 The body of a teleost is denser than water , so fish must compensate for the difference or they will sink . Many teleosts have a swim bladder that adjusts their buoyancy through manipulation of gases to allow them to stay at the current water depth , or ascend or descend without having to waste energy in swimming . In the more primitive groups like some minnows , the swim bladder is open to the esophagus and doubles as a lung . It is often absent in fast swimming fishes such as the tuna and mackerel . In fish where the swim bladder is closed , the gas content is controlled through the rete mirabilis , a network of blood vessels serving as a countercurrent gas exchanger between the swim bladder and the blood . The Chondrostei such as sturgeons also have a swim bladder , but this appears to have evolved separately : other Actinopterygii such as the bowfin and the bichir do not have one , so swim bladders appear to have arisen twice , and the teleost swim bladder is not homologous with the chondrostean one . 
