Spermatogenesis in the carnivorous sponge Lycopodina hypogea (Porifera, Demospongiae)

Carnivorous sponges (family Cladorhizidae) lack the aquiferous system and choanocytes and, therefore, their reproduction and in particular spermatogenesis is unusual for Porifera. We studied spermatogenesis in a carnivorous sponge Lycopodina hypogea using confocal microscopy, SEM and TEM. In brief, spermatogenesis in L. hypogea proceeds as follows. Male cells derive from archaeocyte-like cells. During its first divisions, the spermatogonia are surrounded by a thin follicle made up by a single cell. The spermatogonia divide mitotically, giving rise to next generation of spermatogonia, primary spermatocytes and secondary spermatocytes. Spermatozoa of L. hypogea are long and narrow mono-flagellated cells, tightly packed inside the spermatic cyst. They are unusual for sponges, being dart-shaped, with an anterior filament, which is an electron-dense, rod-like outgrowth terminating in an acrosome. The flagellum is long, and its proximal region is located within a cytoplasmic tunnel. In the course of spermiogenesis, a secondary envelope made up by several cells tightly intertwined by their pseudopodia is formed around the first unicellular envelope. Symbiotic bacteria are usually present between the sperm cells. During maturation, the sperm cyst, which may be called the spermatophore, migrates from the sponge body to the filaments and acquires two bundles of forceps spicules. Mature spermatophores are released from the filaments. To sum up, we showed that spermatogenesis and fertilization in L. hypogea are strikingly different from these processes in other sponges. The differences mostly concern the origin of male germ cells, the structure of mature spermatozoa and the formation of spermatophores.