To prevent fish from sinking and having to work hard to stay a float, they possess a unique organ called a swimbladder.

The swimbladder is a sack, filled with oxygen and nitrogen, linked to the fish's bloodstream. It works a bit like a balloon providing floatation when inflated. If air is removed from the swimbladder, the fish sinks. Fish constantly adjust their buoyancy by carefully controlling the amount of oxygen and nitrogen in the swimbladder. The swimbladder is filled either by air from the throat or oxygen from the bloodstream.

Some catfish travel over land from pond to pond, using their front fins to push themselves along the ground. When out of the water, they release a slimy substance that covers their bodies and keeps them moist. In addition to their gills, these catfish have swim bladders that they use like lungs when they are on land. The swimbladder is connected to the intestinal tract. Here the surface of the swimbladder is enlarged to accommodate gas exchange.

Other fish also have alternative uses for their swimbladders. Some fish also use their bladder as an eardrum to pick up sounds. Others use it to amplify their own sounds.

Sharks and rays have no swim bladders. Instead body oils help to keep these fish float. However, most of these fish rely on swimming constantly to overcome their tendency to sink.