Acta Zoologica (Stockholm) 87: 305– 313 ( October 2006) © 2006 The Authors Journal compilation © 2006 The Royal Swedish Academy of Sciences  Abstract Veríssimo-Silveira R., Gusmão-Pompiani P., Vicentini C. A., Quagio-Grassiotto I. 2006. Spermiogenesis and spermatozoa ultrastructure in Salminus and Brycon, two primitive genera in Characidae (Teleostei: Ostariophysi: Characiformes). — Acta Zoologica (Stockholm) 87: 305–313 In Salminus, spermiogenesis is cystic and gives origin to a type I aquasperm. Spermatid differentiation is characterized by chromatin condensed into thick fibres, nuclear rotation, nuclear fossa formation, cytoplasmic channel formation, mitochondrial fusion producing long and ramified mitochondria, and the presence of several membranous concentric rings around the plasma membrane that encircles the cytoplasmic channel. In Salminus and Brycon, spermatozoa are very similar. They exhibit a spherical nucleus and chromatin condensed into fibre clusters, and a deep nuclear fossa.They show a long midpiece with few elongate mitochondria at the initial region and a cytoplasmic channel completely encircled by one or two membranous concentric rings. The flagellar axis is perpendicular to the nucleus and exhibits the classic axoneme (9 + 2). The very strong similarity observed between Salminus and Brycon spermatozoa supports the hypothesis that these subfamilies are likely to have a monophyletic origin. Irani Quagio-Grassiotto, Departamento de Morfologia, IB, Unesp, Botucatu, SP, Brasil, CP 510, CEP 18618-000. E-mail: morfologia@ibb.unesp.br Blackwell Publishing Ltd Spermiogenesis and spermatozoa ultrastructure in Salminus and Brycon, two primitive genera in Characidae (Teleostei: Ostariophysi: Characiformes) R. Veríssimo-Silveira, 1,4 P. Gusmão-Pompiani, 2 C. A. Vicentini 3 and I. Quagio-Grassiotto 4 1 Departamento de Biologia e Zootecnia, FEIS, Unesp, Ilha Solteira, SP, 2 Unidade de Ensino de Coxim, UEMS, Coxim, MS, 3 Departamento de Biologia, Unesp, Bauru, SP, 4 Departamento de Morfologia, IB, Unesp, Botucatu, SP, Brasil Keywords: Characidae, Characiformes, spermatozoa, spermiogenesis, ultrastructure Accepted for publication: 27 June 2006 Introduction Several phylogenetic questions remain unanswered at the diverse taxonomic levels found in the Characiformes and, consequently, the Characidae (Vari 1998; Weitzman and Malabarba 1998), a family considered today as non- monophyletic (Buckup 1998). Especially controversial are the position of the genus Salminus and its relationship with the other Characidae, specifically with the genus Brycon (Géry 1977; Portugal 1990; Uj 1990; Ortí 1997; Margarido and Galetti. 1999; Zanata 2000; Malabarba and Weitzman 2003). Salminus Agassiz, 1829, which was long considered to be the only genus in the subfamily Salmininae (Morais Filho and Schubart 1955), is now regarded as incertae sedis in the Characidae (Lima and Malabarba 2003). The species of this genus, long appreciated for their culinary uses and in game fishing, are carnivores that may reach 116 cm in length and up to 31.6 kg in weight (Godoy 1987). They can be found throughout the basins of the Prata, the São Francisco and the Madaglena, up to the edges of the Amazon/Orinoco basins (Lima 2003) in South America. Brycon, the only genus of the subfamily Bryconinae, comprises about 60 species (Howes 1982; Géry and Mahnert 1992) and is one of the largest genera in the family Characidae.The species of this genus are also appreciated for cooking and game fishing. They feed on fruits and seeds and their lengths range from 15 to 70 cm. They are found from southern Mexico through Central and South America as far south as the Prata River in Argentina. The comparative osteological study conducted by Géry (1977) placed these two genera in different tribes of the subfamily Bryconinae: Bryconini and Salminini; he also included Triportheini. Uj (1990) and Portugal (1990) also considered Brycon to be closer to Salminus and Triportheus than to other South-American characids. Ortí (1997), however, based on molecular analyses, as well as on the study by Mar- garido and Galetti. (1999) based on karyotypic relationships, suggested that Bryconinae and Salmininae form, in Characidae,