Some catfish can make noises with their bladder - just like a set of bagpipes. A few species of fish communicate with each other by using sounds from their bladder. Colouring Many fish are in constant danger of being eaten by other fish or by birds, so they are often coloured to blend in with their surroundings, which hides them from predators' eyes. The backs of fish that live near the surface of the open sea are blue like the surface of the sea, while river perch are marked with bars that make them difficult to see amongst weeds. Most fish can actually change colour to match the background.
Some tropical fish can change almost instantly from yellow to scarlet or from red to green.
Safe colours Some fish are brightly coloured or have striking markings, but most are camouflaged - that is, coloured to match their surroundings. Stingrays have broad fins that make them almost square, and a long, whip-like tail which can inflict a fearsome sting. They usually live in shallow tropical waters and feed on creatures on the sea-bed, such as shellfish and crabs. Sharks are the most ferocious fish in the sea. There are over 250 kinds, some quite small, but nearly all sharks are hunters, with a mouth full of rows of razor-sharp teeth.