There are approximately 2,000 species of sea star, all of which live in marine waters.
Sea stars live underwater, but that is where their resemblance to fish ends. They do not have gills, scales, or fins. Sea stars live only in saltwater. Sea water, instead of blood, is actually used to pump nutrients through their bodies via a 'water vascular system.'
Also, sea stars move by using tiny tube feet located on the underside of their bodies. Adult sunflower sea stars can move at the astonishing speed of one meter per minute using 15,000 tube feet. Tube feet also help sea stars hold their prey.
Sea stars are related to sand dollars, sea urchins, and sea cucumbers, all of which are echnioderms, meaning that they have five-point radial symmetry. However, this does not mean that all sea stars have five arms and species with 10, 20, or even 40 arms exist! If one of these arms is lost, a sea star has the amazingly ability to regenerate it.