Anatomy of finfish and shellfish

Unit 4 - Gastro Intestinal Tract OR Digestive system and associated digestive glands

4.2.4. Gas bladder
The Gas bladder is a thin walled sac typically found in the upper part of the body cavity immediately below the kidney. In many fishes the shape is simple, usually some what torpedo shaped, but there are many variations, minnows and carps (cyprinidae) have anterior and posterior chambers, connected by an opening controlled by a splinter. Feather backs (Notopteridae) have air bladder divided laterally, but two chambers communicate anteriority most croackers (scieanidae) have unusual air bladders in that variously shaped sacs or branching caeca may be arranged along each side of the organ. In Herrings (chupeidae) the gas bladder has a posterior opening to the exterior near the anus.
In the embryology of bony fishes the gas bladder originates as an outgrowth of the alimentary canal and remains attached to the esophagus or stomach via the pneumatic ducts such fishes are known as physostomes. Some groups of fishes lose the connection to the alimentary tract. These fishes are known as physoclistous. Some of such fishes may retain the connection till larval or juvenile stages.
Some bottom-dwelling stream fishes such as darter (Etheostoma) and Sculpin (cottus) lack the gas bladder. Various other bathy pelagic fishes have also lost gas bladder. Agnathus and cartilaginous fishes also lack gas bladder.