Morphometry, frequency and ultrastructure of male germ cells in morphotypes of the freshwater prawn Macrobrachium amazonicum (Decapoda: Palaemonidae)

Males of the freshwater prawn species, Macrobrachium amazonicum, have been staged histologically into morphotypes, Translucent claw (TC), Cinnamon claw (CC) and Green claw (GC). However, information on reproductive system anatomy and spermatogenesis among morphotypes is scarce. Our aim was to describe the frequency of spermatogenic cells, morphometry of seminiferous tubules and spermatogenic cells, and spermiogenesis to establish any differences among morphotypes. Specimens were captured and sexed. Testes were dissected, processed, and analyzed using light, transmission, and scanning electron microscopy. Macroscopically, testes were symmetrical, elongated, and translucid, with long vasa deferentia ending in ampoules. Histologically, testes exhibited seminiferous tubules with the germinal epithelium containing male germ cells in different stages of development and supporting cells. The diameter of seminiferous tubules was largest (P < 0.05) in the GC morphotype. Germ cell types were evaluated based upon size, as well as their cytoplasm and nuclear chromatin organization. Spermatozoa were the only germ cells displaying no differences in size among morphotypes. According to shape and localization, we classified spermatids into three subtypes: St1 located in the germinal epithelium with scant cytoplasm, a concave nucleus, and condensed chromatin; St2 sickle-shaped, with a concave, thin nucleus; and, St3 cup-shaped, with a developing acrosomal vesicle in the convex portion of the cell. Notably, St2 and St3 were found in the lumen of the seminiferous tubules and vasa deferentia. Spermatids and spermatozoa were floating in a large amount of amorp's glycoprotein-rich material. Spermatozoa were cup-shaped and displayed an acrosome fashioning a long spike with regular and compact cross-striations. In summary, we established differences in size and frequency of male germ cells among morphotypes and described spermatid characteristics not yet observed in other prawn species. Altogether, this information contributes to expanding our knowledge on taxonomy, phylogenetic relationships, and reproduction among Palaemonidae species.