Evolution
Swim bladders are evolutionarily closely related (i.e. homologous) to lungs. It is believed that the first lungs, simple sacs connected to the gut that allowed the organism to gulp air under oxygen-poor conditions, evolved into the lungs of today's terrestrial vertebrates and some fish (e.g. lungfish, gar, and bichir) and into the swim bladders of the ray-finned fish. In embryonal development, both lung and swim bladder originate as an outpocketing from the gut; in the case of swim bladders, this connection to the gut continues to exist as the pneumatic duct in the more "primitive" ray-finned fish, and is lost in some of the more derived teleost orders. There are no animals which have both lungs and a swim bladder.
The cartilaginous fish (e.g. sharks and rays) split from the other fishes about 420 million years ago and lack both lungs and swim bladders, suggesting that these structures evolved after that split. Correspondingly, these fish also have a heterocercal fin which provides the necessary lift needed due to the lack of swim bladders. On the other hand, teleost fish with swim bladders have neutral buoyancy and have no need for this lift.
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