Atlantic sturgeon (Acipenser oxyrhynchus) and lake sturgeon (Acipenser fulvescens) sperm-cell morphologies were examined using scanning electron microscopy. Major differences were found in four of nine metrics, all in the head region of the cell. Atlantic sturgeon sperm cells were much shorter than those of lake sturgeon. Anterior head width exceeded posterior head width, in contrast to the arrangement in lake sturgeon sperm cells. Lake sturgeon sperm cells are nearer in size to those of other sturgeons than are Atlantic sturgeon sperm cells. Comparisons were made with sperm-cell structures known from other sturgeon species, including the Russian sturgeon (Acipenser gueldenstaedti colchicus), stellate sturgeon (Acipenser stellatus), Chinese sturgeon (Acipenser sinensis), and white sturgeon (Acipenser transmontanus). Variation in cell morphology may indicate evolutionary relationships. In addition, the fine structure of Atlantic sturgeon sperm cells was examined using transmission electron microscopy and selected metrics are described. The cell possesses a distinct acrosome, a midpiece, and a single flagellum. A comparison is made with ultrastructural details of the sperm cells of stellate and white sturgeons. Similarities among these species include radial symmetry about the longitudinal axis, an elongate shape, a distinct acrosome, and the presence of endonuclear canals. Noteworthy differences include a smaller total length and width than stellate and white sturgeon sperm cells. The main sperm-cell body is approximately 4 µm long and the flagellum about 37 µm long, resulting in a total cell length of about 41 µm. Also, the Atlantic sturgeon sperm cell possesses only two membraned endonuclear canals, in contrast to the arrangement in white and stellate sturgeons, where three such organelles are found. A structural connection of unknown function between the nuclear fossa and proximal centriole is also present in the Atlantic sturgeon sperm cell. Sperm-cell nuclei of white and stellate sturgeons are elongate trapezoids, with the anterior end narrower, whereas in Atlantic sturgeon the anterior portion of the trapezoid is wider than the posterior. Structural similarities between species may indicate a commonality of ancestral and evolutionary relationships that may have taxonomic implications. Ultrastructure suggests a closer evolutionary relationship between the white and stellate sturgeon than between either of these species and the Atlantic sturgeon. The present findings may be used by biologists studying the reproductive physiology, forensics, taxonomy, and genetics of sturgeons.
