You’re walking along a tide pool in the Pacific Northwest. You see a bright purple starfish—technically a sea star—clinging to a rock. It looks totally peaceful. Static. Almost like a plastic toy someone dropped. But underneath that stiff, bumpy skin, there is a biological horror movie happening. Honestly, if you saw a human eat the way a sea star does, you’d never sleep again.
Ever wondered how do sea stars eat when they don't even have a visible face? They don't have teeth. They don't have a jaw. They don't even really have a head. Instead, they’ve evolved a method of consumption so metal it belongs in a sci-fi flick: they push their entire stomach out through their mouth to digest their prey alive, right there in the open water.
The Stomach That Leaves Its Body
Most animals put food into their stomachs. Sea stars do the opposite. They put their stomach into their food. This is called "stomach eversion," and it's basically their superpower.
When a sea star finds something it wants to eat—usually a mussel, a clam, or an oyster—it uses its hundreds of tiny tube feet to get a grip. These tube feet aren't just for walking; they are powerful hydraulic suckers. Once the sea star has a firm hold on a bivalve, it starts a slow-motion tug-of-war. The mussel tries to stay closed. The sea star pulls. It doesn't need to win by brute force all at once; it just needs a tiny gap. We’re talking less than a millimeter.
Once that tiny crack opens, the sea star goes for the kill. It relaxes its cardiac stomach—one of its two stomachs—and pushes it out of its mouth. The stomach slips through that paper-thin gap in the mussel's shell like a ghost passing through a wall.
Inside the shell, the stomach starts pumping out digestive enzymes. It literally turns the mussel’s body into a soup while the mussel is still technically "home." Once the prey is sufficiently liquefied, the sea star sloshes that nutritious "mussel smoothie" back into its body. It’s efficient. It’s gross. It’s brilliant.
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Not All Sea Stars Are Slow-Motion Snails
We tend to think of these creatures as slow. Lazy, almost. But some of them are active hunters that move faster than you’d think. Take the Sunflower Sea Star (Pycnopodia helianthoides). This thing is a beast. It can have up to 24 arms and can move at a "sprinting" pace of about three meters per minute. For a sea star, that’s basically breaking the sound barrier.
The Sunflower star doesn't just wait for a slow clam. It hunts sea urchins. If you’ve ever seen footage of a sea urchin trying to run away from a Sunflower star, it’s surprisingly high-stakes. The urchin uses its spines to try and fend off the predator, but the sea star just wraps those dozens of arms around it and starts the eversion process.
Why the extra stomach?
It’s a matter of plumbing. Sea stars have two main stomach compartments:
- The Cardiac Stomach, which is the one that takes a field trip outside the body.
- The Pyloric Stomach, which stays inside to finish the job.
By having two stages, the sea star can handle food that is much larger than its mouth. Think about that for a second. If you could eat like a sea star, you could just lean over a whole pizza, drop your stomach onto it, and walk away twenty minutes later fully fed without ever taking a bite.
What’s on the Menu? (Hint: Almost Everything)
Sea stars aren't picky. While mussels are the classic example, their diet is incredibly varied. Depending on the species and where they live, they might go after:
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- Barnacles: Hard to crack, but the sea star's enzymes don't care about shells.
- Snails: A slow-speed chase that usually ends poorly for the snail.
- Coral Polyps: The Crown-of-Thorns sea star is famous (or infamous) for eating through entire reefs.
- Each Other: Yes, some sea stars are cannibals. If food is scarce, a smaller sea star might just become lunch for a bigger one.
The Crown-of-Thorns (Acanthaster planci) is a particularly interesting case. It’s covered in venomous spines and can eat its way through massive chunks of living coral. In places like the Great Barrier Reef, outbreaks of these stars can devastate the ecosystem. Scientists are actually working on ways to cull them because they eat so much, so fast.
The Role of Water Pressure
You might wonder how they move those arms and push out a stomach without muscles like ours. It’s all hydraulics. Sea stars use a "water vascular system." They pull in seawater through a little porous plate on their top side called the madreporite.
This water gets pumped through a series of canals, which is what allows them to extend and retract their tube feet. It’s a very high-pressure system. It’s the same pressure that allows them to pry open a mussel that is trying to stay closed with its life on the line. They aren't using "muscle" in the way a human bodybuilder does; they are using liquid physics to exert constant, unrelenting force.
The Aftermath of the Meal
Once the cardiac stomach comes back inside, the sea star has to process the nutrients. The food moves into the pyloric stomach and then branches out into the arms through pyloric caeca. These are basically digestive glands that run down the length of each arm.
Wait. Why the arms?
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Because sea stars don’t have a centralized circulatory system with a big heart pumping blood everywhere. By sending the "food tubes" down each arm, they can distribute nutrients directly to where the movement is happening. It’s a decentralized way of living. If a sea star loses an arm, it doesn't just lose a limb; it loses a part of its digestive system. Luckily, they can usually grow them back, provided the central disk is still mostly intact.
Common Misconceptions About How They Eat
People often think sea stars have "teeth" because they see the hard bits near the mouth. Those aren't teeth. They are just protective ossicles. They can't chew. If a sea star can't liquefy it, it can't eat it.
Another big one: "Starfish eat through their feet."
Not really. They catch with their feet. They smell with their feet. (Technically, they have chemoreceptors on their tube feet that "taste" the water to find food). But the actual eating? That’s all stomach.
Why This Matters for the Ocean
Sea stars are what biologists call "keystone species." The term actually came from studies of sea stars (Pisaster ochraceus) by Robert Paine in the 1960s. He found that when he removed sea stars from a section of the shoreline, the mussel population exploded. The mussels took over everything, crowded out all the other species, and the biodiversity of the tide pool collapsed.
When you ask how do sea stars eat, you’re actually asking how the ocean stays balanced. By being the "terrors of the tide pool," they keep the aggressive species in check. Without the sea star’s weird, inside-out stomach, our coastlines would look completely different—likely just carpets of mussels as far as the eye can see.
Actionable Insights for Your Next Beach Trip
- Observe, Don't Touch: If you see a sea star that looks like it's "hugging" a rock or a mussel tightly, it's likely mid-meal. Prying it off won't just stress the animal; you might actually damage its stomach if it's currently everted.
- Check the "Mouth": If you find a sea star that has been washed up (and is still alive), you might see a jelly-like substance underneath it. That’s the stomach. Don't poke it. It's incredibly delicate.
- Look for the "Sign": Empty, clean mussel shells that are still attached to rocks often indicate a sea star has recently visited. If the shell isn't broken but is completely empty, you’ve found the remains of a "stomach smoothie" session.
- Support Reef Conservation: If you're traveling to tropical areas, look for groups managing Crown-of-Thorns populations. Keeping these "super-eaters" in check is vital for the survival of coral reefs in the 21st century.
Knowing how these creatures function changes how you see the beach. It’s not just a pretty landscape; it’s a battlefield where the winners are the ones who can turn their bodies inside out to get the job done.