Ultrastructural study of the spermiogenesis in two species of Naididae, Paranais litoralis and Nais communis (Annelida, Oligochaeta)

Ultrastructural studies of sperm cells of two naidid species, Paranais lito ralis and Nais communis were carried out, with the purpose of providing kno wledge on this oligochaete family, because they are predominant species in Argentina (South America). Mature specimens were collected in the Rio de la Plata (Buenos Aires) and processed for transmission electron microscopy. P aranais litoralis presents a straight nucleus, a well developed microtubula r system, an acrosome provided with an acrosomal tube, five mitochondria an d a complex flagella with different electrondense zones with a central shea th corresponding to the prominent type. Nais communis, on the other hand, h as a twisted nucleus, cytoplasmic microtubules with a particular dispositio n; only one mitochondrion and also showing the flagellar axonema with a tet ragon configuration in the central sheath. In these species, the acrosome e xhibits some characteristics ascribed to the tubificids species. Finally, sperm cells in both species present a pattern of a modified sperma tozoa as it is referred to some tubificids species. It must be remarked tha t the cytodifferentiations differences found between the two species are in teresting for taxonomy, especially between taxa of the same family.