Re-examination of the spermatozol ultrastructure of eels: observations of the external morphology of spermatozoa in three species

The nuleus of the spermatozoa in both Anguilla anguilla and A. japonica species is asymmetric on the longitudinal axis, being gently curved and elongated with a hook-shaped superior end directed inside the crescent. A single spherical mitochondrion is located at the superoanterior portion of the nucleus. Nine striae extending from the basal portion of the flagellum run along the right side of nuclear surface. These are always oriented toward the mitochondrial portion and their ends are located on the mitochondrion. These features are seen not only in both Anguilla species but also in Muraenesox cinereus and in other species including other genera of Anguilliformes. It is suggested that these features be considered as common to Anguilliformes.