Sea stars clustering on the chalky landscape. The animal in the foreground shows strength and flexibility in the arms. Sea stars mostly move on hundreds of small tube feet located on the underside of the arms. The feet are operated by a hydraulic system, and a sea star can use them to get a very strong hold on rock, or prey such as clams or mussels.

Sea stars are related to sea urchins, sand dollars, sea cucumbers and crinoids. All of these groups of animals are radially symmetrical. These sea stars each have five arms radiating from a central axis.