Location

• The swim bladder is present in some but not all types of fish, and can generally be found on the body's dorsal side within the body cavity. The swim bladder contains blood vessels that add oxygen and other gases to the cavity through diffusion. In some fish species there is a connecting tube between the digestive tract and the swim bladder through the pharynx, and what this indicates is that the swim bladder may, in some species, aid in the process of respiration.
  
  

Function

• The swim bladder can serve several different purposes depending on the species of fish. Most bony fish use the swim bladder to maintain depth without floating to the surface or sinking to the ground. In other species, the swim bladder receives and produces sound by serving as a resonating chamber. While the swim bladder contains gas in most species, some fish have oil in the swim bladder instead; this may allow it to function more as a respiratory aid or lung aid rather than a buoyancy or hydrostatic organ. Most bottom-dwelling species do not have a swim bladder.
  
  

Species Differences

• Fish that do not have a swim bladder have to find a different method for maintaining their balance. For example, sharks, rays and skates have to remain active, swimming in order to maintain balance. They use as much as 20 percent of their energy to maintain their balance in the water. Fish that use a swim bladder for other reasons include salmonids and eels, which have a connection between the esophagus and the swim bladder.
  
  

Open and Closed Swim Bladders

• In some species, the swim bladder is open, while in others it is completely closed off, and this impacts how the fish uses the swim bladder to its benefit in the water. In a fish species with a closed swim bladder, the pressure inside the bladder is controlled by way of gas exchange between the bladder and the blood. If they move too quickly up or down, they can cause unwanted changes in the swim bladder which can throw them off balance. Fish with open swim bladders typically stay closer to the surface and do not experience these rapid changes in the quantity of gas in their swim bladder, allowing them to maintain the right depth in the water.
  
  

Swim Bladder Disease

• A common problem in fish is swim bladder disease, which causes the fish to float to the surface of the water and to struggle to move lower. Swim bladder disease is considered to be a multifactorial illness. Fish with this disease float to the surface against their will, or sink to the bottom and cannot rise again. There are many potential causes for swim bladder disease depending on the species of fish. This disease is especially common among ornamental goldfish kept in captivity.