Influence of darkness on embryonic diapause termination in dormant Artemia cysts with no experience of desiccation.

One of the environmental cues for Artemia embryonic diapause termination (EDT), the nature of which is barely understood, is the absence of light, although light-dark cycles (photoperiod) have been examined in inducing embryonic diapause. The influence of constant darkness (DD) on EDT was investigated in laboratory-produced cysts of Artemia franciscana. In a previous report (Nambu et al., 2008. J Exp Zool 309A:17-24), we described a facilitated effect of pretreatment of DD (5-14 days) on EDT in Artemia cysts exposed to a standard rearing condition. In this article, the influence of pretreatment of DD (1-150 days) was investigated in detail by bioassay of hatching. It was found that all the DD conditions significantly promoted EDT. Either the plateau value of the percent hatch increased in response to each DD condition, or the rearing period to attain the half value of the plateau value became shorter. It was also found that DD (1-100 days) led to cyclic changes of EDT: the logistic functions of hatching were cyclically regulated and 30 days-DD cycles of relative values of their parameters were observed synchronously. The effect of DD-duration was not cumulative, and a couple of optimum conditions for higher EDT levels were obtained. The probability of Artemia's encountering DD and the possible meaning of cyclic regulation of EDT by DD in Artemia are discussed. J. Exp. Zool. 311A:182-188, 2009. (C) 2009 Wiley-Liss, Inc.