The survival of embryos of the annual fish Nothobranchius guentheri exposed to temperature extremes and the subsequent effects on embryonic Diapause

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Abstract
Nothobranchius guentheri is an East African annual fish found in temporary bodies of water in areas of alternating rainy and dry seasons. During the dry season the embryos may be exposed to temperature extremes. Embryos have been reported to undergo diapause (developmental arrest), and are thus able to withstand the variable durations of the dry seasons. Since the embryos appear to endure long durations in diapause, and since in diapause they may be exposed to large variations in temperature, we have attempted to determine the thermal tolerance and responsiveness of embryos at diapause and at stages exclusive of diapause. Embryos were exposed to temperatures ranging from −8‐40°C for variable durations of time, and survival, abnormalities and developmental rates determined. Both Diapause I and Diapause II embryos were much less sensitive to temperature extremes than were the other stages, and Diapause II was much less sensitive than Diapause I. Abnormalities only occurred under certain temperature exposures but showed the highest frequency at the early stages of development. Short exposure of Diapause I embryos to −8 and 3.4°C eliminated Diapause II, while short exposure to 40°C enhanced its duration. Although it has been proposed that the onset and duration of the diapauses are denendent on the oxygen concentration of the water, we propose that temper ature may also be another imposrant factor.
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