TOLERANCE OP HIGH TEMPERATURES BY FISHES, ETC. 



189 



Forbes and Kichardson (1908), Everman and Clark (1920), Pearse and Achten- 

 berg (1920), and Pearse (1921) agree that perch are to be regarded as typically lake 

 fishes, with a decided preference for deep, cool water. When obliged to desert the 

 deepest parts of a lake on account of summer stagnation, perch remain, for the most 

 part, at the bottom in the region of the thermocline (Pearse and Achtenberg, 1920). 

 It was not surprising, therefore, to find that normal perch had a low limit of temper- 

 ture tolerance and relatively little ability to become adjusted to higher temperatures. 

 Neither did it seem strange that such resistance as the perch did possess was very 

 slightly reduced by exposure to a temperature of 10° C. The large-mouthed black 

 bass, sunfish, and bluegill, on the contrary, are to be regarded as typically shallow- 

 water fishes (Pearse, 1921). They were collected in large numbers from waters 3 to 

 5 feet deep, in places where abundance of aquatic vegetation impeded circulation of 

 water, so that the temperature often was considerably higher than that of most of 

 the surface of the lake. Sunfishes, indeed, seldom are found in waters without vege- 

 tation (Pearse, 1921). The results in the present experiments seem to indicate a 

 correlation between the high temperatures occurring in the shallow portions of a 

 lake and a high degree of tolerance and modifiability possessed by the fishes that 

 usually inhabit these waters. 



It is doubtful, however, whether differences in tolerance and in capacity for 

 acclimatization are explainable wholly on ecological grounds. It is of interest to 

 note that bluegills and sunfishes (which are closely related in taxonomic position 

 and have similar but not identical habitat preferences) showed the same normal 

 limit of tolerance and practically the same degree of modifiability. Bass, on the other 

 hand, with habitat preferences quite similar to those of the other centrarchids, 

 appeared to have a limit of tolerance distinctly lower than that of the bluegills and 

 sunfishes, but a greater capacity for rapid increase in resistance by exposure to high 

 temperatures. Although all three species of shallow-water fishes suffered rapid 

 reduction in tolerance by exposure to low temperature in the laboratory, this change 

 might be expected to occur only very slowly in a state of nature because of the relative 

 slowness of fluctuation in the temperature of a lake. 



The conditions of life of toad tadpoles are somewhat different from those of 

 fishes. The shallow pools that they commonly inhabit present the possibilities of 

 wide diurnal fluctuations in temperature and of protracted chilling during periods 

 of cool, cloudy weather. As previously noted, the present experiments seem to 

 indicate that normal tadpoles have a very high tolerance limit, with moderate capacity 

 for further increase in resistance, while their loss of tolerance at 10° C. is relatively 

 slow. When it is recalled that, among fishes, high tolerance limits were subject to 

 rapid lowering by exposure to cold, the possession by tadpoles of a very high and 

 relatively stable degree of tolerance appears to be a distinct adjustment to the vari- 

 able temperature conditions under which they live. 



Regarding the physiological nature of the changes involved in acclimatization, 

 little that is new can be said at present. Evidence secured by Davenport and Castle 

 (1895) and by Loeb and Wasteneys (1912) indicates that resistance to high tempera- 

 tures gained by acclimatization is lost very slowly even at very low temperatures, 

 and several observations to the same effect were made on both fishes and tadpoles in 

 the course of the present work. The heavy breathing of fishes transferred to high 



