Description: The airbladder or gasbladder or swimbladder is a white glistering saccular organ developing as a diverticulum form the wall of the pharynx in the gut. It is often present among the teleosts. It attains its full development among the spiny-rayed teleosts, Actinopterygii. In fishes where the swim bladder functions as an accessory respiratory organ, its structure like more complex and it acts almost like a lung. (Yadev, 1999)
In Notopterus chitala and Notopterus notopterus, the gills are reduced and the air bladder is very much developed and is almost like the lung of the higher vertebrates. The fish can live without water for few hours. When in water, it periodically gulps air. The main portion of the air bladder lies in the abdominal part inside the body cavity and bifurcates of two lateral bronchi like tubes. In the trunk region of the tube, there are 14 to 15 blind pouches with finger like projections. The pouches and their projection have profuse supply of blood capillaries. Thus, the airbladder in this case serves as true lung and makes the fishes almost a air breather (Munchi and Hughes, 1992).
Function: In the transfer phase, gas is passed passively from the gas bladder to the buccal cavity and then the exhalation of air takes place through the mouth due to action of the geniohyoideus muscle, which exerts a positive pressure pulse in the buccopharynx. This is allowed by an active intake phase by action of the sternohyoideus muscle creating expansion of the buccopharynx and a negative pressure pulse. This is followed by the compression of the buccopharynx by the action of geniohyodeus muscle forcing the fresh air into the gas bladder.
Gas exchange: A prominent artery from the dorsal aorta directly supplies blood of swimbladder. A short vein drains blood from the swim bladder into the posterior cardinal vein.
Modification: The gas bladder posses an extensively caudal extension that gives off a series of ventral finger-like projections descending on either side of the anal pterygiophores. There is also a cranial extension of swim bladder found in the part of modification.
Yadev, B.N., 1997. Fish and Fisheries. Daya Publishing House, Delhi-110035. 366p.
Munshi, J.S.D. and G.M. Hughes, 1992. Air-breathing Fishes of India; their structure, function and life history. Oxford & IBH Publishing co. Pvt. Ltd. New Delhi-110001. 338p.