Starfish, more accurately known as sea stars, are marine invertebrates found across all the world’s oceans, from tropical reefs to polar waters. They are part of a larger group called echinoderms, which also includes sea urchins and sand dollars. Sea stars play a significant role in marine ecosystems, often acting as predators that help regulate populations of other invertebrates.
The Eversible Stomach
The sea star’s digestive system can evert its stomach. Sea stars possess two stomachs: a cardiac stomach and a pyloric stomach. The cardiac stomach, located closer to the mouth on the underside of the central disc, is the larger and more flexible of the two, capable of extending outside the body.
Stomach eversion involves muscular contractions and hydraulic pressure from the sea star’s water vascular system. This system, characteristic of echinoderms, uses water pressure to power various functions, including stomach eversion and retraction. This unique adaptation enables sea stars to consume prey that would otherwise be too large to fit through their small oral opening.
Opening Up Clams and Oysters
When a sea star targets bivalve prey, it positions itself over the mollusk. It then uses its tube feet, located on the underside of its arms, to attach firmly to the bivalve’s shells. Each tube foot is equipped with a suction disk, allowing the sea star to create a strong grip on the shell surface.
The sea star then exerts a steady, continuous pulling force on the shells. This persistent effort gradually fatigues the bivalve’s adductor muscles, which are responsible for keeping the shells tightly closed. While this process can take hours, the sea star’s pull eventually causes the shells to gape open slightly, creating a minuscule gap. This slight opening is all the sea star needs to proceed with its feeding strategy.
Digestion Beyond the Body
Once the bivalve’s shells are slightly ajar, the sea star inserts its everted cardiac stomach into this small opening. The stomach then wraps around the soft tissues. The cardiac stomach secretes digestive enzymes onto the prey’s tissues.
These enzymes begin to break down the prey’s soft tissues externally, predigesting the meal into a liquid form. This external digestion allows the sea star to process food that it cannot ingest whole. After the prey’s tissues are broken down and the nutrients are absorbed through the stomach lining, the cardiac stomach is retracted back into the sea star’s body. The remaining undigested material is left behind. The partially digested food then moves to the pyloric stomach for further processing and nutrient absorption within the sea star’s body.