Spiny starfish (Marthasterias glacialis) under water. This large starfish usually measures about 40 centimetres across. It lives between the surface and 150 metres depth, mainly on hard substrates, and also sandy seabeds. Each of its arms (usually five, but only four here) has rows of sharp spines. It moves on hundreds of tube feet (some seen round the edges). Its mouth is under the centre of its body. Starfish mainly eat bivalve molluscs, pulling them apart with their arms.