Study of the embryogenesis of Dunhevedia crassa King, 1853 (Cladocera: Chydoridae) and a comparison of embryonic instar durations in different cladocerans

The aim of this work is to study the embryonic development of a common tropical cladoceran Dunhevedia crassa King, 1853 (Anomopoda:Chydoridae) and compare the durations of embryonic instars and growth during embryogenesis in small-sized chydorids and other cladocerans. The sequence of events in the embryogenesis of Dunhevedia females was studied by means of in vitro observation in the laboratory at 28.5°C. We found that the general pattern of the sequence of events in chydorid embryogenesis is similar to that in other anomopods. However, the relative durations of the embryonic instars I and II+III are markedly short, while that of instar IV is long (delayed) when compared with other cladocerans. The observed pattern of ontogenesis in chydorids may therefore be apomorphic within the cladocerans and could reflect a neotenic evolution. Another peculiar trait of Dunhevedia is the very small increase of the size of the embryo during embryogenesis, which is correlated with a very large size of the egg relative to the mother. Therefore, our study revealed some peculiarities in the ontogenesis of small-sized chydorids, which could be a result of family-specific evolution.