SPERM AND SPERMIOGENESIS IN OPISTHOTEUTHIS-PERSEPHONE (OCTOPODA, CIRRATA) - ULTRASTRUCTURE, COMPARISON WITH OTHER CEPHALOPODS AND EVOLUTIONARY SIGNIFICANCE

Ultrastructural features of spermiogenesis and mature spermatozoa are described for the first time in a cirrate octopod. Comparison with other investigated cephalopod species shows that spermatozoa of Opisthoteuthis persephone Berry most closely resemble those of the genus Octopus. The shared features include: (1) an elongate, solid acrosome with internal banding and a prominent helical keel; (2) a straight, rod-like nucleus; (3) a short midpiece and (4) a post-mitochondrial ('annular') skirt. Of these, the acrosomal morphology is most significant taxonomically, as the internal banding of the acrosomal vesicle only occurs in the Octopoda. Spermatozoa of Opisthoteuthis and Octopus differ however in the extent of the internal banding of the acrosome (poorly developed in Opisthoteuthis) and substructure of the acrosomal keel (more complex in Opisthoteuthis). Surprisingly, the extensive fibrous plug of Octopus and Vampyroteuthis sperm, is not developed either in Opisthoteuthis or in Eledone. Results provide additional support for the monophyly of the Octopoda but also hint at the possibility that some groups of incirrates may not be as advanced as generally believed.