Swim bladder

The swim bladder (gas bladder, air bladder) is an internal gas-filled organ. It helps many bony fish (but not cartilaginous fish^[1]) to control their buoyancy.

The swim bladder of a rudd

Internal positioning of the swim bladder of a bleak
S: anterior, S': posterior portion of the air bladder
œ: œsophagus; l: air passage of the air bladder

Fish with a swim bladder can stay at their current water depth without having to waste energy in swimming.^[2] The dorsal position of the swim bladder means the center of mass is below the center of volume, so it acts as a stabilizing agent. Also, the swim bladder is a resonating chamber, to produce or receive sound.

Swim bladders are evolutionarily closely related (i.e., homologous) to lungs. Traditional wisdom has it that the first lungs (simple sacs connected to the gut) allowed the fish to gulp air in oxygen-poor conditions. They evolved into the lungs of today's terrestrial vertebrates and some fish (lungfish, gar, bichir) and also into the swim bladders of the ray-finned fishes.^[3]