GENETIC AND PHYSIOLOGICAL FLEXIBILITY 463 



As Heinle (1978) pointed out, however, not much work has been 

 done on zooplankton. There are several comprehensive reviews that 

 include discussions of acclimation, e.g., Kinne (1970), Vernberg and 

 Vernberg (1972), and Gilles (1975). Acclimation has been demon- 

 strated in several ways, at the molecular level (Baldwin and 

 Hochachka, 1970; Bowler, 1963), as change in respiration rate 

 (Fitzpatrick and Atebara, 1974), as change in respiration and heart 

 rates (Markel, 1974), as change in other rate functions or develop- 

 ment times (Landry, 1975; Bernard, 1970), and simply as survival 

 (McLeese, 1956). As mentioned earlier, the temperature at which the 

 animal entered a heat coma was used by Hamby (1975) as a criterion 

 to show acclimation. 



Not much work has been done on acclimation to cold tempera- 

 tures. McLaren (1966) found that the constant describing a 

 "biological zero" in Belehradek's equation (which is used to describe 

 the relationship between the rate of a process and temperature) 

 varied less than 1°C among populations of copepod species over a 

 wide range of temperature environments. From this and other studies 

 on acclimation to high temperatures, Heinle (1978) speculated that 

 adaptation to extreme low temperatures may be primarily genetic 

 and adaptation to high temperatures may be both genetic and 

 nongenetic. Variation between populations may have little to do 

 with variations within populations, however. The work reported here 

 (and some not reported) suggests that variation in heat and cold 

 tolerance within populations of Eurytemora, and hence adaptation 

 to both extremes of temperamre, can be both genetic and non- 

 genetic, the relative importance depending on the sex of the animals. 



The sexual dimorphism in genetic variance in high-temperature 

 tolerance (Table 1) has some precedence in other copepod traits 

 studied by McLaren (1976). He found that females had significantly 

 higher heritabihty of age at maturity (at 15°C) than males and that 

 males had a higher heritability of adult size (also at 15°C) than 

 females. Heritabilities of these characteristics were not significant in 

 either sex at 10 and 12.5°C. McLaren attributed this to natural 

 selection occurring at these "normal" temperatures. Enfield, 

 Hartung, and Hefeneider (1975) reported differential gene expression 

 in pupal weight in male and female Tribolium and cited other 

 examples of sexual dimorphism. 



The importance of sexual dimorphism in both genetic variance 

 and physiological flexibility (Tables 2 and 3) in the adaptation of 

 Eurytemora to variable temperatures is not obvious. It suggests that 

 males are more vulnerable in the short term. If females stored sperm 

 and produced multiple egg sacs per mating, males might be more 



