A Starfish Walking on the Beach

Sea stars, commonly known as starfish, are star-shaped organisms usually found in sea water. Starfish are found in oceans all over the world ranging from the warm, tropical regions to the frigid, polar regions. These species are found at regions 6,000 m (20,000 ft) below the sea surface. Approximately 1,900 species of starfish are known.

As brightly-colored organisms, starfish may be red, orange, brown, blue, or even grey in color. Starfish have a central disc and five arms which extend from the center. However, more than five arms may also be found in a few species of starfish. Starfish also have the ability to regenerate damaged parts or lost arms.

The upper surface of starfish is covered by overlapping plates and may be smooth, granular, or spiny. The lower surface of starfish, also called the oral surface, consists of the mouth which is located at the center of this surface. Starfish are known to feed on tiny organisms found at the bottom of the sea.

The lower surface also contains special appendages called tube feet which work for movement, feeding, and breathing. Tube feet are found in the grooves along the arms of starfish and function through the water vascular system. The water vascular system is a hydraulic system or a system which works on the pressure of the fluid. This system is made of canals joining the many tube feet together.

Water enters the system through an opening on the upper surface of starfish called the madreporite. This water is then transported from the madreporite to a ring canal and to each of the arms of the starfish through radial canals. Shorter canals arise from the radial canals in each arm as well. Finally, these canals lead to structures called ampullae in each arm. The inside of the canal system is lined by hairlike structures called cilia.

When muscles in the ampullae of starfish contract, valves in certain canals close and water is forced into the tube feet. The tube feet in starfish attach onto and move in a wavelike manner. As one arm attaches to the surface, another arm simultaneously releases.

Watch this video to get a closer look at a starfish as it walks on a beach!