244 THE CONDITIONS OF LIFE IN THE SEA [PART III 



The results of this series of estimations are of great interest, 

 for they indicate that the bacteria may exert a powerful influence 

 in reducing the proportion of oxygen contained in the warmer 

 seas. The mean value as indicated by the above figures at 13°*1 C. 

 is 1*2 milligrammes, and from this Putter calculates that 1000 

 millions of bacteria — a number which may be present in one 

 cubic metre of Mediterranean water — may use up about 300-times 

 their own weight of the dissolved oxygen. 



So also with the other organisms dealt with by Putter. Thus 

 the calcareous alga Lithothainnion racemus, when kept in the 

 dark, uses up oxygen, and the amount of the latter utilised by 

 the plant is almost exactly proportional to the temperature, 

 increasing with the latter. The oxygen-consumption of the holo- 

 thurian Cucumaria gruhei increases with the temperature, and as 

 we have seen (p. 232) the nature of the nutrition process also 

 changes, so that at the higher temperatures the oxidation pro- 

 cesses become more prominent than those of the fermentation 

 character^. 



Salinity. Precisely how changes in the concentration of the 

 salts of the sea water affect the habits and metabolism of marine 

 animals we do not know. It is of course easy to see how changes 

 in the density of the water may have powerful effects in the 

 distribution of certain animals in the sea. Many organisms, such 

 as pelagic fish eggs, have almost exactly the same specific gravity 

 as the water in which they float ; and any alterations in the 

 density of the latter (such as may be caused by changes in salinity, 

 in temperature, or both) may cause these organisms to rise or sink 

 in the sea; and since the strength and direction of the currents 

 may vary at different levels, the distribution of the fish eggs, or 

 of the larvae hatched from them, may be affected. Further, such 

 eggs or larvae may be carried by currents into other parts of the 

 sea, the estuaries of rivers for instance, where the density is so 

 low that they at once sink to the bottom, become smothered in 

 mud, and die. It is probable that considerable numbers of pelagic 

 fish eggs may be destroyed in this way in the Baltic, where the 

 density of the water may fall so low that fish eggs and larvae do 



1 Vergleich. Phys. Stqffwechsel, p. 52. 



