FRESH-WATER MUSSELS- 117 



DISSOLVED GASES. 



Air is inconspicuous, yet nothing is more important to man. Without it he dies; 

 and his comfort, health, and normal development depend upon the purity of the air 

 by which he is surrounded. This is because of the absolute necessity for oxygen, and 

 the deleterious effect of too much carbonic-acid gas. The gases dissolved in water 

 are as invisible as air, but the mussels are as dependent upon the free oxygen in solution 

 in the water as man is dependent upon the oxygen of the air. The water of streams 

 and lakes dissolves air at the surface from the atmosphere and derives it from the 

 physiological action of plants in light. Cold water will hold more free oxygen than 

 warm, but the absorption of oxygen at the surface is favored by increased evaporation, 

 with warm dry air and the pre^•alence of winds (W. E. Adeney, in Report of the Metro- 

 politan Sewerage Commission of New York, 1912, p. 81). Falls, rapids, and swift 

 currents promote the absorption of oxygen, and circulation currents lead to its better 

 distribution into the deeper parts and throughout the whole body of water. Even 

 without the aid of circulation currents, a measure of distribution of oxygen dissolved 

 at the surface is effected by diffusion and "streaming" of the gas within the water 

 (W. E. Adeney, loc. cit., p. 82). 



Carbon dioxide (COj), commonly called carbonic-acid gas, which is given off as a 

 waste product of mussels and other animals, and which is also formed by the decom- 

 position of animal and vegetable matter, is helpful in small quantities, but is poisonous 

 to animals when present in too great quantities (Shelford, 1913, p. 59; 1918, pp. 39, 40; 

 and 1919, p. 106). It is used up by green plants in sunlight and is also given off to the 

 atmosphere at the surface of the water. The same conditions that are favorable to the 

 absorption of oxygen are also favorable to the loss of COj. 



Carbon dioxide is of especial significance sometimes because of its tendency to 

 unite with calcium carbonate to form the bicarbonate, which is soluble in water. Since 

 the shell of a fresh-water mussel is composed principally of calcium carbonate it is liable 

 to be attacked by free carbon dioxide in the water and taken up into solution. The 

 homy covering of the shell is a protection against the action of the gas, but if that 

 becomes broken or worn off in spots, as frequently occurs, the shell is exposed to the 

 destructive effect of the acid. This leads to little harm in hard waters where the CO, 

 may unite with the calcium carbonate derived from rocks or soils, but in soft waters, or 

 in any waters where there is an excess of gas over dissolved calcium, the shells are 

 partially or completely destroyed by corrosion. On many rivers "baldhead" shells 

 are commonly encountered, and sometimes the shells are full of pits or even eaten clean 

 through in the older parts. 



Nitrogen, though an important element in the composition of mussels, can not be 

 used by them in the form of a gas, and its presence in water (unless in excess) is pre- 

 sumably a matter of indifference to them, just as the nitrogen which composes the 

 bulk of the atmosphere is uninjurious to men and not directly utilized by them (Shelford, 

 1 91 8, p. 36). Othei gases found in water are ammonia, methane (CH^) and other 

 hydrocarbons, and hydrogen sulphide (HjS), which are formed in certain processes 

 of decomposition (Noedham and Lloyd, 1916, p. 47). These are of importance only 

 when occurring in sufficient quantity to be injurious. 



