Morphological aspects of fertilization in Phallusia (Ascidia) nigra (Ascidiacea, Tunicata).

The spermatozoa of Phallusia (Ascidia) nigra have an elongated head (approximately 5 μm in length) in which a nucleus and a single mitochondrion are located side by side. There is no midpiece. The apex of the head is wedge-shaped. Acrosomal vesicles (approximately 55-65 nm in diameter) and moderately electron-dense material (MEDM) are present between the plasmalemma and the nuclear membranes in the anterior tip of the head. The MEDM occupies a central position and three or four acrosomal vesicles are seen in a line alongside it. The acrosomal vesicles disappear as the sperm makes contact with the surface of the chorion. Gamete fusion most likely occurs between a small process extending from the peripheral margin of the sperm apex and the egg surface, resulting in incorporation of the sperm into the egg from the anterior region of its head.