Features

• Inflated swim bladder.
  
  The swim bladder (labeled as "5" in the image above) lies just under the kidneys and above a fish's stomach, liver and intestines. Some fish are able to self-regulate the pressure in their swim bladder.
  
  

History

• The swim bladder likely developed from lung-like organs found in evolutionary ancient fish. Like lungs, the swim bladder develops from the esophagus.
  
  

Other Functions

• In some fish, the swim bladder is connected to the inner ear and might play a role in hearing or producing sound. Some fish may also use their swim bladder to aid in respiration.
  
  

Warning

• Swim bladder disease can be fatal to goldfish and other pet fish. Various ailments might cause swim bladder disease, including viral or bacterial infections, diet, dirty water or poor anatomical conformation.
  
  

Fun Fact

• Not all fish have swim bladders; sharks and other cartilaginous fish lack the organ. as do some bottom-dwelling bony fish. The lack of a bladder is part of the reason sharks must swim nearly constantly.