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NGSS: Core Idea: LS2.A

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Hungry Sea Stars

How this animal's appetite for urchins could save underwater forests.

September 2023
By Stephanie Warren Drimmer

Shutterstock.com (Background); Brent Durand/Getty Images (Sunflower Sea Star); Justin Meyers 

ARM FULL:

Sunflower sea stars begin life with only five arms. They can grow as many as 26 as adults.

Visit Orford Reef off the coast of Oregon and you’ll find a seafloor covered in a carpet of purple spines. As far as the eye can see, millions of Pacific purple sea urchins scuttle around, munching on everything in their path. There are so many urchins that little else can survive. But it wasn’t always this way.

The reef was once a thriving kelp forest. These groups of large seaweeds are home to many creatures. Animals and plants live there for shelter and to find food.

Today kelp forests are disappearing around the world. One reason is that the number of urchins has grown out of control. A team of scientists think they may have found a way to help kelp. They want to unleash sunflower sea stars to hunt down urchins.

STAR FIGHTER

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?

Douglas Klug/Getty Images (Kelp)

UNDERWATER COMMUNITY:

A healthy kelp forest can contain thousands of plant and animal species.

Tank Test

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.

KELP HELPERS?

Here’s how raising sunflower sea stars and releasing them into the wild could help kelp forests.

Illustration by Jim Starr

WITHOUT SUNFLOWER SEA STARS
Urchin numbers are out of control. The urchins eat all the kelp, destroying the ecosystem.

Illustration by Jim Starr

WITH SUNFLOWER SEA STARS
Sunflower sea stars eat urchins. That allows kelp forests and wildlife to flourish.

YUMMY RESULTS

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.

STAR SOLUTION?

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.

Hungry Sea Stars
Students will watch a video from a lab experiment showing sea stars hunting sea urchin.

Justin Meyers (Aaron Galloway)

Aaron Galloway

Scientists conduct experiments to find answers to questions about phenomena they observe. Think about the tests Galloway and his team created to learn about sea stars and urchins. Answer these questions.

1. What problem did Galloway try to solve?

2. How did the team figure out how many urchins the sea stars could eat in a week?

3. How did Galloway test whether sea stars could help control urchin populations?

4. What are the next steps for the team’s research?

Life Science
Animal Anatomy & Physiology, Habitats & Ecosystems, Adaptations, Animal Behavior

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