The Variable Effects of Ambient and Artificial Light:Dark Cycles On Embryonic Diapause in A Laboratory Population of the Annual Fish Nothobranchius Guentheri

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ABSTRACT The effects of light:dark cycles (L;D) on embryonic development in a laboratory population of the East African annual fish Nothobranchius guentheri were studied. Under ambient light conditions (40° N) there was a low frequency of embryos entering diapause between June and October. Beginning in November there was an increasing frequency of diapausing embryos with a peak in December, and a lower frequency by February. Under artificial light conditions there was an increasing frequency of diapausing embryos as the L:D changed from 16:8 to 9:15. When individual fish or groups of fish were followed it was found that, even under the same light conditions, variable frequencies of diapausing and non-diapausing embryos were produced and that the frequencies often changed with time. The L: D cycle under which the embryos were incubated had no effect on diapause. As in some species of insects the ‘diapause factor’ was of maternal origin. The ability of the fish to produce both diapausing and non-diapausing embryos under the same and variable L : D is most likely an adaptive trait related to the survival of the fish in the harsh environments of alternating rainy and dry seasons.