You've probably heard it before, maybe in a documentary or at an aquarium: "Sea stars have three stomachs!" It's one of those cool, weird animal facts that gets passed around. But here's the thing I've learned after years of poking around tide pools and reading far too many scientific papers—it's not exactly true. The reality is more nuanced, and honestly, way more interesting. So, do sea stars have three stomachs? The short, direct answer is no. They have two. But understanding why the myth persists and how those two stomachs pull off one of the most bizarre feeding routines in the ocean is where the real story is. Let's clear up the confusion.

Two Stomachs, Not Three: The Core Misconception

Let's get the anatomy straight right from the start. A typical sea star has a digestive system centered in its central disc, and it features two primary, sequential stomachs:

1. The Cardiac Stomach: This is the showstopper. It's a large, sack-like stomach that can be turned inside out (everted) through the mouth. It's used for external digestion.
2. The Pyloric Stomach: This sits below the cardiac stomach. It's a more internal, second-stage stomach that receives the soupy, partially digested food from the cardiac stomach once it's retracted.

That's it. Two. So where did the "three stomachs" idea come from? It almost certainly stems from a misunderstanding of the pyloric ceca (also called digestive glands). These are pairs of long, branching glands that run down the length of each arm. They connect to the pyloric stomach, and their job is absorption and storage of nutrients—not active digestion in the way a stomach does. Calling them a stomach is like calling your liver a second heart because it processes blood. It's confusing function with form.

The Cardiac Stomach in Action: Eating Outside the Body

This is the part that blows people's minds. The cardiac stomach isn't just an internal bag. It's a deployable digestive organ. When a sea star finds prey—let's use the classic example of a mussel—it doesn't have the strength to pry the shell open with its arms alone.

Here's what happens instead, a sequence I've timed watching Ochre stars in the Pacific Northwest:

The sea star arches over the mussel and attaches its tube feet to both shells. It applies steady, relentless pull. After hours, the mussel's adductor muscle tires, and the shell gaps open just a millimeter or two. That's all the opening needed.

The sea star then extrudes its cardiac stomach out through its mouth. It's not vomiting. It's a controlled eversion using fluid pressure from the water vascular system. The stomach slides into the narrow gap in the shell. Once inside, it envelops the mussel's soft body. The stomach lining then secretes powerful digestive enzymes, which begin to liquefy the mussel's tissues right there in its own shell.

Think about that. The sea star is digesting its meal outside its own body, inside the prey's home. It's a completely alien strategy to us.

The Pyloric Stomach's Role: Finishing the Job

After several hours of external digestion, the cardiac stomach, now filled with a nutrient-rich soup, is retracted back into the body. This slurry isn't ready to be distributed just yet. It passes into the pyloric stomach.

The pyloric stomach acts as a central hub. Its primary role is to further process this liquid meal and distribute it to the pair of pyloric ceca in each of the sea star's arms. These ceca are where the bulk of nutrient absorption happens. They also serve as storage sites, allowing the sea star to survive lean periods—something I suspect is crucial for species in temperate zones with seasonal food scarcity.

The waste material, what little solid matter might remain, is eventually compacted and expelled back out through the mouth. Yes, the mouth serves as both entrance and exit.

The Digestive Glands: The Source of the "Third Stomach" Myth

Let's demystify the pyloric ceca, the likely culprits behind the "three stomachs" myth. If you dissect a sea star arm (or look at a detailed diagram from a source like the Smithsonian Ocean Portal), you'll see these large, branched, often dark-colored glands. They are impressive and take up a lot of space.

Their functions are critical but distinct from a stomach:

• Nutrient Absorption: They have a huge surface area to absorb sugars, amino acids, and fats from the digested soup.
• Nutrient Storage: They store lipids and glycogen, acting like internal pantries for each arm.
• Gamete Production: In many species, these glands also produce sperm or eggs during spawning season.

Calling them a stomach is a misnomer because they don't initiate digestion with enzymes. They don't evert. They are a post-processing plant, not the initial factory. This distinction matters because it changes how you understand the sea star's body plan. It's not a creature with three identical digestive chambers; it's a creature with a two-stage, inside-out stomach system feeding a distributed network of吸收 organs.

A Step-by-Step Look at the Feeding Process

To tie it all together, here’s the complete sequence from prey detection to waste expulsion. This table breaks down what each part of the system is doing at each phase.

Notice how the "three stomachs" never appear. It's a seamless, two-phase stomach process (cardiac -> pyloric) supported by the absorbing glands.

Common Mistakes & Expert Insights

After talking with other marine life enthusiasts and guides, I see a few patterns in how people get tripped up.

The "Instantaneous" Misconception: Videos often speed up the process. In reality, a sea star eating a mussel is an all-day (or all-night) affair. The slow, relentless pressure is key. If you try to force the shell open, you'd fail. The sea star wins through patience and hydraulics.

The Vulnerability of the System: This is a point rarely mentioned. That everted cardiac stomach is incredibly delicate. If a fish nips at it or a sudden wave dislodges the sea star mid-meal, it can be a disaster. They can retract it quickly, but injury is possible. More critically, the entire enzymatic process is finely tuned to specific water temperatures and chemistry. This is why sea stars are such important indicator species—their complex feeding is easily disrupted by pollution or ocean acidification.

Species Variations: Not all sea stars use this method for every meal. Some smaller species, or those that feed on detritus or surface films, may ingest small particles directly. Others that eat coral polyps (like the infamous Crown-of-Thorns) use a similar eversion strategy but on a different scale. The two-stomach framework, however, is consistent across species.

For authoritative details on these variations, institutions like the NOAA Fisheries provide resources on specific sea star species and their roles in the ecosystem.

So, the next time someone shares the "three stomachs" fact, you can kindly correct them. Sea stars have two—a deployable external stomach and an internal processing hub—working in concert with a network of吸收 organs. This setup isn't just a weird trivia answer; it's an elegant, if bizarre, solution to the problem of eating armored prey without jaws or claws. Understanding it correctly gives you a deeper appreciation for the hidden complexities of even the most familiar tide pool dwellers.