394 ADAPTATIONS TO MEDIA AND SUBSTRATES 



Many deep-sea fishes are really, primarily, cold-water fishes. The same 

 species, or closely related forms, may live at different depths in different 

 places, but will be found obedient to isothermal lines drawn through 

 that whole portion of the sea. Some genera, which are characteristic of 

 shallow waters in polar seas, are still to be found — living far deeper in 

 the water — in the temperate regions. Approaching the tropics, some of 

 these arctic types live beneath two miles or more of water. A few genera, 

 such as Raja, are found from pole to pole. 



To a layman, the most startling feature of the bathic environment is 

 the hydrostatic pressure. Computations are complicated by the fact that 

 the weight of a given volume of water increases with depth — the pres- 

 sures become so great that the water is actually compressed. At a depth 

 of four miles — and a few fishes exist even there — the water is 3% heavier 

 than at the surface. Roughly, one ton per square inch is added with each 

 1000 meters of depth. 



Most of us have read popular accounts of submarine rescue work by 

 skilled divers, and we know that great difficulties are involved in send- 

 ing a man safely to a depth of even 100 meters in a regulation diving 

 dress. We tend to assume that if a fish can go blithely down to many 

 times this depth, he must have some pretty remarkable adaptations to 

 enable him to withstand the pressure. Yet, during storms, many pelagic 

 fishes which have no special provisions for it, sink some hundreds of feet 

 into calmer water, later returning unharmed to their accustomed level. 

 Though deep-sea fishes cannot be brought quickly to the surface without 

 their 'exploding', this is because the gases in their spongy remnants of 

 the swim-bladder, or present in solution in their body fluids, expand when 

 released from pressure and proceed to blow the viscera out through the 

 mouth. A fish has no great air-filled chest to be crushed, and so does not 

 need to be kept distended by an air-compressor at the far end of a hose. 

 He is not receiving such volumes of compressed air that his blood foams 

 with nitrogen if he rises quickly, and no excess of oxygen in his brain 

 makes him light-headed. For him to go downward for a few hundred feet 

 is not at all the same as for a human diver to attempt to do so. And, for 

 a surface fish to go down is not at all the same thing as for an abyssal 

 one to come up (see also pp. 415-6). 



Nor does the eye require any special devices for withstanding pres- 

 sure, though in a captured deep-sea fish it may be bulged from the orbit 

 by a big bubble of nitrogen which has formed behind it. The tissues of 

 the eye, and of the whole body, are permeated by a fluid continuum in 



