The Air-BUdder 133 



The tarpon has perfectly good gills, but it also has a fine 

 large open tube leading from the upper side of its gullet to 

 its air-bladder, and the inside of the air-bladder is well sup- 

 plied with blood-vessels. Young tarpon are frequently found 

 in lagoons which have been cut off from the sea by sand-bars 

 for so long that their water is stagnant, brackish, even sul- 

 phurous, and the tarpon's ability to survive there seems with 

 little doubt to be due to its ability to come to the surface 

 and take atmospheric air into its air-bladder. 



But aside from the fish which actually "breathe" air in 

 this way, a great many others use the air-bladder as a sort 

 of respiratory reservoir. Deep-sea fishes have an unusually 

 high percentage of oxygen in the air-bladder, and in some 

 species, the deeper the fish the more oxygen. The oxygen 

 content of the water is low at these depths, and the fish 

 extracts from the water more than its current requirements 

 and stores it up in the air-bladder against emergencies. 

 Further, when fish are experimentally suffocated, the oxy- 

 gen in the air-bladder is greatly reduced, showing that fish 

 do fall back on this reserve when the normal oxygen supply 

 is cut off. 



In the second place we definitely know that, in those fish 

 which have it, the air-bladder plays a part in the permanent 

 equilibrium between the fish and the water. This is self- 

 evident, and has nothing to do with adjustment of pressure 

 to changing levels. It has to do with density. If a fish has an 

 air-bladder, that air-bladder must be of such a size that the 

 total volume of water occupied by the fish will weigh just 

 about what the fish weighs, if he is not to be in constant strug- 

 gle with his environment. The actual tissue of which most 

 fishes are made is denser than water, and all fish would weigh 

 more than water and tend to sink if they did not have some 

 way of buoying themselves up. The air-bladder supplies this 

 need, but paradoxically, all fishes do not have to have air- 



