36 VERTEBRATE RESPIRATION 



changes in depth of the fish or the effect of sound vibrations in 

 the water. The detection of sounds, however, is best done by 

 those fishes which have some connection between the swim- 

 bladder and the inner ear. For example, cyprinoid fishes such 

 as carp, tench and minnow possess Weberian ossicles which 

 transmit vibrations of the anterior sac to a part of the inner ear. 

 Conditioning experiments have shown that these fish are able 

 to respond to higher frequencies (e.g., 10,000 c.p.s.) than most 

 other fish which do not possess them. This apparatus also serves 

 to detect slow changes in volume of the more extensible anterior 

 sac resulting from slow changes of ambient pressure such as the 

 fish will experience if its depth below the surface changes. In 

 other fishes the swimbladder is concerned in the production of 

 sounds, usually by the operation of special muscles attached to 

 the swimbladder, but in other cases it functions as a resonator to 

 amplify such sounds. 



The most important and primary function of the swim- 

 bladder, however, is to regulate the density of the fish relative 

 to the surrounding medium. Evidence strongly suggesting such 

 regulation is fairly clear when a bony fish is observed freely 

 floating in an aquarium tank, for it maintains its position 

 neither moving up or down in a state of neutral buoyancy with- 

 out making any active movements. A second piece of evidence 

 comes from measurements showing that the density of fish 

 tissues is about 1-076. In fresh water a fish of this density would 

 sink to the bottom, but by containing a bubble of gas the density 

 of the whole fish is reduced to that of the external medium. It 

 can be calculated that the volume of such a bubble must be 

 about 7 ml. for a 100-gm. fish in freshwater and 5 ml. for a 

 marine fish. Determinations of the volume of swimbladders 

 (Table 5) of many bony fishes have confirmed that this size of 

 bladder is generally found. 



The presence of such a bubble of air in an organism means 

 that it will be affected when the hydrostatic pressure surround- 

 ing it is altered. Increasing the pressure will decrease the volume 

 of gas and consequently the density of the whole fish will be 

 reduced. Conversely, reductions in pressure will cause the swim- 



