CHAPTER IV 



CONDITIONS OF EXISTENCE OF AQUATIC ANIMALS 



I. Introduction: Comparison of Land and Aquatic Animals 



The conditions of existence of aquatic plants and animals are very 

 different from those of land plants and animals. Some of the most 

 important differences are as follows: 



a) Water, the surrounding medium, is about 768 times as heavy 

 as atmospheric air at the sea-level. 



b) The necessary gases are in solution in the water and their diffusion 

 is much less rapid than in the atmosphere. 



c) The necessary inorganic salts are in solution in the surrounding 

 medium. 



d) The necessary organic food substances for plants and some of the 

 carbon compounds necessary for animals are in solution in the water and 

 are taken directly by the plants and animals (47). 



e) Vegetation rooted to the bottom is important in most bodies of 

 water. In large lakes like Lake Michigan, however, there are very few 

 attached or rooted plants, and therefore nothing comparable to the 

 vegetation of the land, or to the plant-eating animals which live on it, 

 is to be found. Most of the plants float freely in the water. Such 

 plants are present also, however, where rooted vegetation occurs. 



II. Chemical Conditions 



I. DISSOLVED CONTENT OF WATER 



In order to support animals and plants, water must contain certain 

 minerals and gases in solution (71). Salts (carbonates, sulphates, and 

 chlorides) of magnesium, calcium, and sodium and salts of potassium, 

 iron, and silicon are practically always present in solution in water, and 

 their presence in definite proportions is essential to the life of the animals 

 (72). Water without these has been shown to kill fish (71). Dissolved 

 gases in definite proportions are also necessary. 



Gases. — The chief facts regarding the occurrence of gases in nature 

 and their solubility under experimental conditions are shown in Table II. 

 The standard method of expressing quantity of gas in solution is in cubic 

 centimeters per liter at o° C. and 760 mm. of mercury (73). All values 

 are therefore given in these terms. 



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