GENETIC AND PHYSIOLOGICAL FLEXIBILITY 

 OF A CALANOID COPEPOD 

 IN THERMAL STRESS 



BRIAN P. BRADLEY 



Department of Biological Sciences, University of Maryland, Catonsville, 



Maryland 



ABSTRACT 



The copepod Eurytemora affinis has considerable potential for both physiologi- 

 cal and genetic adaptation to temperature stress. Physiological variance was 

 expressed more in females, and genetic variance was expressed more in males. 

 Estimates of both variances were obtained by several methods, all giving 

 essentially the same results. The assay used to measure temperature tolerance 

 depended on time to enter a coma after a temperature shock. Information was 

 obtained on individuals that could later be used for breeding or retested. The 

 assay was short term (30 min maximum) but was closely related to long-term 

 survival in high temperatures. On the basis of field observations and laboratory 

 experiments in cycling temperatures, the copepod populations seem to be at or 

 near their limit of survival in summer temperatures in the Chesapeake Bay. This 

 raises the question of whether all the genetic and physiological flexibility 

 observed is usable in further adaptation or whether reproductive capacity, for 

 example, becomes limiting. 



The copepod Eurytemora affinis is present throughout the year m 

 the Chesapeake Bay and tributaries but is most abundant in winter 

 and e£irly spring. Since no resting stages have been found, the 

 free-swimming adults and preadults are adapted to the entire range of 

 environmental variables, such as temperature and salinity. 



Temperatures in the bay range between and 30° C. Above 26° C, 

 the species seems to be under considerable stress, judging by the very 

 low densities found. On the other hand, there is considerable 

 flexibility in temperature tolerance at both individual (physiological 

 differences within individuals) and population (genetic differences 



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