It contains air and develop as a small outgrowth from wall of the gut. Swim bladder regulating buoyancy allows control of depth in water without using muscles to fight gravity (saves energy). When the swim bladder expands it increases in volume and therefore displaces more water. This increases the fish’s buoyancy, causing it to float upward. When the swim bladder deflates, the fish’s buoyancy decreases, causing it to sink as it displaces less water.
A swim bladder may be single or double. It may open into the esophagus, dorsally or ventrally or not at all. It may be very small or quite large, extending through the entire length of the coelom. It often lies above the gut, directly beneath the vertebral column.
In some species, such as the pirarucu, Arapaima gigas, the swim bladder is highly vascularized and functions as a respiratory organ.
The swim bladder may be connected to the digestive tract, more specifically with the esophagus and stomach through a structure called the pneumatic duct.
Air bladder is supplied with blood from dorsal aorta or from the branch of coeliaco-mesenteric artery. The deoxygenated blood is drained into the hepatic portal system or into posterior cardinal vein.
Air bladder of fish
