The sea star (starfish) is a marine invertebrate belonging to the phylum Echinodermata, characterized by its spiny skin and radial symmetry. They inhabit diverse environments across all the world’s oceans, from shallow intertidal zones to the deepest abyssal plains. Their feeding methods are highly varied, leading to common misunderstandings. Most sea stars employ active and complex feeding strategies rather than simply straining the water column.
Clarifying the Filter Feeder Question
Sea stars are not generally classified as filter feeders, which are animals that siphon water through specialized structures to trap suspended particles like plankton. Suspension feeding is common among other echinoderms, such as crinoids, which use feathery arms to passively catch food drifting in the current. Most of the nearly 1,900 sea star species are active predators or scavengers. Rare exceptions exist, particularly in the deep sea. Some species, such as those in the order Brisingida, are specialized suspension feeders, using numerous long, flexible arms held up in the water column to passively capture zooplankton and organic matter.
The Unique Mechanism of Stomach Eversion
The most widely recognized feeding method involves active predation using stomach eversion. This process is frequently employed when a sea star encounters hard-shelled prey like bivalves (clams and oysters). The sea star uses its tube feet to secure itself over the bivalve and exerts a steady, powerful pull on the shells, often applying up to twelve pounds of pressure. This persistent force tires the bivalve’s adductor muscle, creating a tiny gap between the shells.
Once the opening is achieved, the sea star pushes its large, thin-walled cardiac stomach out of its mouth and into the prey’s shell cavity. This external digestion allows the sea star to consume prey much larger than its small central mouth. Digestive enzymes are secreted directly onto the soft tissues, liquefying the meat into a nutrient-rich “soup.” The semi-digested material is absorbed by the stomach tissue before the organ is retracted back into the body, leaving the empty shell behind. The process is regulated by specific neuropeptides, such as SALMFamides (triggering relaxation and eversion) and NGFFYamide (stimulating contraction and retraction).
Diverse Diets Beyond Active Predation
While stomach eversion is common, sea stars exhibit a wide range of other feeding behaviors, demonstrating their opportunistic nature. Many sea stars are effective scavengers, consuming dead or decaying organic matter, which helps recycle nutrients within the marine ecosystem. For certain species, scavenging detritus and carrion is a primary feeding strategy.
Other species are deposit feeders, ingesting the substrate (such as sand or mud) to extract organic detritus. They process large quantities of sediment, absorbing the thin layer of organic coating and microscopic organisms present. Some sea stars also graze on less mobile food sources, using their everted stomach to digest bacterial films, sponges, or algae directly on the substratum. This dietary flexibility ensures their survival across diverse habitats, allowing them to shift strategies based on food availability.
The Role of Tube Feet in Locomotion and Capture
The sea star’s ability to locate and capture its varied diet depends on its unique water vascular system and associated tube feet. The water vascular system is a hydraulic network that draws seawater in through the madreporite, using fluid pressure to operate the numerous tube feet lining the ambulacral grooves on the underside of each arm. These tube feet extend and contract through hydraulic action, serving multiple functions, including locomotion.
Critically for feeding, the tube feet generate a powerful, temporary chemical adhesion, allowing the sea star to firmly grip surfaces and prey. This grip is essential for predation, providing the necessary leverage and suction to pry open bivalve shells. The tube feet also help manipulate smaller food particles, detritus, or sediment, guiding them toward the central mouth.