82 Causes of Emigrations 



through the air, but none of these animals has become estabUshed 

 on land. The absence of molar agents may have profound effects 

 on aquatic animals, chiefly through the stagnation of water, which 

 causes animals to die or take up breathing from or through the 

 surface of the water. 



Temperature 



In aquatic habitats there are certain inherent advantages and 

 disadvantages that are associated with temperature. As water 

 grows warmer it can hold less gas in solution and more of most 

 salts. Polar oceans therefore contain an abundance of oxygen for 

 respiratory needs and lack available lime, but animals in shallo^v 

 tropical waters often produce calcareous deposits and at times find 

 breathing diflicult. The rate of metabolism in poikilothermic 

 animals is influenced markedly by temperature. An aquatic ani- 

 mal which lives in a tropical puddle therefore may often lack 

 sufficient oxygen, especially at night when photosynthesis is not 

 replenishing that which is used up by the metabolic activities of 

 organisms. If such an animal gradually becomes adapted to breath- 

 ing air and is thus able to spend more or less time actually out of 

 water, it meets new dangers in the way of temperature changes, 

 for in any latitude such variations are wider and more rapid on 

 land than in water. Land temperatures, according to Johnstone 

 (1908), may vary between -90 °C. and -|- 65 °C.; sea temperatures, 

 -2.8 C. to -f- 31 C. This gives a range of 155 C. for land and of 

 33.8 C. for sea. Probably Johnstone's range for the ocean is too 

 limited, but there is no doubt that land temperatures are much 

 more variable than those in water. The specific heat of water is 

 great, and aquatic habitats therefore change temperatures slowly. 



Poikilothermic animals live longer at low temperatures, and this 

 fact in part accounts for the abundant populations on the bottoms 

 of some cold oceans and the preponderance of plankton organisms 

 in polar as compared to tropical seas. Oysters live shorter lives at 



