Effects of single exposures of X-radiation on survival were studied in Gammarus duebeni, G. salinus and G. zaddachi under constant conditions of temperature (150 C) and salinity (10 ‰). Effects on moulting were investigated in G. duebeni. The amphipods were irradiated with doses between 625 and 20,000 R. G. salinus and G. zaddachi are equally radioresistant (LD50/30 : 1,700 R) but less resistant than G. duebeni (LD50/30(males) : 3,900 R; LD50/30(females) : 3,500 R). At high doses, half of the test individuals die within a few days; at medium doses survival time is dose-independent; at lower doses survival time again increases with decreasing doses up to dose-ranges (below 1,250 R for G. duebeni, 1,000 R for G. salinus and G. zaddachi), at which only a few individuals die before their natural death. Subadult gammarids are less resistant;G. duebeni of 4 to 7 mm body length have a LD50/30 of 2,200 R. Death distribution after medium doses indicates that at least two mechanisms are involved in acute mortality. A first mortality maximum occurs shortly after irradiation; subsequently, mortality decreases for a few days, and is followed by another maximum. At lower doses no individual dies during the first critical period. Shortening of survival time at higher doses results from survival of fewer individuals to the second critical period. Few or no moults occur in G. duebeni irradiated with 20,000 R following the second day after irradiation and after the 30th day in those individuals irradiated with 2,500 to 10,000 R; moulting is delayed after exposure to 5,000 to 10,000 R. Individuals of G. duebeni taken from the field during the mild winter 1966/67 were more resistant to radiation, and moulting was less affected, than in laboratory-reared amphipods, or in those collected in the field during other seasons. Besides on irradiation dose, survival time of an individual depends on the time of its moulting in the course of an experiment. © 1969 Biologischen Anstalt Helgoland.
