Starfish, more accurately called sea stars, are predators that engage in highly specialized hunting and feeding behaviors. These invertebrates belong to the class Asteroidea within the phylum Echinodermata. While their slow movement might suggest a passive lifestyle, the majority of species actively hunt, relying on unique biological features to capture and consume prey. Their methods allow them to exploit food sources protected by tough shells or hard exteriors.
The Starfish Diet: What They Consume
The diet of a starfish is concentrated on organisms that are slow-moving or permanently attached to a substrate, given the predator’s own lack of speed. Preferred food sources are typically bivalve mollusks, such as clams, oysters, and mussels. These sessile organisms provide a rich, stationary meal.
Starfish also prey on other invertebrates, including marine worms, small crustaceans, and sea snails. Some species also act as scavengers or detritivores, consuming settled organic material. However, their ability to access the soft tissues inside a tightly sealed shell makes them formidable carnivores.
The Unique Eating Strategy
The method a starfish employs to consume its prey is one of the most unusual feeding mechanisms in the animal kingdom, relying on the tube feet and the eversible stomach. When a starfish locates a bivalve, it maneuvers its body to straddle the shell, positioning its central mouth over the seam. The hundreds of tiny, suction-cupped tube feet lining the underside of its arms are used to grip both sides of the shell.
The starfish uses its water vascular system to generate a steady, immense pulling pressure on the bivalve’s shell. This sustained force, which can reach up to 12 pounds, eventually causes the bivalve’s adductor muscles to fatigue, forcing the shell open by just a fraction of a millimeter.
This tiny gap is enough for the starfish to execute stomach eversion. The sea star pushes its stomach out through its mouth, extending the organ into the narrow opening of the prey’s shell. Once inside, the stomach secretes powerful digestive enzymes directly onto the soft body tissues.
This process of external digestion breaks down the prey into a liquefied substance while it is still inside its own shell. The partially digested meal is then drawn back into the starfish as the stomach retracts, where final nutrient absorption occurs in the digestive glands. The entire eversion and retraction process is regulated by specific neuropeptides, such as SALMFamides and NGFFYamide.
Starfish as Keystone Species
The predatory habits of certain starfish species have profound consequences for the structure of their environment, leading to their classification as keystone species. A keystone species is an organism whose effect on its ecosystem is disproportionately large compared to its abundance.
The Ochre Sea Star (Pisaster ochraceus) in the rocky intertidal zones of the Pacific Northwest is the most famous example of this ecological role. This species feeds heavily on mussels, which are dominant competitors for space. Without the sea star’s predation, mussels would quickly monopolize the rocks, crowding out other organisms and reducing the total number of species. This predatory action maintains the high biodiversity of the ecosystem.