Sea stars move with tube feet powered by a water vascular system.

Rows of tube feet lie on the underside of the arms. Their repeated extension, attachment, and withdrawal allow a sea star to move while maintaining contact with the seafloor.

Fluid pressure inside the water vascular system helps extend the feet, while muscles contribute to their movement. The animal uses a coordinated combination of hydraulics and muscular action, not water pressure alone.

Sea stars belong to the echinoderms, alongside sea urchins and sea cucumbers. Their familiar name does not make them fish, and their locomotion reflects a very different body plan.

Looking down at a sea star can hide the machinery that moves it. The broad arms are conspicuous, but much of the action happens underneath, where numerous tube feet project from grooves along the arms. A sea star advances through the activity of these small appendages rather than by taking large steps with the arms themselves. The arms carry the feet and help position the body.

The feet connect to a system of internal fluid-filled canals. In the typical arrangement, a ring canal surrounds the mouth region and radial canals extend into the arms. Local muscular bulbs called ampullae help force fluid into the tube feet. As a foot extends and makes contact, its muscles and attachment mechanism help produce traction. Repeated activity across many feet moves the animal.

Calling this arrangement hydraulic describes how fluid transmits force, not an absence of muscles. The internal system is controlled by living tissues, and the animal must coordinate extension, attachment, pulling, and release. A useful comparison is a collection of small fluid-operated supports, but it is not a rigid mechanical machine. Feet can perform different tasks as the sea star encounters an uneven surface.

Attachment also deserves care. Tube feet are often described as little suction cups, yet adhesive secretions play an important role in gripping surfaces. Their ability to hold on should not be reduced to suction alone. Secure attachment helps sea stars resist being dislodged and supports feeding activities as well as travel. Some species use their feet to exert a sustained pull on the shells of bivalve prey.

NOAA points out that sea stars have no fins or gills and are not fish. Their closest familiar comparisons are other echinoderms, including sand dollars, sea urchins, and sea cucumbers. The water vascular system is a defining feature of that broader group, though the shape and use of its appendages vary. A sea cucumber does not need to resemble a star to share this underlying relationship.

Five arms are common, but they are not a universal rule. Some sea stars have many more, so counting five points is not a reliable definition of the group. Nor does every species occupy the same habitat or eat the same food. The shared lesson is the use of an echinoderm body plan with specialized appendages, not one exact outline, speed, or feeding strategy.

A sea star's arms are conspicuous, but watching the underside reveals how it travels. Small tube feet make and release contact with the surface in a coordinated sequence. The water vascular system helps operate them, linking internal fluid mechanics to external movement. The result is not a miniature fish swimming along the bottom but a very different solution to locomotion.