Starfish or sea stars are echinoderms belonging to the class Asteroidea. The names "starfish" and "sea star" essentially refer to members of the class Asteroidea. However, common usage frequently finds these names also applied to ophiuroids, which are correctly referred to as "brittle stars" or "basket stars". About 1,800 living species of starfish occur in all the world's oceans, including the Atlantic, Pacific, Indian, Arctic and Southern Ocean regions. Starfish occur across a broad depth range from the intertidal to abyssal depths of greater than 6,000 m (20,000 ft).
Starfish are among the most familiar of marine animals found on the seabed. They typically have a central disc and five arms, though some species have many more arms than this. The aboral or upper surface may be smooth, granular or spiny, and is covered with overlapping plates. Many species are brightly coloured in various shades of red or orange, while others are blue, grey, brown, or drab. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the oral or lower surface. They are opportunistic feeders and are mostly predators on benthic invertebrates. Several species having specialized feeding behaviours, including suspension feeding and adaptations for feeding on specific prey. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged or lost arms.
The Asteroidea occupy several important roles throughout ecology and biology. Starfish, such as the ochre sea star (Pisaster ochraceus), have become widely known as an example of the keystone species concept in ecology. The tropical crown-of-thorns starfish (Acanthaster planci) is a voracious predator of coral throughout the Indo-Pacific region. Other starfish, such as members of the Asterinidae, are frequently used in developmental biology.[Source]