A Thermal Race of Daphnia atkinsoni Baird, and its Distributional Significance

Abstract

Experiments were conducted to determine the temperature at which 50% mortality occurred on 24 hr. exposures. For specimens of the low-temperature race obtained from Berrylands, Surrey, England, this lethal point was about 26.8oC. In all other races of species of Daphnia investigated the lethal point was over 30[degree]C , and for a "normal" race of D. atkinsoni occurring at Godshill, Hampshire, it was 32.9[plus or minus]0.210.