C. L. PROSSER 36] 



sulation in hair or subcutaneous fat. Thus, short-term regulation 

 and long-term acclimation are accomplished by very different 

 mechanisms. Long-term changes are often morphological — tad- 

 poles have larger gills when reared in low oxygen, certain mos- 

 quito larvae have larger anal papillae in freshwater than in salt 

 water. There may also be behavioral changes, such as aggregation 

 by hive bees in winter. 



In addition, many physiological and biochemical adaptations 

 are known. Lungfish in estivation shift their products of nitrogen 

 excretion to urea from the ammonia they excrete when actively 

 swimming. Daphnia and Artemia reared in low oxygen develop 

 high levels of hemoglobin whereas in high oxygen their hemo- 

 lymph is colorless. Also, in low oxygen there is an increase in 

 tissue cytochrome (Fox, 1955). The amount of "free" water di- 

 minishes in cold-hardened insects and molluscs. 



We have referred above to the increased metabolic rate in 

 aquatic poikilotherms acclimated to cold either seasonally or 

 latitudinally. That this change is at least partly cellular is shown 

 by the fact that tissue slices from certain organs of cold-acclimated 

 fish and crustaceans consume more oxygen than do those from 

 warm-water individuals (Piess and Field, 1950; Freeman, 1950; 

 Roberts, 1953). 



It appears likely, therefore, that environmental stresses can re- 

 sult in biochemical adaptations at the cellular level. The enzymes 

 of thermophilic bacteria show higher temperatures of inactivation 

 than corresponding enzymes of mesophilic bacteria, and the in- 

 activation temperature differs for different enzymes from the 

 same organism; these differences are clearly genetic. However, 

 Precht and his associates ( Christophersen and Precht, 1950, 1951) 

 report that the highest temperature of activity in yeast measured 

 by oxygen consumption and by methylene blue reduction is 

 higher (65° C.) for yeast reared at 41° C. than it is (60° C.) for 

 yeast reared at 20° C.; also the Qo 2 of 20° yeast is higher than 

 that of 41° yeast when measured at the same temperature. The 

 experiments with yeast agree with data from poikilothermic 

 animals that acclimation temperature may change the upper 

 limiting temperature, the absolute level of oxygen consumption 



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