The Missing 24-Limbed Animals That Could Help Rescue the Ocean’s Forest

The kelp forests off the West Coast are dying, and with their decline, an entire ecosystem of marine plants and animals is at risk. A large starfish with an appetite for sea urchins could come to the rescue.

One reason for the disappearing kelp is the tremendous expansion of the sea urchin population that feeds on it — including an estimated 10,000 percent increase in their numbers over the past few years in a reef surveyed off the coast of Oregon. And it may be that sea urchins have multiplied because one of their chief predators, the sunflower sea star, has been nearly wiped out by disease. (Scientists prefer “sea star” to “starfish” because the animals are not fish.)

A team of scientists suggests that the population explosion in sea urchins could not have happened if sunflower sea stars had been there to prey on them, and that restoring the population of the colorful creatures may help in the recovery of the kelp forest and the ecosystem it supports. The study appeared last month in Proceedings of the Royal Society B.

Scientists estimate that there once were as many as five billion sunflower sea stars along the coast from Alaska to Baja California. They come in varying shades of purple, brown, orange and yellow, and can grow as large as three feet across, with up to two dozen arms. They move quickly, at least for a sea star — up to 200 feet in an hour. But sea star wasting disease, possibly caused by a virus, has killed most of them.

To test whether introducing captive-bred sunflower sea stars could help, the researchers collected 24 sunflower sea stars and 300 purple sea urchins near the San Juan Islands in Washington and observed them under experimental conditions, recording hunting activity and food preferences. These were healthy sea stars, survivors unaffected by the wasting disease, possibly because they were resistant to the illness. The researchers hope that their offspring will share their characteristics.

The scientists found that sea stars were passionate consumers of both juvenile and large adult sea urchins. When sea stars attack, urchins fight back, often pinching off pieces of the sea star’s arms and making the attacker back off. If the sea star can persist, it surrounds the urchin and ingests it through the mouth on its underside. After about 18 to 24 hours, it spits out the empty shell, having digested the soft parts, including the roe, which is also a delicacy among sea otters and human sushi eaters.

Is re-establishing the sunflower sea star population with captive-bred animals practical? Aaron W.E. Galloway, who is an associate professor of marine biology at the University of Oregon and an author of the paper, believes it could be.

“Just a few sunflower sea star individuals can produce millions of larvae,” he said. If they are successful, he says, even a small restoration effort “could easily lead to millions of sea stars returning to the wild.”

Dr. Galloway acknowledged that there are many other factors besides the diminishing sea star population that affect the health of the kelp forest, like climate change and increased periodic heat waves. And he makes no claim that a healthy sea star population is the ultimate solution.

“There are a lot of things you might try to do,” he said. “But the restoration of sea stars is one of the most efficient levers we can pull. If we can help sea stars recover naturally, it could have ecosystem-scale effects, and it works without human intervention after it gets started.”