How Do Sea Stars Eat? Pushing Their Stomach Out, and Other Ways

1. How Sea Stars Open Closed Clams to Eat Them

One common food for sea stars is bivalves (clams with two shells). Encyclopedias state that most sea stars are carnivores that prefer slow-moving animals, especially bivalves [2]. If a clam closes its shell tight, it seems safe. However, sea stars have many small feet called tube feet on the underside of their arms, which move using water pressure. They stick to the prey and pull on both sides of the bivalve’s shell to open it little by little [3].

You do not need to pry the shell wide open. According to MarLIN, a marine life information network, for common starfish in Europe, a gap of just 1 millimeter is enough to insert stomach folds into the clam [3]. This is about the width of one mark on a ruler. However, the size of the gap needed varies by species, so this is just one example. What they insert is not feet or claws, but their "stomach." But what does it mean to push the stomach out?

2. External Digestion: Pushing the Stomach Out of the Body (Not All Sea Stars Do This)

The most strange part of sea star eating is how they use their stomach. Encyclopedias say that many sea stars perform "external digestion" by turning their stomach inside out, pushing it out through the mouth, and digesting prey outside their body [2]. They wrap the prey with the exposed stomach, dissolve it, and absorb nutrients. The Shirahama Aquarium at Kyoto University notes that some sea stars, like the red sea star, do this, and large prey may take a few days to a week to digest [1]. They can also insert just their stomach through the gap in a shell to eat only the inside of the clam [1]. This means they do not have to swallow the whole shell.

It is important to note that "not all sea stars push out their stomachs." The same source states that many sea stars simply eat with their mouths [1]. Pushing out the stomach is just one type of eating method, not a rule for all. So, what else do they eat besides clams?

3. Other Foods Sea Stars Eat Besides Clams

Sea star food is not limited to clams. According to Shirahama Aquarium materials, many sea stars are carnivores that eat shrimp, crabs, and fish carcasses, while others eat organic matter found in mud [1]. This means some sea stars do not just hunt living prey but also consume dead bodies on the sea floor. While this suggests a cleanup role, the source describes food items rather than explicitly defining an ecological role.

The number of species is not fixed. Encyclopedias range from over 1,200 to about 2,000 species worldwide, depending on the reference [2]. In waters near Japan, it is said there are about 200 to 300 species. With so many species, eating habits vary, which makes it hard to say "sea stars eat like this" in one sentence. Some species that push out their stomachs affect human life significantly. One example is the crown-of-thorns starfish.

4. How Crown-of-Thorns Starfish Eat Coral with Their Stomachs

The largest scale of sea star feeding may be seen in the crown-of-thorns starfish, which eats coral. The Ministry of the Environment explains that these starfish have a diameter of 30 to 50 cm and 14 to 18 arms [4]. They push their stomach out to wrap around the coral for digestion and absorption. If you turn over a feeding individual, you can see the exposed stomach [4].

They eat the living parts (polyps) of stony corals. One estimate suggests a single starfish can eat up to about 6 square meters of coral per year [5]. This is like 3 to 4 tatami mats. However, this is a "maximum" estimate. Field measurements show large individuals eat hundreds of square centimeters per day in summer (about 2 to 3 postcards) and about 60 square centimeters per day in winter [5]. The amount changes greatly by season.

If their numbers increase, the impact grows. Okinawa Prefecture explains that past outbreaks of crown-of-thorns starfish caused major damage to coral reefs, and removal efforts continue today [6]. Since the end of 2001, large outbreaks have occurred again around Okinawa Island and the Kerama Islands [6]. If there are too many eaters, the eaten ones decrease. What happens if the eaters disappear?

5. What Happened to Rocky Shore Life When Sea Stars Were Removed

In the 1960s, Paine conducted an experiment on rocky shores in Washington State, USA [7]. He removed sea stars (Pisaster) from some areas and left neighboring areas as controls. In the places without sea stars, barnacles first covered the rocks and pushed out many other species. The account says mussels later spread and displaced barnacles. It is reported that the number of species in those areas dropped [7]. Specific numbers are not used here because they rely on secondary sources.

Sea stars ate clams and barnacles. Because the predators were present, many species could live on the rocky shore. Organisms that strongly influence the community of an ecosystem were later called "keystone species." This was one experiment at one coast and may not apply to all sea stars everywhere. Still, in this experiment, barnacles and mussels spread where sea stars were gone, and other species declined.

6. How to Watch Sea Star Feeding Yourself

In aquariums, if you watch a sea star walking on the tank wall, you might see its underside (where the tube feet are). Take time to observe how the tube feet move like waves. If you are there during feeding time, you might see what and how they eat.

If you find a sea star at the seaside tide pools, do not turn it over. Just observe it gently, because doing so causes burden or danger [1]. If you want to learn more, materials like "Inside Sea Stars" from Kyoto University Shirahama Aquarium or explanations on crown-of-thorns starfish from the Ministry of the Environment and Okinawa Prefecture are available in Japanese [1][4][6]. Looking for sea star food and tube feet information in books at home is also fun.