Many starfish have evolved a remarkable method of feeding: they extrude their stomachs out of their mouths and begin the process of digesting their prey outside the body. Some species are able to open the tightly clamped shells of doubled-valved molluscs, such as mussels, oysters and scallops.

After locating its prey by sensing chemicals that the mollusc releases into the water, the starfish wraps its arms around the tightly closed shell. The arms are equipped with rows of little tube feet, sticking out through the starfish's armour plating. Each tube foot bears a tiny, but powerful sucker at its tip. When the starfish has a secure grip on its victim, it anchors the ends of its arms firmly to the sea bed. It then retracts its tube feet, a manoeuvre that exerts sufficient force for the shell to gradually open. This forced entry may take some time to succeed, as the shell muscles of molluscs are very strong.

Then the starfish extrudes its stomach into the body cavity of the mollusc and digests its victim with enzymes released from its intestine. Once the food is partly digested, thin hairs on the surface of the stomach create a current that carries the food into the starfish's body, where digestion is completed.

Many common starfish are orange in colour, but some are brown, red, or even purple. Colour variation is frequent among these creatures. Most starfish live low on the shore or in deep water. Those washed up by heavy seas and stranded out of the water may not survive until the tide returns. Starfish can grow new arms as well. If an arm is crushed by a wave swept boulder or mauled by a predator, it can be cast off and a new one grows in its place. In fact, provided most of the central disc is intact, one remaining arm can grow four new ones.