Sea stars, commonly known as starfish, are marine invertebrates belonging to the class Asteroidea. With nearly 2,000 identified species inhabiting all the world’s oceans, from tropical reefs to the abyssal depths, these organisms play a varied role in their respective ecosystems. They are classified primarily as opportunistic predators or scavengers, and their diet is as diverse as their global distribution, ranging from large mollusks to microscopic organic matter.
The Unique Mechanics of Sea Star Feeding
A sea star’s ability to consume prey much larger than its mouth is due to its specialized digestive system, which features two distinct stomachs. The most remarkable part of this process involves the cardiac stomach, located at the center of the body. When feeding on shellfish, the sea star uses its rows of tiny, suction-cupped tube feet to exert a steady, immense force, slowly prying open the bivalve’s shell just a fraction of a millimeter.
Once a small gap is created, the sea star everts its cardiac stomach out through its mouth, inserting it directly into the shell opening. The stomach then envelops the soft tissues of the prey, such as an oyster or clam, and secretes powerful digestive enzymes to begin liquefying the meal externally. This partial, external digestion allows the sea star to process food that would otherwise be too large to swallow whole.
After the prey has been sufficiently broken down into a nutrient-rich fluid, the cardiac stomach is retracted back into the body. The partially digested matter is then passed into the smaller, non-eversible pyloric stomach, located above the cardiac stomach. From the pyloric stomach, ducts called pyloric caeca extend into each arm, where the final stages of digestion and nutrient absorption occur.
Common Prey and General Diet Categories
For the majority of species, the diet on slow-moving or sessile benthic invertebrates. Mollusks, particularly bivalves like clams, mussels, and oysters, form a large part of the diet for many sea stars in coastal and intertidal zones. They also routinely prey on other invertebrates, including marine worms, snails, and sponges.
Many sea stars are also detritivores, consuming decomposing organic matter, debris, and dead animals. Species in the genus \(Astropecten\), for instance, are known to be generalist molluscan feeders that will swallow smaller prey items whole, digesting the soft tissues and later spitting out the shells and other indigestible hard parts. Some primitive species, such as those in the genus \(Luidia\), lack the ability to evert their stomach, meaning they must consume their prey entirely before beginning digestion.
These generalist species are opportunistic and will take advantage of any available food source, including microalgae and small crustaceans. This broad, carnivorous, and scavenging diet ensures their survival across diverse marine habitats.
Specialized Feeders and Ecological Impact
While many sea stars are generalist feeders, some species exhibit highly specialized diets that have significant ecological consequences. The most well-known example of a specialized feeder is the Crown-of-Thorns starfish, which feeds almost exclusively on the soft polyps of hard coral. This sea star everts its stomach over a section of coral, dissolving the polyps and leaving behind a bleached, white coral skeleton in its wake.
Other sea stars are micro-feeders, relying on tiny particles rather than large prey. Species like \(Henricia\) and \(Echinaster\) are suspension feeders, using their specialized tube feet and ciliary action to collect phytoplankton, small organic particles, and bacterial films from the water column or from the surface of rocks.
Beyond their varied diets, certain predatory sea stars are recognized for their disproportionate influence on the structure of their marine communities. The ochre sea star is a notable example of a keystone species in the rocky intertidal zone of the Pacific Northwest. By preying on competitively dominant organisms, primarily mussels, it prevents them from monopolizing space and allows a greater diversity of other species, such as barnacles and algae, to coexist.