ИСТИНА

In scyphozoans a polyp forms medusa larvae (ephyrae) by the process called strobilation. We aimed to make a comparative analysis of the strobilation in two scyphozoans: Aurelia aurita (Semaeostomeae) with the polydisk strobilation and Cassiopea sp. (Rhizostomeae) with the monodisk strobilation. We already made a time-table of the ephyra development for Aurelia. To obtain the data for comparative analysis we created a time-table of Cassiopea strobilation. We characterized the cellular bases of the strobilation in this species. We revealed a set of events which are independent of the type of strobilation but have some species-specific features. These events are: the demarcation of the ephyra anlage, the enlargement of the ephyra disk, the morphogenesis of the ephyra margin, the development of the ephyra gastric system and the detachment of the ephyra. The mechanism of the demarcation seems to be similar in both species: the cell layers invaginate to make a transverse circular constriction between the future ephyra and a basal part of the polyp. In both species the enlargement of the ephyra is based on cell shape changes and epithelial cell intercalation. The exact role of cell proliferation is not clear. However, in Aurelia we detected very intense proliferation at the earliest stages of the ephyra formation. The morphogenesis of the ephyra margin is different in Aurelia and Cassiopea. Aurelia forms stare-shaped ephyra by the gradual development of eight long lappets. In Cassiopea, which forms roundish ephyra with multiple small lappets, this morphogenesis is relatively simple. There are differences in the ephyrae gastric system development. The Cassiopea ephyra forms well developed manubrium and gastric cavity with branches long before separation, while the detached ephyra of Aurelia has only one gastric pouch and poorly developed manubrium. In general, the newly detached Cassiopea ephyra seems to be more developed than the Aurelia ephyra.