How Do Starfish Move About?
A starfish (or sea star) creeps along the bottom of the sea in a rather peculiar way. It has hundreds of tiny tube-like “feet” on the underside of its many arms. To take a step, the starfish lets water into its tube feet. The water pressure makes the tube feet stretch out, pushing the starfish forward. This water is taken in through a tiny spot on the upper part of its body, called the madreporite.
To move, each tube foot swings like a leg, lifting up and swinging forward, then planting itself on the ground and pushing back. At the tip of each tube foot (in most species) is a suction cup. These aren’t used when walking on level ground, but can be used when walking up sheer surfaces.
Suckers on the ends of the tube feet hold onto hard surfaces on the sea bottom. Most starfish have five arms, although there are starfish with as many as fifty arms. If a starfish loses an arm, it is able to grow another arm to take its place.
But these tube feet are capable of more than just movement. They also play a role in respiration and in collecting food. One of the sea stars favorite foods to dine on is bivalves–dual-shelled mollusks with strong muscles that keep their shells shut. That doesn’t stop sea stars, however. They can grasp a mollusk and pull the shells apart slightly using their strength and the suction of the tubed feet.
All sea stars need is to get the shells open just enough to spit their stomachs out through their mouth and right into the mollusk! Digestive juices can turn the mollusk’s body to liquid inside the shells, making for a tasty snack. Cilia lining the arms of the sea star or the star fish guide the liquidy mollusk into its mouth, which is located on the underside of its body.