Effects of lifetime exposure to chronic irradiation on reproductive success were assessed for laboratory populations of Neanthes arenaceodentata. Exposure was initiated upon the spawning of the parental ( P1) female and was terminated upon spawning of the first filial ( F1) generation female; broods from the F1 pairs were sacrificed before hatching occurred. Groups of worms in the experiments received either no radiation (controls) or 0.19, 2.1 or$17\ {\rm mGy}\ {\rm h}^{-1}$. The total dose received was either background or approximately 0.55, 6.5 or 54 Gy, respectively. The mean number of embryos in the broods from the F1 females exposed to$17\ {\rm mGy}\ {\rm h}^{-1}$ was statistically significantly different from the mean number of embryos from control females; however, the mean number of embryos in the broods from the F1 females exposed to 0.19 and$2.1\ {\rm mGy}\ {\rm h}^{-1}$ was not significantly different from the mean number from control females. For all the radiation-exposed groups, there was a statistically significant reduction in the number and percentage of live embryos in the broods from the F1 pairs as well as a statistically significant increase in the numbers and percentages of abnormal embryos. Results on embryo abnormalities and mortalities indicate that dominant- and recessive-lethal mutations were most likely induced in the germ cells and that these mutations had an adverse effect on reproductive success by affecting the survival of early-life stages. Except for pairs exposed to$17\ {\rm mGy}\ {\rm h}^{-1}$, there was no evidence of gemete killing or reduced fertilization success, because the number of developing embryos in the broods did not decrease with increased dose. Data for the estimated hatch number and actual hatch number indicated that doses as low as$0.19\ {\rm mGy}\ {\rm h}^{-1}$ can reduce significantly the number of larvae that hatch when lifetime doses are given.

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