All live in the ocean, on the sea floor. Many starfish live in deep water, others in shallow water. Some live in the intertidal zone, between low and high tide. They have five or more arms and can be quite large. The Sunflower sea star ("Pycnopodia helianthoides") is the largest: fully grown, its arm-span is about a metre. This is a bit larger than the famous Crown-of-thorns starfish, which lives on coral polyps.