. The Biological bulletin. Biology; Zoology; Marine biology. 186 R. G. STROSS AND J. C. HILL The first stage of diapause development is refractory and may require an interval of low temperature ( 4° C.) for completion. The second stage is light-demand- ing ( positively photoblastic) and diapause release follows only after an appro- priate exposure to white light. Previous reports of winter diapause have shown an extensive refractory period requiring low temperature (Vollmer, 1912; Stross, 1966) but no need of light to permit diapause release. That light influences embryos in winter di
. The Biological bulletin. Biology; Zoology; Marine biology. 186 R. G. STROSS AND J. C. HILL The first stage of diapause development is refractory and may require an interval of low temperature ( 4° C.) for completion. The second stage is light-demand- ing ( positively photoblastic) and diapause release follows only after an appro- priate exposure to white light. Previous reports of winter diapause have shown an extensive refractory period requiring low temperature (Vollmer, 1912; Stross, 1966) but no need of light to permit diapause release. That light influences embryos in winter diapause is indicated through several lines of evidence. When diapause is terminated through a shock treatment, such as with sodium hypo- chlorite, exposure to light is essential, as it is when embryos which are held in prolonged diapause, through storage in a stagnant medium in constant dark, are transferred to fresh medium at a higher temperature (, 12° and 20° C). The holding action of a stagnant medium and constant dark is nullified if the embryos are exposed to light. Also, there is some indication, to be explored below, that a short-day photoperiod may delay slightly the termination of diapause in embryos provided the medium is sufficiently well aerated. It is the purpose of the following to describe the influence of light and temperature on the termination of winter diapause in both the laboratory and Paul Lake. LAKE f < ojlilii::;;: 80-1 70- 60- Q- 50-1 40- 30' 5 3 20-1 Z ?jjiiJJjJjjjitijititiip'iA pause 10- f;=:::^-=-^-i-;-iit=:=iv;f L SEPT OCT NOV DEC u LABORATORY TERMINATION. ? |< PARTHENOGENESIS 1 I I L FEB MARCH APRIL MAY JUNE JULY AUG Figure S. Life-cycle of Daphnia pulex and {D. rosea) in Paul Lake. Sexual reproduc- tion begins in early autumn and the resulting embryos remain in diapause until late March or early April of the year following. Termination of diapause proceeds in constant dark (solid line) and apparently at a somewhat different rate than
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