Starfish, correctly known as sea stars, are marine invertebrates belonging to the phylum Echinodermata, which also includes sea urchins and sea cucumbers. These animals inhabit all of the world’s oceans, from tropical reefs to frigid deep-sea environments. The sea star’s internal anatomy, particularly its digestive system, is highly specialized to accommodate its predatory lifestyle, featuring an unusual arrangement for feeding and processing nutrients.
The Dual Digestive Organs
A sea star possesses two distinct stomach structures—the cardiac stomach and the pyloric stomach—which work in sequence to process food. This dual-stomach system is centrally located within the animal’s body disc.
The cardiac stomach is a large, sac-like organ positioned closest to the mouth on the lower, or oral, surface. This stomach is notable for its ability to be pushed out of the body during feeding.
Resting directly above it is the smaller, flatter pyloric stomach, which remains inside the body at all times. The pyloric stomach serves as a junction, connecting to the central body and branching out into the arms, allowing for efficient nutrient distribution.
Eversion and External Feeding Mechanics
The cardiac stomach’s capacity for eversion—turning inside out through the mouth—is a key adaptation. Sea stars use this mechanism to consume prey much larger than their small central mouth opening, such as clams, oysters, and mussels.
To begin feeding, the sea star uses the powerful suction of its tube feet to pry open the shells of its prey. Only a gap of a fraction of a millimeter is required between the shells. Through this tiny opening, the cardiac stomach is extended into the prey’s shell cavity.
Once the soft tissues are enveloped, the stomach releases powerful digestive enzymes directly onto the food source. This allows the sea star to pre-digest its meal externally, dissolving the prey’s flesh into a nutrient-rich liquid. External digestion means the sea star is not limited by the size of its mouth.
The neuropeptide SALMFamide triggers the necessary relaxation and eversion of the cardiac stomach. After the enzymes have broken down the prey, the muscular cardiac stomach contracts, pulling the partially digested food and the stomach itself back inside the body.
Internal Processing and Nutrient Absorption
Once the liquefied meal is retracted, it passes from the cardiac stomach into the smaller pyloric stomach. This upper stomach acts as a central processing station for the semi-digested material. From here, the food is routed into specialized extensions that run the length of each arm.
These extensions are known as the pyloric cecae, or digestive glands; there are two in each arm of the sea star. The pyloric cecae are the primary sites for the final stages of digestion and nutrient absorption. They are lined with cells that secrete additional enzymes and absorb broken-down sugars, proteins, and fats.
The pyloric cecae also function as nutrient storage areas, similar to the liver in other animals, holding reserve food during periods of scarcity. A short intestine and a small anus on the upper (aboral) surface allow for the expulsion of minimal undigested waste. The extensive network ensures that nutrients are distributed efficiently, supporting the animal’s metabolism and regenerative abilities.