Oogenesis and larval development of Ephydatia fluviatilis (Porifera,Spongillidae)

Summary In Ephydatia fluviatilis young oocytes already appear in autumn. They pass the winter in the highly reduced sponge, but vitellogenesis and further development do not take place before following spring. The fact that the young oocytes appear before the normal period of reproduction makes E. fluviatilis different from all other local freshwater sponges, which reduce totally in autumn. E. fluviatilis seems to be a gonochorist. The oocytes originate from archaeocytes and during the first growth phase they reach a diameter of approximately 40 m. In the second growth phase the oocyte is enclosed in a single-layered follicle epithelium and grows to 170–180 m by phagocytosis of trophocytes. The fully developed egg cell finally shows a distinct layering of the incorporated yolk material. Cleavage is totally equal to unequal so that macro- and micromeres appear in some cleavage stages. Cleavage leads to a solid embryo consisting of uniform cells. At this stage of development the first scleroblasts appear. As the cells develop they are surrounded by companion cells, managing the transport of the scleroblasts. The further development to the larva is marked by the appearance of the larval cavity, typical for larvae of Spongillids, which finally occupies about half the volume of the larva at emergence. The periphery of the larva consists of a single-layered ciliated epithelium. After emergence the larva forms flagellated chambers, which are integrated into the primordia of the excurrent canal system. This system connects with the larval cavity and ensures that it becomes part of the excurrent canal system of the young sponge. Particularly in the region of the larval cavity the ciliated epithelium of the free larva is reduced. Here a new larval surface epithelium is formed by pinacocytes.