Spermatogenesis and ultrastructure of the spermatozoa of Seison nebaliae (Syndermata)

The spermatozoa of Seison nebaliae are characterized by an elongated sperm body, a filiform nucleus, and an anteriorly inserting external cilium with a 9x2+2 axoneme pattern. In the sperm body a frontal, middle, and hind region can be distinguished. The frontal region contains an acrosomal vesicle, a perforatorium, a basal body, and a pair of apical dense bodies; an accessory centriole is absent. The middle region is characterized by several so-called filamental plates. One large mitochondrion and one pair of accessory tubular structures are located in the middle and hind region. The hind region also contains two rows of dense bodies. Accessory tubular structures and filamental plates are autapomorphies of S. nebaliae. The shared appearance of the dense bodies in spermatozoa of species of the taxa Seison and the Acanthocephala founds their sister-group relationship, while the anterior insertion of the cilium in the spermatozoa of these taxa and in the Rotifera confirms the monophylum Syndermata Ahlrichs, 1995.