The spermatozoon of the Chinese bitterling, Rhodeus sericeus sinensis (Cyprinidae, Teleostei)

The bitterling spermatozoon has been examined by electron microscopy using sectioned material and freeze-fracture replicas. Its structure is characterized by a higher degree of order than is seen in spermatozoa of related teleosts. The intramembrane particles in the cell membrane are arranged in a large (around 1 micron in diameter) hexagonal grating pattern at a certain site of the sperm head. The flagellum protrudes from a site slightly posteriorly to this hexagonal grating and is surrounded by a cytoplasmic sleeve that contains regularly spaced filaments in a helical arrangement. The sperm midpiece contains a large mitochondrion enclosed in an elaborate system of membranous cisternae and narrow tubules. Fertilization in the bitterling takes place in the gill chamber of freshwater mussels--a unique brood chamber and site of fertilization for a vertebrate.