Most starfish, also known as sea stars, eat by prying open the shells of prey such as clams or oysters with their arms, pushing their stomachs out their mouths and into the prey's shell, partially digesting the animal and then pulling their stomachs back into their mouths. Starfish that don't have suction disks swallow prey whole and afterwards eject undigested parts.
The unusual method of digesting prey outside their bodies allows starfish to eat animals considerably larger than their tiny mouths. Besides oysters and clams, they feed on mussels, arthropods and fish. Some species supplement their diets with algae and other organic matter.
The digestive system of a starfish occupies not only the disc but also part of the arms. The cardiac stomach that engulfs its prey releases enzymes to aid digestion. Once the partly digested prey is retracted into the disk, the food is passed on to the pyloric stomach. This has extensions into the arms lined with glands that absorb nutrients from the food. A short intestine, rectum and anus are at the top of the disk. Starfish lack distinct sense organs and centralized brains, but they have complex nervous systems and respond to light, temperature, touch and water conditions. Unable to plan their actions, if starfish detect prey, they instinctively move toward it.
The majority of starfish are generalist predators, feeding mainly on bivalves such as clams and mussels. In addition to bivalves, some starfish also eat barnacles, snails, microalgae, sponges and other small sea creatures. A few species of starfish subsist on decomposing fecal matter and other organic materials. When prying its prey's shell open, the starfish is able to push its stomach through a space the size of just a fraction of a millimeter.