Sea stars digest their meals outside their bodies by inverting their stomachs
When hunting clams and mussels, a sea star does not bother swallowing its prey whole. Instead, it uses hundreds of suction-tipped tube feet to pry open the mollusk's shell by less than a millimeter. The sea star then pushes its cardiac stomach out through its mouth, sliding the organ inside the shell. Powerful digestive enzymes liquefy the prey alive inside its own armor, allowing the sea star to slurp the nutrient-rich soup back into its body.
An Anatomy Split Between Two Stomachs
Sea stars possess a body architecture fundamentally distinct from most other active marine predators. Built around radial symmetry, typically arranged in five radiating rays or multiples thereof, their bodies lack a centralized brain, a traditional head, or any form of jawed mouthparts. The mouth sits directly in the center of the underside, termed the oral surface, while the opposite side, facing upward into the water column, is known as the aboral surface. Because a sea star cannot bite, chew, or shear its food, its internal digestive tract is organized around an unconventional physical movement: the ability to relocate its own digestive tissue outside its skeletal frame.
The sea star digestive system is divided into two distinct compartments situated within the central disc. The lower compartment, connected directly to the mouth opening, is the cardiac stomach. Highly flexible, pleated, and muscular, the cardiac stomach is the organ responsible for venturing into the outside world. Directly above it sits the smaller pyloric stomach, which connects to pairs of elongated digestive glands, known as pyloric caeca, that extend down the length of each arm. This anatomical separation allows the sea star to initiate breakdown in the open environment while reserving the secondary stages of absorption, enzyme synthesis, and energy storage for the interior cavities of its limbs.
This structural arrangement solves an essential biomechanical problem. Most armored marine invertebrates, such as mussels, clams, and oysters, are shielded by hardened calcium carbonate shells that resist external crushing by all but the strongest predators. By having an expandable stomach located adjacent to a wide oral opening, the sea star does not need to crack, break, or ingest the entire armored shell of its quarry. Instead, it turns the interior of the prey's shell into an extension of its own gut.