Comparative analysis of the late embryogenesis of Sida crystallina (O.F. Muller, 1776) and Diaphanosoma brachyurum (Lievin, 1848) (Crustacea : Branchiopoda : Ctenopoda)

The embryonic development of two ctenopods Sida crystallina and Diaphanosom a branchyurum has been investigated by observing living embryos removed fro m the female brood pouches. The sequence of morphological changes was analy sed, as was the time at which the activity of certain organs began. The tim ing of these events at 21-22 degrees C is documented for both species. Duri ng embryogenesis four membranes are cast off. The growth in length of embry os began only after the shedding of the external egg envelope. Growth rates throughout embryogenesis are documented. A new outline of ctenopod embryogenesis is proposed. It includes four stage s (= instars) demarcated by the shedding of membranes (= moults), as is com monly accepted for juvenile and mature animals in ctenopods and other crust acean groups. These instars are well distinguished morphologically. Their c haracteristics are presented. Differences in the duration of embryogenesis in Sida and Diaphanosoma are explained by greater extension of the first, a nd especially the fourth, instars of Sida, while the duration of the second and third instars are approximately the same in both. A list of instar fea tures is given which may be used to determine the instar (and approximate a ge) of embryos in the brood pouch of ctenopod females from natural populati ons.