Oogenesis and ultrastructure of the ovary in Neotrigonia margaritacea (Lamarck ) (Bivalvia, Mollusca)

The solitary mode of oogenesis in the Australian marine bivalve, Neotrigonia margaritacea, observed in specimens sampled over a 5-month period, is described. The structures of oocytes and gonad tissue are described for the first time in this genus, using electron microscopy and histology. The ovary was found to contain oocytes in various developmental stages throughout the study period. Oocytes develop from oogonia derived from protogonia and then undergo three distinct stages of oogenesis: previtellogenesis, vitellogenesis and postvitellogenesis with mature oocytes. Vitellogenic oocytes contain large and numerous rough endoplasmic reticulum and Golgi bodies in the cytoplasm, suggesting that autosynthetic vitellogenesis may occur in this species. Mature oocytes are large in size and have a thick vitellogenic layer. The vitellogenic layer could provide a barrier to polyspermy and has a single passage, a micropyle to permit sperm entry for fertilization. Based on gonad tissue and oocyte structure, we hypothesize that N. margaritacea can be considered as sequentially tachitictic, with an extended period of reproductive activity, using trickle (continuous) spawning over the summer season.