AIR BLADDER AND SPECIFIC GRAVITY OF FISHES. 



125 



accumulating fat, the effect on specific gravity of diving would be correspondingly 

 reduced. 



In the following table there is shown for the several percentages of fat the correspond- 

 ing depth to which a fish must descend in fresh and salt water, respectively, so that the 

 pressure of the water would equalize the displacement by compressing the air bladder. 

 There is also given the excess weight in grams over the displacement which the body of a 

 10 kg. fish would acquire by diving 10 m. in water whose density is 1.026 if the fish is in 

 equilibrium at the surface. 



The air bladder is present in the great majority of fishes. In the Selachii it is 

 absent, in the teleosts it is generally present, although the Heterosomata or flatfishes, 

 Xiphias, the swordfish, Menticirrhus, all the Alepocephalidae, and a few other families 

 or genera are without air bladders. In all the spiny-rayed fishes (which are typically 

 marine) the air bladder when present has no outlet duct in the adult fish. Any reduc- 

 tion in volume of the air bladder must therefore be accompanied, at least temporarily, 

 by pressure. The pressure might be relieved by absorption of gas into the blood. It 

 does not seem at all unlikely that the varying salinities of ocean water guide such fishes 

 as the mackerel, tuna, herring, bluefish, sharks, and many others. Temperatures, 

 oxygen and CO, content, plankton, and other food supply have been studied as directing 

 influences, but it is difficult in every case to show what the immediate effect of the in- 

 fluence is on the fish. In the case of specific gravity the direct effect is obvious and 

 unavoidable. 



It would be exceedingly unsafe to make assumption as to what method the fish 

 uses to maintain itself in equilibrium with the water. We know from the work of 

 Tower (1902) and others that the composition of the gas in the air bladder of fishes 

 varies. It contains more and more oxygen with increasing depths, so that fishes taken 

 from great depths have nearly pure oxygen in the air bladder. It was shown that the 

 air bladder very probably performs an important respiratory function. Certainly the 

 loading into the blood stream or the removing therefrom of large quantities of gas 

 could not possibly fail to have a profound effect on the physiological functions of the 

 fish even if fatal embolisms did not occur. 



The following conclusions are drawn : 



1 . Fish on migrating from water of low salinity to that of high salinity may adjust 

 specific gravity by reducing the size of air bladder. In the reverse direction there is 

 no apparent means for voluntary adjustment. 



