The critical stage for inducing oviparity and embryonic diapause in parthenogenetic Artemia (Crustacea: Anostraca): an experimental study

Abstract

The brine shrimp Artemia exhibits two reproductive modes: 1) oviparity, producing diapause embryos; and 2) ovoviviparity, producing free-swimming nauplii. Previous studies have suggested the existence of a critical stage that determines the reproductive mode. Physicochemical factors, such as photoperiod, temperature, and salinity, have been suggested to irreversibly affect the reproductive mode of oocytes during this critical stage. In this study, experiments were carried out using a photoperiod and temperature-sensitive parthenogenetic Artemia clone where maternal Artemia were shifted bidirectionally between ovoviviparity (18 h L:6 h D, 27°C) and oviparity (6 h L:18 h D, 19°C) culture conditions. In the main experiment (Artemia shifted at six different stages including the post-larva II to adult II), the reproductive mode of first brood was converted when shifting was performed on post-larva II and III but was not converted when females were shifted after post-larva III. A supplementary experiment further revealed that the reproductive mode of first brood could be altered when shifting females at an “early phase of post-larva IV”, characterized by a developing ovisac reaching the middle of the third abdominal segment, ventral spines, and some oocytes growing larger than the others. In both experiments, reproductive modes of the second brood were significantly affected when the shifting was performed on post-larva IV. These results suggest that the critical stage for inducing oviparity and embryonic diapause is at the previtellogenic stage of oocytes, or at maternal “early phase of post-larva IV” for the first-brood offspring. During this stage, differential gene expression patterns of the two destined oocytes may be triggered by the token stimuli signals received by the oocytes.