GENETIC AND PHYSIOLOGICAL FLEXIBILITY 453 



between individuals) levels which ought to allow further adaptation 

 when necessary. 



This paper presents and discusses data on genetic and physiologi- 

 cal flexibility in temperature tolerance and on the range of 

 adaptation of individuals, in the context of widening the range of 

 temperatures by adding waste heat from electric-power generating 

 plants. Practical measurements of (1) the temperature tolerance of 

 individuals, (2) the genetic variance in temperature tolerance, and (3) 

 the physiological variance in temperature tolerance, at least in 

 organisms that can be cultured in the laboratory, are also described. 



METHODS 



Eurytemora affinis has a short life cycle (12 days at 20°C), 

 always reproduces sexually, has clearly distinguishable sexes, and can 

 be cultured in the laboratory in filtered bay water at to 7%o 

 salinity with a mixture of several species of algae for food. Stock 

 cultures were routinely kept in 3-liter flasks; matings were set up 

 in shell vials; and individual broods were reared in 125-ml flasks. The 

 animals used were collected from Bear Creek near Baltimore Harbor 

 with a No. 10 plankton net. Each experiment included only animals 

 from a single collection, so that seasonal differences did not 

 contribute to measured genetic variability. 



Temperature tolerances of individual copepods were measured as 

 the time to inactivity or to succumb (TS) to a temperature shock. 

 Individual animals were placed in 2 ml of water in vials in racks of 12 

 and partially immersed into an aquarium initially at 32°C. In the 

 experiments measuring genetic variance, the temperature was raised 

 0.5°C each 5 min aiid held constant between adjustments of the 

 thermostatically controlled heating— stirring unit. Vials containing an 

 animal that had fallen to the bottom were removed, and the animals 

 were allowed to recover. The tests took a maximum of 0.5 hr, and 

 different broods were equally represented, as much as possible, in 

 each test. The few animals not inactivated after 30 min were assigned 

 the maximum score (30). In the experiments measuring physiological 

 variance by acclimation, the temperature was held constant (34.5°C) 

 throughout the 30-min test; the animals remained in the aquarium; 

 and the time to recover (TR) was also noted. Tolerance in this case 

 was measured as an index (30 + TS — TR) having a maximum value 

 of 60 for animals that did not succumb at all. Although they 

 obviously have different variances, the two measures of temperature 

 tolerance are closely related since TR actually adds little information 

 because of its high negative correlation with TS. Both the index and 



