790 BLAYLOCK AND FRANK 



27°C, instead of 25°C, inadvertently maintained for a period of 1 

 week before the 15-day test. The effect of a different accHmation 

 temperature was also observed in the irradiated fish at the 1500-rad 

 dose level, in which the 15-day mean survival time was also 

 significantly greater than the 5- and 30-day mean survival time. The 

 acclimation temperature was returned to 25°C after the 15-day tests, 

 and the mean survival time of the controls at 30 days returned to a 

 level that was not significantly different from the controls at 5 days. 

 The tissues of three Gambusia irradiated with 500 or 1500 rads 

 were examined for pathological changes at 5, 15, and 30 days after 

 irradiation. No pathological effects were observed at any time period 

 at the 500-rad dose level. At 1500 rads after 5 days, atrophy of the 

 hematopoietic tissues and gonads was observed, but, as expected at 

 this dose level, no effects were observed on the intestinal epithelium. 

 At day 15 postirradiation, atrophy of the gonads was complete, and 

 some regeneration of the hematopoietic tissues was observed. At day 

 30 regeneration of the hematopoietic tissues was continuing. 



DISCUSSION 



Our experiment demonstrated by use of CTM and mean-survival- 

 time tests that acute doses of ionizing radiation significantly reduce 

 the thermal tolerance of Gambusia. These results are in opposition to 

 those of Ophel and Judd (1966), who reported that irradiation from 

 internal emitters increased the thermal tolerance of goldfish. Ophel 

 and Judd suggested that impairment of the heat-transfer properties 

 of the gills or reduction in blood circulation caused by irradiation 

 injury would result in a slower increase of body temperature, thereby 

 prolonging the mean survival time. 



In our experiment Gambusia suffered heat shock immediately 

 after being placed in a temperature bath at 39. 5° C, but they 

 recovered after a short time before succumbing to the lethal 

 temperature. After the initial recovery from heat shock, the fish 

 showed a great deal of variation in their activity, and this appeared to 

 influence their survival time. Very active fish died in a shorter time 

 than did fish that remained inactive until death. In the experiment of 

 Ophel and Judd (1966), a high body burden of ' ^ ' I or '^"Sr could 

 have possibly influenced the response of goldfish to lethal tempera- 

 ture; however, it was difficult to relate increase in mean survival time 

 of the fish to high doses of radiation. Conversely, the results of our 

 experiments, in which an acute dose of 1500 rads significantly 

 decreased the mean survival time, can be related to radiation effects. 



