Study of the late embryogenesis of Daphnia (Anomopoda, 'Cladocera', Branchiopoda) and a comparison of development in Anomopoda and Ctenopoda

The embryonic development of Daphnia galeata and D. hyalina ('Cladocera', Anomopoda, Daphniidae) has been investigated by observing living embryos removed from 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 22 degreesC is documented for both species. These data were compared with similar information, previously obtained for two representatives of the Ctenopoda (Kotov & Boikova, 1998). The sequence of events is basically similar in the two groups during early and late phases of their development, but the time of shedding of the embryonic membranes is different in the Anomopoda and Ctenopoda. The ctenopod embryo hatching from the second egg membrane is covered by the third membrane, which will be cast some hours later. The anomopod embryo hatches from the second egg membrane approximately simultaneously with the shedding of the third membrane, and it is covered already by the fourth membrane after the shedding of the second egg membrane. Earlier (Kotov & Boikova, 1998), we determined four embryonic instars in the course of the development of the Ctenopoda. Two of them are passed within the egg membranes, the next two instars occur after the shedding the egg membranes within the mother's brood pouch. However, in anomopods, one of the latter (the third) occurs within the second egg membrane, one is incorporated into the egg. Thus, the development of the Anomopoda is more embryonized in comparison with that of the Ctenopoda.