ULTRASTRUCTURE OF THE EMBRYONIC DEVELOPMENT AND METAMORPHOSIS OF RHOPILEMA ECULENTUM

This study investigated cell ultrastructure of Rhopilema esculentum during embryogesis and metamorphosis into a polyp using scanning and transmission electron microscopy combined with protargol staining method. Our study showed that (1) Cell cleavage was almost equal from fertilized egg to gastrula. All divided cells were similar in morphology, and cell junctions were observed among adjacent cells; (2) Vacuoles in cell were firstly observed at ectoderm of early planula, and became bigger as developing to 4-tentacle polyp, while the vacuoles of entoderm were firstly found at 4-tentacle polyp stage. The suspected apoptotic body were observed at scyphostoma and polyp stages; (3) Immature cnidocytes were firstly observed in ectodermal area closed to mesoglea ectoderm at early planula stage, moved to outside, located at outermost layer, and matured at polyp stage; (4) Cilium was observed at early planula stage, gradually degraded, and completely disappeared at polyp stage; (5) Lots of yolk bodies with dark stain were observed at all developmental stages, and the phagocytosis of ectodermal cell of planula was firstly observed. These phenomena suggest that planulae of R. esculentum were both planktotrophic and lecithotrophic, which would provide the scientific basis for illuminating the developmental pattern of embryonic and metamorphosis of cnidarians.