Sea Stars are commonly referred to as Starfish, but they actually are not fish at all but echinoderms. Starfish characteristically have five arms extending from a central disk like a star, hence the common name Starfish or sea stars. Represented by over 2,500 species, families of importance to the aquarium include the Oreasteridae, Himerometridae, Ophidiateridae, and Ophiomyxidae. Found in all oceans, they prefer rocky crevasses and sandy regions in reefs. Most are found in inter-tidal zones, but a few are found in deep water. Most prefer to live singly, though they will congregate for sexual reproduction. They are rarely bred in the home aquarium.
Perhaps the most intriguing feature of the Starfish is their ability to regenerate lost arms. Under ideal conditions, the lost arms will turn into a complete Starfish. Each arm of a sea star has two rows of "feet" called podia that are used for attachment, locomotion, and respiration (gas exchange). Many have an additional sucker attachment at the end of each arm. Movement is achieved by a hydraulic system of water-filled channels. Serpent Starfish use their arms to push and pull themselves along the substrate and rocks.
It is important to keep Starfish under water during acclimation and placement in the aquarium. Serpent and Brittle Starfish are popular additions to reef aquariums that contain live sand as they constantly stir the sand bed and provide oxygen to the beneficial bacteria living in the sand.
The mouth and stomach is located on the underside of the animal in the central disc. Before food is absorbed, the stomach is often pushed out of the body and placed over the food to digest it. Smaller Starfish eat the microalgae found on rocks and algae mats; larger ones will eat other Starfish, Urchins, Clams, and other bi-valves, and some Anemones. In the aquarium, they can be fed small pieces of fish, placing the food near or even under the Starfish.