When the fish swims higher again, this extra gas is let out. All these adjustments are made automatically by the fish's nervous system. In some fish, the swim bladder does not only keep the fish afloat. In lungfish, for instance, the bladder acts as a lung. Lungfish live in tropical rivers and the bladder enables them to breathe air when they are stranded on the mud in the dry season.
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.