other predators, like otters, who are picky when it comes to choosing what urchins to eat, preferring healthy urchins that are less common in a barren. 

[Related: A virgin birth in Shedd Aquarium’s shark tank is baffling biologists.]

“Eating less than one urchin per day may not sound like a lot, but we think there used to be over 5 billion sunflower sea stars,” Sarah Gravem, a research associate at Oregon State said in a release. Although there’s no consensus on just how devastating sea star wasting disease has been, most estimates place the loss at around 90 percent of the population. “We used a model to show that the pre-disease densities of sea stars on the U.S. West Coast were usually more than enough to keep sea urchin numbers down and prevent barrens,” Gravem adds.

With this knowledge in mind, future research can focus on how exactly to use sunflower sea stars to keep sea urchin populations in check—and hopefully restore kelp forests in the process.