Swim bladder
S: anterior, S': posterior portion of the air bladder
œ: œsophagus; l: air passage of the air 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.
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]
Swim Bladder Media
Swim bladder from a bony (teleost) fish 
How gas is pumped into the swim bladder using counter-current exchange.
The West African lungfish possesses a lung homologous to swim bladders
Most mesopelagic fishes are small filter feeders which ascend at night using their swimbladders to feed in the nutrient rich waters of the epipelagic zone. During the day, they return to the dark, cold, oxygen deficient waters of the mesopelagic where they are relatively safe from predators.
Swim bladder display in a Malacca shopping mall
delicious!
Swim bladder disease has resulted in this female ryukin goldfish floating upside down
References
- ↑ "More on Morphology". ucmp.berkeley.edu.
- ↑ "Fish". Microsoft Encarta Encyclopedia Deluxe 1999. (1999). Microsoft.
- ↑ Kardong K.V. 1998. Vertebrates: Comparative anatomy, function, evolution. 2nd edition, WCB/McGraw-Hill, p. 12 ISBN 0-697-28654-1