Starfish, also known as sea stars, are marine invertebrates belonging to the class Asteroidea, found in all the world’s oceans, from tropical zones to polar regions. These organisms are typically recognized by their central disc and multiple arms and are a familiar sight in various coastal and deep-sea habitats. A common assumption is that these creatures graze on kelp or other large seaweeds, especially since they often inhabit the same marine environments. This idea misrepresents their feeding habits, which are far more predatory and specialized.
Starfish are Not Herbivores
The idea that starfish consume kelp or large marine plants is a widespread misconception that overlooks their classification as primarily carnivorous or omnivorous organisms. Starfish are benthic, meaning they live on the ocean floor, and they are characterized as opportunistic feeders that prey mostly on slow-moving or stationary invertebrates. Their presence in kelp forests or near large algal beds often leads people to incorrectly assume they feed directly on the vegetation.
While some juvenile starfish or specific species may consume microalgae or plankton, the vast majority of adult starfish do not feed on kelp, seaweed, or seagrass. Their body structure and unique digestive system are not adapted for processing the tough, complex carbohydrates found in large marine plants. Instead, they focus on locating and consuming other animal life, positioning them as significant predators within their ecosystems. This predatory role defines their ecological impact, sometimes even protecting kelp forests by eating kelp-grazing sea urchins.
The Actual Menu: Primary Prey and Scavenging
The diet of a starfish is highly diverse, centered on invertebrates with an emphasis on shelled organisms. Mollusks are a primary food source, with bivalves such as mussels, clams, and oysters being common targets for many species. Starfish locate these prey items using chemical senses and then employ their specialized tube feet to initiate the feeding process.
They also consume other marine life, including snails, small crustaceans like barnacles and crabs, and various types of worms. Different species exhibit variations in their diets; for instance, the large sunflower sea star is a predator of sea urchins, which helps maintain the health of kelp forests by controlling the urchin population. Some starfish act as detritivores, consuming organic waste and decomposing material, making them important scavengers.
A notable exception is the Crown-of-Thorns starfish, a species found in the Indo-Pacific region. This predator feeds on coral polyps, making it a significant factor in the ecology of coral reefs. This specialized diet highlights the adaptability of starfish, which are generally predators of benthic invertebrates but can target a wide range of available food sources.
The Science of Starfish Digestion
The ability of starfish to consume shelled prey stems from a remarkable biological process known as stomach eversion. The starfish has two stomachs: a smaller pyloric stomach and a larger, eversible cardial stomach. When a starfish finds prey, it uses the thousands of tiny, suction-cupped tube feet on its underside to hold the organism firmly.
If the prey is a bivalve, the starfish uses its tube feet to exert a steady, prolonged pulling force on the shell, overcoming the bivalve’s powerful adductor muscles. The required gap between the shell valves needs to be only a fraction of a millimeter wide for the next step to occur. The starfish then pushes its cardial stomach out of its mouth and into the opening or over the soft tissue of its prey.
Once the stomach is everted, it releases potent digestive enzymes directly onto the prey, initiating external digestion. These enzymes break down the soft tissues of the prey into a liquid, soup-like substance that the stomach then absorbs. This method of eating allows the starfish to consume prey much larger than its small mouth opening, such as a whole clam.
After the meal is complete, a neuropeptide called NGFFYamide triggers the contraction and retraction of the stomach back into the starfish’s body. This process leaves the hard, indigestible parts of the prey behind.