Most bony fish have swim bladder, which is a slender sac located at the back of the body cavity. The swim bladder is usually divided into two parts, which contain oxygen, nitrogen and carbon dioxide respectively. The swim bladder has a swim bladder tube communicating with the esophagus, called the throat swim bladder, which belongs to lower bony fish, such as carp. The swim bladder without swim bladder tube, called closed swim bladder, belongs to the swim bladder of higher bony fish, such as perch.
The volume of swim bladder accounts for about 5% of the body. Its shapes are oval, conical, heart-shaped, horseshoe-shaped and so on. The gas filled in the swim bladder is mainly oxygen, ammonia and carbon dioxide, with the most oxygen content. Therefore, in an anoxic environment, the swim bladder can be used as an auxiliary respiratory organ to provide oxygen for fish.
By adjusting the contraction and expansion of the swim bladder through its muscles, the fish can adjust its body density and float or sink in the water.