Aquatic Environment 255 



do without oxygen for short periods, as when the soil is rain soaked, 

 but for the most part earthworms migrate to the surface when oxygen 

 becomes depleted. Many organisms encyst, or go into a dormant 

 condition, when oxygen becomes too scarce for the species concerned. 

 However, some of the microorganisms inhabiting the soil can follow 

 an anaerobic existence (Waksman, 1932). Included are members of 

 the genera Clostridium and Rltizobiiini that are important as nitrogen- 

 fixing bacteria; Clostridium tetani is of particular concern to men as 

 the microbe that causes lockjaw. We realize then that the highly 

 variable conditions of oxygen availability often exert a critical control 

 over the distribution and activity of both aerobic and anaerobic 

 organisms involved in the ecology of the soil. 



Aquatic Environment. We have seen that the supply of oxygen 

 in the aquatic environment varies over the complete range from super- 

 saturation to total exhaustion, but that, even when saturated, water 

 contains a much lower concentration of oxygen than the atmosphere. 

 Some fresh-water organisms have hit upon certain dodges by which 

 they can live in the aquatic medium and still breathe air. Many 

 water bugs such as Notonecta and beetles come to the surface, entrap 

 bubbles of air beneath their wing cases, and swim down with a supply 

 of oxygen that will last for a period of time. Other animals, like the 

 water scorpion Ranatra (Fig. 6.12), the mosquito larva, and the 

 rat-tailed maggot, possess breathing tubes that extend to the surface. 

 Some kinds of snails periodically refill their lungs with air at the 

 surface, whereas other water animals are able to bore into the stems 

 or roots of emergent hydrophytes and obtain oxygen from the internal 

 air spaces of the plants. 



Most aquatic organisms, however, must get along with the rela- 

 tively meager supply of oxygen dissolved in the water. Let us con- 

 sider the ecological conditions in well-aerated water before taking up 

 the effects of severe oxygen depletion. Although saturated water 

 contains only a small fraction of the amount of oxygen in an equal 

 volume of the atmosphere, the partial pressure of oxygen in the water 

 is equal to that in the air with which it is in equilibrium. Accord- 

 ingly, the tendency of the oxygen in water to cross the respiratory 

 membranes of organisms is just as great as it is in air. The lower 

 concentration of oxygen in water simply means that less is in reserve. 

 The relatively small amount of oxygen in the water is adequate for 

 aquatic organisms if the medium in contact with the absorbing mem- 

 branes is rapidly replenished. 



Inhabitants of the water environment display a wide variety of 

 adaptations that serve to obtain a sufiicient supply of oxygen from the 



