Why Does a Starfish Turn Its Stomach Inside Out?

A starfish turns its stomach inside out in order to consume food that is too large to fit through its small mouth opening. By wrapping its stomach around an object or inserting it into a shell, a starfish externally digests its food, producing a slurry that it pulls into its body as it withdraws its stomach. The food then passes into its pyloric stomach, which always remains inside its body.

Starfish possess two stomachs: the cardiac stomach and the pyloric stomach. The cardiac stomach is expelled in order to digest large food items. The digestive process finishes in the pyloric stomach.

Starfish have ravenous appetites, their size and maturity dependent not on their age but on their diet. A starfish feeds on mussels, clams, oysters and other bivalves, fastening a few suckers to the shell of its prey and then exerting a long, slow, continuous pull to force open the shell. Large starfish have even been witnessed eating small fish. Some starfish supplement their diets with coral and with algae or food particles in the water.

The starfish's unique ability to retract its stomach once ejected has been linked to a particular neuropeptide. Neuropeptides are molecules that the neurons of the brain use to communicate with each other. Interestingly, this particular neuropeptide is evolutionarily linked to the human neuropeptide that is responsible for regulating arousal and anxiety.