Ultrastructure of spermatozoa in cobia, Rachycentron canadum (Linnaeus, 1766).

Ultrastructure and development of spermatozoa in cobia, Rachycentron canadum are described. Sections through the testis show different developmental stages viz, Spermatocytes, spermatids and sperm. Spermatozoa of R. canadum exhibit the configuration of uniflagellated, anacrosomal Type I aquasperm, typical for externally fertilizing fish. Mature spermatozoon is seen with a prominent head and long cylindrical flagellum. Ultrastructure of sperm shows invaginated 'U' shaped nucleus and other organelles. The mitochondrial matrix is electron-dense with irregular arrangement of the cristae. The nucleus reveals a deep invagination (nuclear fossa) in which the centriolar complex is located. The centriolar complex lies inside the nuclear fossa and is composed of a proximal and a distal centriole. The two centrioles are placed perpendicular to each other. The flagellum has a typical eukaryotic organization (microtubule doublets 9 + 2 pattern) and measures around 36.21 ± 0.42 μm in length. This study for the first time provides a comprehensive detail on the ultrastructure and developmental process of sperm in cobia, R. canadum.