The swim bladder, commonly known as the fish bubble, is a position adjuster in the water when fish swim, and it is also a "life buoy" when fish swim. It can adjust the specific gravity of fish by inflating and deflating.

The swim bladder, commonly known as fish bubble, contains air, which can make fish adjust their body density and float or sink in the water. Most teleost fish that live 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.

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 and control the ups and downs of the body.