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The fossil record for starfish is ancient, dating back to the Ordovician around 450 million years ago, but it is rather poor, as starfish tend to disintegrate after death.Only the ossicles and spines of the animal are likely to be preserved, making remains hard to locate.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.With their appealing symmetrical shape, starfish have played a part in literature, legend, design and popular culture.They are sometimes collected as curios, used in design or as logos, and in some cultures, despite possible toxicity, they are eaten.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.
Asteroids have broad-based arms with skeletal support provided by calcareous plates in the body wall Most starfish have five arms that radiate from a central disc, but the number varies with the group.
Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding.
They have complex life cycles and can reproduce both sexually and asexually.
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.