There are about 1,500 living species of starfish to be found worldwide, from the tropics to subzero polar waters and from the intertidal zone down to abyssal depths. Starfish are among the most familiar of marine invertebrates. They typically have a central disc and five or more arms. The upper surface may be smooth, granular or spiny, and is clad in overlapping plates. Many species are brightly coloured in shades of red or orange, while others are blue, grey or brown. Starfish have tube feet operated by a hydraulic system and a mouth at the centre of the lower surface. Most are voracious predators, either swallowing their prey whole or turning their stomachs inside out to engulf it. They have complex life cycles and can reproduce both sexually and asexually. Most can regenerate damaged parts and many can shed arms as a means of defence. Starfish such as the ochre sea star and the reef sea star have become widely known as examples of the keystone species concept in ecology. With their appealing symmetrical shape, starfish are found in literature, legend and popular culture. They are sometimes collected as curios, used in design or as logos, and in some cultures they are eaten.