Sea stars breathe using simple diffusion through their tube feet, as well as with dermal gills that keep water moving past them with cilia. Oxygen is carried through the body via canals filled with fluid that differs little from the surrounding water, without any specialized blood cells or compounds to help carry oxygen or carbon dioxide. All echinoderms, such as starfish, have very simple respiratory and circulatory systems.
Sea stars, like all echinoderms, are extremely slow moving. Their low energy requirements are a large part of what permit them to operate with such inefficient respiratory systems. They rely on their toughness and regenerative abilities, rather than any speed or ability to fight, to protect themselves. Despite this, they are voracious predators, feeding on prey that is sessile or at least as slow moving as they are, including bivalves, such as clams, as well as other sea stars in some cases.
The same water-based circulatory system that sea stars use to carry dissolved gases is also used in motion via their hydraulic motion system. Each arm on a sea star bears many tube feet, which are tiny moving tentacles with suction cups on the end. These are extended by forcing water into them, then drawn in with muscles.