Spermatogenesis and unique spermatozoa in the viviparous phoronid Phoronis embryolabi

Knowledge of gametogenesis and structure of mature gametes is important for understanding the reproductive biology of a species under study. Among phoronids, Phoronis embryolabi is distinguished by a most unusual type of development: larval viviparity. In the present work, we characterize spermatogenesis in this species using transmission and scanning electron microscopy. The results indicate that the development of male gametes occurs within the vasoperitoneal tissue. Early spermatocytes have a well-developed secretory apparatus comprising a Golgi body and endoplasmic reticulum. The Golgi body, which is retained until the late spermatid stage, produces acrosome material, thick glycocalyx, and membrane vesicles that are presumably necessary for the elongation of initially small cells into filiform mature spermatozoa. The germ cells retain their connections with each other until the late spermatid stage. The mature sperm of P. embryolabi can be regarded as an introsperm, which is characteristic of species with internal fertilization. The structure of mature sperm differs from that of other phoronid species, whose sperm can be described in terms of the form and location of organelles. P. embryolabi sperm differs in the location of the acrosome, in the presence of a collar around the base of the flagellum, and due to the close attachment of the flagellum to the non-flagellar part of the cell. Due to this attachment, a mature P. embryolabi spermatozoon is probably able become an undulating cell capable of movement in the densely packed inner space of an animal. The filiform shape of the mature sperm is likely correlated with the specificity of the P. embryolabi reproductive strategy, i.e. viviparity of larvae, when thousands of embryos, larvae, germ cells, and cells of vasoperitoneal tissue occupy the body cavity; thus, the spermatozoon has to squeeze in narrow spaces.