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Most can regenerate damaged parts or lost arms and they can shed arms as a means of defence.
The Asteroidea occupy several significant ecological roles.
The water vascular system of the starfish is a hydraulic system made up of a network of fluid-filled canals and is concerned with locomotion, adhesion, food manipulation and gas exchange.
They are found from the intertidal zone down to abyssal depths, 6,000 m (20,000 ft) below the surface. They typically have a central disc and five arms, though some species have a larger number of arms.
Starfish, such as the ochre sea star (Pisaster ochraceus) and the reef sea star (Stichaster australis), have become widely known as examples 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, and the northern Pacific sea star is considered to be one of the world's 100 worst invasive species.
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 or brown.