NATURE ADRIFT 



about 75 per cent is between 2,000 and 6,000 metres. Pressures in deep water 

 are extremely high, increasing by about one atmosphere in every 10 metres, 

 so that the pressure at 1,000 metres is 1,470 lb per square inch, or 100 atmos- 

 pheres. Yet depth itself is only partially a controlling factor as organisms 

 living under these pressures have the same internal pressure as outside, so that 

 there is no sensation of pressure there any more than we feel the air pressure 

 of 1 s lb per square inch. It is sudden changes of pressure that are trouble- 

 some and only severely so if there is an air bladder as there is in some fish (and 

 of course in divers !). Water is so little compressible that delicate animals can 

 be brought up from the bottom quite quickly with little or no apparent 

 damage, and will swim about merrily in a tank. Gases are very compressible. 

 This means that a small swim bladder of air, say 100 cubic millimetres, at 

 500 metres depth, will try to expand to 850 cubic millimetres at only 50 

 metres depth, and 5,100 cubic millimetres at the surface. A fish with a swim 

 bladder wishing to change its depth must do so slowly allowing time for the 

 expanded gas to be absorbed, or additional gas produced for a downward 

 journey. If it climbs too quickly, or is forced to do so when caught in a net, 

 the bladder expands and as it is a very tough elastic structure it forces the 

 stomach and other internal organs out through the mouth causing the death 

 of the fish even if the swim bladder does not reach bursting point. 



The most important factor, associated with depth, that controls distri- 

 bution is light. Marine animals in the plankton are usually extremely sensitive 

 to light and vary their depth to give them the most suitable amount of light. 

 This results in an up and down movement by night and day that is more fully 

 discussed in Chapter 12 on behaviour. Plankton adapted to everlasting 

 twilight or eternal dark in the depths can be killed on capture merely by the 

 unaccustomed brightness at the surface. Watching the stars at night one can 

 readily see how the fainter stars become visible only as the last remnants of 

 twilight fade and the night gets really dark. Stars of the first magnitude can 

 be seen soon after dusk, those of the third magnitude when it seems quite 

 dark, yet only when it is really dark does one see the fourth magnitude stars. 

 Experiments have shown that some planktonic worms, for example, are 

 sufficiently sensitive to small light intensities that their reactions change very 

 markedly in intensities that correspond to the difference that makes fourth 

 magnitude stars visible to us instead of only the third. 



The depth that light will penetrate into the sea depends on the angle of 

 the sun and the clarity of the water. An overhead sun will penetrate deeper 

 than when it is lower in the sky, and the clarity of the water is associated with 

 the amount of mud in inshore waters and the amount of plankton. Light 

 therefore penetrates much deeper in the blue ocean water than in the green 

 shelf water. Even in the clear tropical waters of the Mariana Trench the divers 



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