The acrosome reaction inStyela plicataspermatozoa occurs through vesiculation in which a vesicle bounded by a hybrid membrane, formed from the acrosomal membrane and the overlying plasma membrane, is generated as a byproduct. The head of the spermatozoon ofS. plicatais about 6 μm long. In the apex of the head an acrosome is present, which appears as a flattened vesicle (about 150x 150x50 nm). The apical part of the acrosomal membrane and the overlying plasma membrane are in close contact with each other. The acrosome reaction appears to involve the fusion of the acrosomal membrane and the overlying plasma membrane along the marginal region of the acrosomal vesicle. Membrane fusion results in the formation of a vesicle bounded by hybrid membranes of the acrosomal membrane and the overlying plasma membrane, which eventually detaches from the tip of the sperm head.