The effects of acute ionizing radiation on Daphnia Magna : a possible hormetic response?

The purpose of this study was to examine the effects of acute ionizing radiation on Daphnia magna. Third, fifth, and sixth instar Daphnia magna females were irradiated with acute doses of Co⁶⁰ gamma rays, ranging from 0 to 5000 rads, in order to examine the hermetic response—the hypothesis that low doses of radiation can exert positive or beneficial effects. Fifteen females, followed throughout their complete lifespans, were used at each dose. The effects of radiation were measured on the total number of offspring/female, total number of broods, individual and mean brood sizes, individual and mean longevity, and the occurrence of anomalies.

With the exception of sixth instar animals irradiated at 5000 rads, no experimental runs produced significant differences in the total number of offspring/female. The 5000 rad/sixth instar dose caused a greater than 50% reduction in young production when compared to all other doses. No significant differences were found in an analysis of the total number of broods and mean survival times. At doses of 100, 500, and 1000 rads, significant increases in mean brood sizes were found between the second and sixth broods in the third, fifth, and sixth instar studies. Maximum individual brood sizes equaled or sur passed the controls in all experiments, and trends toward a greater number of broods and survival time were observed in irradiated animals. Total abortions of eggs and young were more prevalent in irradiated females, especially at the highest dose level.

The results of the present study, therefore, enhance the concept of a hormetic response, but also indicate that the highest dose may have exceeded a critical radiation level for reproductive fitness in Daphnia magna.