Ultrastructure of oogenesis in two tropical oviparous Demospongiae (Porifera): Cinachyrella apion and Tethya maza

Reproduction is a key step for propagation of any species. Consequently, gametogenesis is crucial, as it links one generation to the other. Oogenesis is influenced by different factors, but it is usually related to the quality and quantity of the food and the capacity of the female to convert these resources into egg production. In Demospongiae (Porifera), oocytes vary in several aspects (e.g., origin, size, and vitellogenic pathways). However, data on oocyte morphology is still fragmentary, and the ultrastructural organization of reproductive cells has been investigated only in a few species, mainly of viviparous sponges. Here, we aimed to comprehend the oogenesis of two tropical oviparous demosponges (Cinachyrella apion and Tethya maza) using light and electron microscopy. In both species, oocytes seemed to originate from archaeocytes. Oocytes of C. apion were surrounded by a collagenous matrix and nurse cells containing many lipid vesicles. The increase of biosynthetic organelles, concomitantly with the presence of yolk vesicle in the ooplasm, indicated that the vitellogenesis was carried out through the mixed pathway. The oocytes of T. maza were surrounded by a follicle cell membrane and nurse cells containing yolk vesicles. The absence of characteristic biosynthetic organelles in the egg of this species indicated that vitellogenesis occured through the heterosynthetic pathway. The oogenesis of C. apion is similar to other species of the genus, while the follicle membrane and nurse cells surrounding the oocytes of T. maza are not observed in any other species of Tethya. These accessory cells were considered to have a trophic role during the oogenesis of the studied species. Moreover, the presence of these accessory cells may have ecological significance, as they accelerate the egg's production through trophic support of the growing oocyte.