Long-term irradiation of a marine fish, the plaice Pleuronectes platessa: an assessment of the effects on size and composition of the testes and of possible genotoxic changes in peripheral erythrocytes

Abstract

Purpose: Previous investigation showed the very significant effects of chronic gamma -radiation on plaice testes at mean absorbed dose rates as low as 1.3mGyh-1 given over a period of 168 days (accumulated dose 4.7Gy). The present paper examines the effects on the testes of exposure to even lower dose rates of gamma -radiation given over periods of 73 days or 197 days. In addition, the use of micronucleus counts and flow-cytometric measurement of nuclear DNA content in samples of peripheral blood for monitoring genotoxic effects has been assessed. Materials and methods: In Experiment 1, adult male plaice were exposed at mean absorbed dose rates of 0.25, 0.5 or 1.2mGyh-1 for 73 days (mean accumulated doses of 0.43, 0.85 and 2.03Gy, respectively) and in Experiment 2 to 0.24, 0.5 or 1.0mGyh-1 for 197 days (mean accumulated doses of 1.07, 2.24 and 4.57Gy, respectively). At termination the testes were removed, weighed and sections were prepared and examined histometrically. In addition, in Experiment 2, blood samples were taken during exposure and at termination. Blood smears were scored for micronuclei and samples processed and examined for nuclear DNA content by flow cytometry. Results: Significant reductions in testis weight were seen in all radiation groups after 197 days of exposure, which were predominantly due to decreased amounts of sperm. In plaice killed after 73 days (at an earlier stage of spermatogenesis), there were no significant differences in weight compared with controls but amounts of spermatogonia were significantly reduced in irradiated fish. Conclusions: Exposure to dose rates as low as 0.24mGyh-1 of gamma -radiation given over a period of 197 days significantly reduced the weights of plaice testes, this being consequent on reductions in the amounts of sperm. Although there was some evidence of radiation affecting the numbers of spermatogonia it was not possible to determine the primary target for radiation damage which eventually caused the sperm reductions. Along with the related work described by Greenwood and Knowles (1996) this is the first investigation of a marine fish and it indicates that plaice testes are probably more radiosensitive than those previously described in tropical fish and of a similar radiosensitivity to mammalian testes. Although significant effects were observed after the lowest dose rate used of 0.24mGyh-1, this is still a factor of about 400 times greater than the estimated absorbed dose rate to plaice testes in the north-east Irish Sea off Sellafield at the present time. Micronucleus counts and flow-cytometric analysis of blood DNA both failed to show any evidence of genotoxic damage.