150 ANIMAL LIFE OF THE DEEP SEA BOTTOM 



sight; but the theory is no longer held, and the significance of this for 

 us it that the deep sea must have been deep for millions of years. For 

 that matter, as mentioned previously, more than half the sea area lies 

 over depths greater than 4,000 metres, so that even a drastic lowering of 

 the sea-level would still leave an extensive field for deep-sea research. 



In order to convey an impression of the animal life out there as we 

 got to know it on the Galathea, and correlate it with the discoveries of 

 previous expeditions, it will be best to go out to between 4,000 and 5,000 

 metres. The fact is that between the fauna of the continental slope and 

 that of the deep sea there is no very sharp dividing-line. Yet so many 

 changes have taken place once we have passed the 4,000-metre curve 

 that we could always see whether a trawl had been really deep, or had 

 only touched the lowermost part of the slope or its vicinity. 



Nor is there any sudden change if we go still deeper, down to about 

 6,000 metres; but it will be best to deal separately with the special fauna 

 which we found in the oceanic trenches. This will make it easier to appre- 

 ciate the special characteristics of the hadal fauna, as it has now been 

 called. 



Let us first consider the facilities for life in general. 



The perpetual darkness, the temperatures near freezing-point, and the 

 slight variation in the chemical composition of the water have already 

 been discussed. We have also seen how, before the Galathea Expedition, 

 it had been shown that the water nearly everywhere is adequately supplied 

 with oxygen, so enabling higher animals to thrive even at the greatest 

 depths. In short, the characteristic feature is uniformity. 



Yet certain things do change; above all, the pressure. For the purposes 

 of the present discussion it is sufficiently accurate to regard a column of 

 water 10 metres deep as corresponding to a pressure of one atmosphere, 

 which is the pressure of the air around us. This means that 26 per cent, 

 of the bottom of all the seas is subject to a pressure of 200 — 400 atmo- 

 spheres, 56 per cent, to 400 — 600 atmospheres, and 1.3 per cent to 600 — 

 1,000 atmospheres, according to depth. It is still difficult to say anything 

 definite about the significance of pressure. True, the French physiologist 

 M. Fontaine has demonstrated, in laboratory tests, that the tissues of 

 organisms which have lived under low pressure suffer irreparable damage 

 under high pressure; but it will be obvious that what really is of interest 

 here is a study of the organisms which actually derive from the abyssal 

 depths, such as the bacterial cultures which Professor Claude E. Zobell 

 is engaged in studying. However, there are, as we shall see, both animal 

 species which only live at great depths and species which have a wide 



