Inter- and Intraspecific Variation in Acute Thermal Tolerance of Daphnia

The acute thermal tolerances of six Daphnia species were compared at three acclimation temperatures. Of the species tested, all but one showed an increase in thermal tolerance when acclimated at higher temperature. Comparison of the regression relationships between thermal tolerances and acclimation temperature revealed no significant interspecific differences among species in their improvement of thermal tolerance caused by acclimation. However, significant intraspecific differences in thermal tolerance were noted. Daphnia obtusa had the highest tolerance, while D. pulex had the lowest. Among the other species acclimated at 10 C, thermal tolerance declined in a sequence of D. ambigua, D. magna, D. carinata, and D. nivalis. The differences in thermal tolerance noted among species were correlated with maximum temperatures of the environment from which they originated. While comparison of clones of D. pulex from several geographic localities revealed up to 2 C differences in acute thermal tolerance, there was no evidence of clonal variation in thermal tolerance of D. magna.