An ultrastructural study of oogenesis in a planktonic and a direct-developing species of Siphonaria (Gastropoda: Pulmonata)

Oogenesis and vitellogenesis were compared at an ultrastructural level in Siphonaria capensis (a planktonic developer) and S. serrata (a direct developer). Except for some months in winter, most stages of oogenesis were observed during the year within a gonad, although oogenesis was asynchronous between the gonad acini. Previtellogenic oocytes, which contained few organelles, were surrounded by follicle cells. During vitellogenesis three types of storage products were accumulated in the ooplasm: yolk, lipid and glycogen. In S. capensis yolk formation begun before lipid synthesis and the yolk was produced autosynthetically. By contrast in S. serrata lipid was deposited before yolk synthesis. Morphological evidence (production of yolk by Golgi bodies and rough endoplasmic reticulum; endocytotic pits along the oolemma) was found for yolk formation by both auto and heterosynthesis. In both species as the oocytes grew the follicle cells became hypertrophic and then gradually withdrew from the oocytes. Results from this study add further support to the suggestion that siphonariid limpets had a marine ancestry.