GENETIC AND PHYSIOLOGICAL FLEXIBILITY 459 



stress, both genetically and physiologically. Of course, physiological 

 potential may well depend on the animal's genotype. 



As suggested earlier, in the context of adaptation to thermal 

 stress, a more appropriate measurement of physiological variability 

 may be the variance in temperature tolerance between different 

 temperature treatments (the third method in Appendix B). 



Some of the data on which the latter estimates of physiological 

 variance are based were reported elsewhere (Bradley, 1978a). 

 Variance components are shown separately between 10°C and the 18 

 and 24°C classes combined, between the 18 and 24°C classes, and 

 among all three classes. In every case the variance in mean tolerances 

 of female progeny in the different classes is greater than that in the 

 means of male progeny. The critical range of temperature seems to 

 be 18 to 24°C; every variance between 18 and 24°C is greater than 

 that between 10 and 18 + 24°C, except that for male progeny at 7 

 days. Acclimation, or physiological adjustment to an extreme 

 temperature, seems to be virtually complete by 4 days or earlier. This 

 conclusion can also be drawn from the quite different measurement 

 of acclimation to high temperatures shown in Fig. 1. Thus, on the 

 basis of both variances among exposure temperatures and mean 

 tolerances after exposure to different temperatures, Eurytemora, and 

 especially Eurytemora females, can adapt to thermal stress to a 

 significant extent at the individual level. 



Analogous experiments were done on tolerance to cold tempera- 

 ture, but the emphasis here is on high-temperature tolerance since 

 Eurytemora affinis, a north-temperate species, is probably under 

 greater stress at the warm end of the temperature range. Similar 

 methods were used to measure physiological flexibility in cold 

 tolerance; the results are shown in Table 3 (variances) and Fig. 2 

 (mean tolerances). Females seem to have greater physiological 

 flexibility in cold tolerance as well as heat tolerance. The sizes of the 

 components of variance between the 10 and 15°C classes indicate 

 that little acclimation to low temperature (1.5°C) occurs in this 

 range. 



The discussion of adaptation to increased temperatures from 

 thermal addition raises the further question of the relationship 

 between heat tolerance and cold tolerance. In one set of experi- 

 ments, 24 copepods were tested for heat tolerance and then for cold 

 tolerance, or vice versa, with a lapse of 3 hr to allow recovery. Each 

 animal was assayed twice, once for heat and once for cold, each day. 

 The three correlations among heat and cold tolerances averaged 0.5, 

 which was comparable to the cold— cold retest correlation (0.6) and 

 the heat— heat retest correlation (0.5). However, in other experi- 



