Spermiogenesis in the curimatid species, Steindachnerina insculpta, Cyphocharax gillii, C. modestus, C. spilotus, and Potamorhina altamazonica, is characterized by lateral development of the flagellum, nuclear rotation, eccentric nuclear fossa formation, and chromatin compacted into thick fibers. These spermatozoa exhibit a spherical head containing a nucleus with the chromatin highly condensed into thick fibers with small electron-lucent areas, and no acrosome. The nuclear fossa is of the moderate type and eccentric and penetrated by the centriolar complex. The midpiece is small, has many elongate vesicles, and a short cytoplasmic channel. Mitochondria may be elongate, branched or C-shaped, and are separated from the initial segment of the axoneme by the cytoplasmic channel. The flagellum contains the classical axoneme structure (9+2) and has a membranous compartment in the initial region; it does not have lateral fins. Only small differences were observed among the analyzed species and genera of the Curimatidae. Spermiogenesis and spermatozoa in the Curimatidae have many of the characteristics found in almost all other characiform species. On the other hand, the presence of a membranous compartment in the initial region of curimatid flagella, a structure common in many Cypriniformes spermatozoa, is unknown in other characiforms. Spermiogenesis and the spermatozoa of the Characiformes are discussed.
fish; spermatid; sperm; ultrastructure
