THERMAL TOLERANCE OF MOSQUITOFISH 791 



Except for those receiving a 1500-rad dose at day 15, the CTMs 

 for Gambiisia in our experiment were very consistent. Acclimation 

 temperatures can influence the CTM of Gambusia (Otto, 1973); 

 however, in our experiment the irradiated and control fish were 

 maintained at the same acclimation temperature. Thus the difference 

 in CTM values was attributed to radiation injury. 



Irradiation death of fish receiving doses less than 2000 rads is 

 usually attributed to damage to the hematopoietic tissues (Aoki, 

 1964; Aoki, Egami, and Arai, 1966; Egami and Etoh, 1966; Etoh, 

 1969; Cosgrove et al., 1975). An early and rapid depletion of 

 hematopoietic cells takes place in proportion to radiation dose, with 

 recovery from sublethal doses beginning in about 2 weeks. Depres- 

 sion of blood cells in the circulatory system reportedly lags behind 

 the loss of cells in the hematopoietic tissues (Cosgrove etal., 1975). 

 The period, 2 weeks after irradiation, appears to be a crucial time for 

 the survival of irradiated fish kept at about 25° C (Egami, 1969). Fish 

 that survive this period usually show regeneration of tissues and have 

 a good chance of recovering from radiation damage. 



An acute dose of 500 rads did not influence the CTM values of 

 Gambusia at 5, 15, or 30 days. Pathological studies of the 

 hematopoietic tissues support these results; no damage to the tissues 

 was observed at a dose of 500 rads. Atrophy of the hematopoietic 

 tissues occurred 5 days after a dose of 1500 rads, but regeneration of 

 the tissue had started by day 15 and continued at day 30. Only at 

 day 15 was the CTM influenced by radiation effects. Apparently 

 radiation lesions had not developed sufficiently after 5 days to 

 produce an effect, and by the end of 30 days regeneration of tissue 

 and recovery from radiation effects had taken place. 



Even doses of 3000 and 5000 rads produced no effect on the 

 CTM values 3 days after irradiation. At these dose levels the majority 

 of deaths are attributed to intestinal damage (Hyodo, 1964; Egami 

 and Etoh, 1966). The development of radiation lesions in fish is 

 temperature aiid time dependent, however, and apparently de- 

 velopment of the lesions had not progressed sufficiently to influence 

 CTM at 3 days. Mean survival time and CTM type tests were not 

 conducted at 15 and 30 days after irradiation for fish that had 

 received doses of 3000 and 5000 rads because insufficient numbers 

 of Gambusia survived. 



The effect of radiation on the temperature tolerance of Gam- 

 busia was more evident when mean survival time was used as a test. 

 Although the lethcd temperature obtained with the CTM type test 

 was affected only at day 15 postirradiation, the effect on the mean 

 survival time was expressed at both the 5- and 15-day intervals. 



