Starfish are marine invertebrates . They typically have a central disc and usually five arms, though some species have a larger number of arms. 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. 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.