Pressure Increase with Depth 31 



are confined to relatively narrow zones, but these restrictions of range 

 are probably due primarily to other factors, such as temperature, 

 light, or food. Nevertheless very great changes in pressure have been 

 shown experimentally to alter the rates of certain physiological reac- 

 tions; and a variety of invertebrate animals, fishes without swim 

 bladders, and bacteria are inactivated, or killed, when subjected in 

 pressure chambers to several hundred atmospheres, although the ef- 

 fect is much reduced if the temperature remains nearly constant 

 (Zobell and Oppenheimer, 1950). Thus, although moderate pressure 

 changes do not ordinarily harm such organisms, great changes may 

 exert certain subtle influences on their life processes. Whether or 

 not such physiological action of pressure actually limits the vertical 

 range of aquatic organisms in their natural habitats has not yet been 

 ascertained. 



For animals with air-filled cavities, such as fish possessing swim 

 bladders and diving birds and mammals, the rapid increase in pres- 

 sure with depth in the aquatic environment is a serious matter. 

 The swim bladder of a fish supplies buoyancy, and a fish with this air 

 cavity is similar in its flotation to the Cartesian diver of the physicists. 

 When the fish moves downward, the bladder is compressed, and when 

 it moves toward the surface, the bladder expands. Gas must be re- 

 moved from the swim bladder, or added to it, in order for the fish to 

 maintain control of its buoyancy equilibrium. If the fish moves up- 

 ward so fast that gas cannot be removed from the swim bladder at a 

 rate sufficient to compensate for the reduction in pressure, the con- 

 tained gas will continue to expand and the fish will rise toward the 

 surface at an accelerating rate. If movement toward the surface con- 

 tinues, the swim bladder will eventually burst, and the expanding 

 gas, now in the body cavity, will force the stomach to protrude from 

 the mouth, the intestine from the anus, and the eyes from their 

 sockets. Presumably, under ordinary conditions, an early stage of in- 

 ternal volume change stimulates the fish to return to its former level. 

 The pressure factor thus definitely limits the vertical range of fish 

 with swim bladders as well as the speed with which they can move 

 from one depth to another (Jones, 1952). 



For diving mammals and birds the problem of breathing is added 

 to that of the increased pressure. When a human diver descends into 

 the water in a flexible diving suit he must withstand the increased 

 pressure, but air is supplied to him through a hose or by means of an 

 "aqualung." Pressure alone prevents the diver from descending more 

 than 100 m or so. Whales, seals, and diving birds are forced to go 

 without a renewal of oxygen during the period of their dive. Whales 



