Introduction to the Swimbladder


 What is a swimbladder?
Different Types
Gas Secretion and Absorption
Sensory Functions
Literature Cited

     Have you ever wondered how fish manage to stay afloat all day long yet they still have enough energy left over to go about their every day business?  One particular organ, the swimbladder, is responsible for this unique phenomena in numerous fish.   Without a swimbladder, the fish, with body tissues denser than water, would sink (Howlett 1996).  Most teleosts (ray-finned fish) have over time converted their lungs into swimbladders (Jones 1957).  The swimbladder is an oval-shaped sac found in the fish's abdominal cavity, which at different times can be filled with varying amounts and compositions of gases (same as atmospheric gases: carbon dioxide, oxygen, and nitrogen).  The bladder has developed as an extension of the gut wall (Figure 1). 

 Figure 1: Location of the swimbladder in teleosts- found below the vertebral column. 

     The swimbladder's main function is that of a hydrostatic organ.  Neutral buoyancy, the ability of an organism to use little or no energy to stay at particular levels of water, is achieved through the expanding and shrinking of the swimbladder due to varying gas pressures (Schmidt-Nielsen 1997). The fish can enjoy a sense of near weightlessness when its swimbladder gas capacity is around 5-7% of its total body volume.  Freshwater fishes require a larger swimbladder than those in salt water because freshwater is less buoyant than salt.
     There are two other functions of the swimbladder that are of great importance to the fish.  One is its connection with the ear to facilitate hearing abilities in the water and the other is its sound producing capabilities (discussed further in Sensory Functions). 
     The evolution of the swimbladder is one of the main reasons that teleosts have been so successful.  With this highly functional organ, fish can have more precise control over their movement while expending minimal amounts of energy (Marshall 1966).

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