Hungry Sea Stars

An urchin’s sharp spines are enough to scare away most predators. But not sunflower sea stars. These giant sea stars can grow as big as a bicycle tire. They also have as many as 26 arms. All those arms make them speedy! Of the 2,000 known sea star species, the sunflower sea star gets around the fastest!

When a sunflower sea star catches an urchin, it grabs hold, swallows its victim whole, and digests it alive. “They are fearsome predators,” says marine biologist Aaron Galloway from the University of Oregon.

Galloway wanted to find out whether sunflower sea stars could control the population of sea urchins. So he asked a question: How many urchins can sunflower sea stars eat in a week?

The first step the researchers took was to scuba dive and collect sunflower sea stars to study. That wasn't easy, because there aren’t many sunflower sea stars left in the ocean. They managed to find 24 of them. The scientists also caught many sea urchins.

Back at their lab, they put a sea star and its urchin prey in a tank together. Then they watched. Every time a sea star swallowed an urchin, the researchers tallied the kill.

The scientists found that each sea star ate about five urchins per week. “That doesn’t sound like a lot,” says Galloway, “until you consider that there used to be more than 6 billion sea stars!” Today there are less than 6 million of the animals. Scientists’ aren’t sure why so many sunflower sea stars have disappeared. They believe warming oceans are a major factor causing them to die off.   

The team created a computer model to test the effect 3 billion sea stars would have on today’s urchin population. That many sea stars, they found, could keep urchin numbers under control.

Galloway and his team will now find out if sunflower sea stars can eat as many urchins in the wild as in the scientists' lab. If they can, the researchers may try raising sea stars in their lab. Then they could release them into the wild as Pacific purple sea urchin hunters. That could help bring back kelp forests.