The swim bladder, commonly known as fish bubbles, contains air, which can make fish adjust their body density and rise or sink in the water.

Most bony fishes living in the upper and middle layers of seawater have swim bladders. 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.

The most important function of the swim bladder is to adjust the specific gravity of the fish by inflating and deflating, so as to adjust the water pressure balance inside and outside the bony fish body and control the ups and downs of the body.

In this way, when they swim, they only need a little strength to keep a stable state in the water. The pressure of seawater increases with the depth of seawater, which is very important for a trip to the seabed.