Air bladder

Air bladder is an organ found in many fish species, which is used for maintaining buoyancy. The air bladder, also known as the swim bladder, allows fish to ascend or descend without having to expend significant energy in swimming. The air bladder is a flexible-walled, gas-filled sac located in the dorsal portion of fish. It is derived from an outpocketing of the fish's digestive tube.

Function

The primary function of the air bladder is to provide the fish with the ability to control its buoyancy and thus, its depth in the water. By adjusting the gas pressure within the bladder, the fish can either float higher or sink lower in the water column. This is achieved by the secretion or absorption of gases into the bloodstream.

Structure

The structure of the air bladder varies among different species of fish. In some species, the bladder is a simple sac. In others, it is divided into two or more parts. Some fish have a connection between the air bladder and the digestive tract, known as the pneumatic duct, which allows them to gulp air and fill the bladder when they are in water that is too shallow for them to dive and fill it by absorbing gas from the bloodstream.

Evolution

The air bladder is believed to have evolved from the primitive lungs of early fish ancestors. Some fish, such as lungfish, still retain lungs and use them for respiration in addition to or instead of gills. In these species, the lungs have a similar structure to the air bladder of other fish, suggesting a common evolutionary origin.

See also

• Fish anatomy
• Buoyancy
• Evolution of fish