The fine structure of the sperm cell of the shortnose sturgeon (Acipenser brevirostrum) was examined using transmission electron microscopy and selected metrics. The cell possesses a distinct acrosome, a defined head region, a midpiece, and a single flagellum. The mean length of the sperm cell body (acrosome + nucleus + midpiece) is approximately 9.71 µm, and the length of the flagellum is about 37 µm, resulting in a total cell length of about 46 µm. The sperm cell of the shortnose sturgeon is much longer and slightly wider than that of the Atlantic sturgeon. The nuclei of shortnose, white, and stellate sturgeon sperm cells are elongate trapezoids with the anterior (acrosome) end narrowest, the opposite of that of the Atlantic sturgeon. Although slightly smaller in total length and width than the sperm cells of the stellate and white sturgeons, that of the shortnose sturgeon is most similar to them in overall ultrastructure, as all three cells have three endonuclear canals. A structural connection of unknown function between the nuclear fossa and the proximal centriole, which is similar to the fibrous body in other species, is present in the shortnose sturgeon sperm cell. Our results suggest a more recent evolutionary link between the shortnose, white, and stellate sturgeons than between any of these and the Atlantic sturgeon. This is the first description of sperm cell ultrastructure in the shortnose sturgeon, an endangered species.
