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

Abstract

The embryonic development of two ctenopods Sida crystallina and Diaphanosoma brachyurum has been investigated by observing living embryos removed from the female brood pouches. The sequence of morphological changes was analysed, as was the time at which the activity of certain organs began. The timing of these events at 21–22 °C is documented for both species. During embryogenesis four membranes are cast off. The growth in length of embryos 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 stages (= instars) demarcated by the shedding of membranes (= moults), as is commonly accepted for juvenile and mature animals in ctenopods and other crustacean groups. These instars are well distinguished morphologically. Their characteristics are presented. Differences in the duration of embryogenesis in Sida and Diaphanosoma are explained by greater extension of the first, and 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 features is given which may be used to determine the instar (and approximate age) of embryos in the broud pouch of ctenopod females from natural populations.