Characterization of zebra mussel (Dreissena polymorpha) sperm morphology and their motility prior to and after spawning.

In order to understand more about the reproductive physiology of the zebra mussel (Dreissena polymorpha), we have examined the effect of spawning on the kinematics and fertilization ability of their spermatozoa. Sperm morphology was characterized using scanning electron microscopy, differences in sperm kinematics were examined using CASA following manual collection of sperm from the gonad or spawning induced by 5-hydroxytryptamine (5-HT), and the fertilization ability of spawned and manually removed spermatozoa were compared. D. polymorpha sperm have a tail 50 microns long, a midpiece consisting of four spherical mitochondria-rich regions approximately 0.6 micron in diameter, a conical nucleus 2.52 microns in length, and a cylindrical acrosome 0.72 micron in height and width. Kinematic analysis revealed that manually removed sperm exhibit significantly lower motion characteristics as compared to spawned sperm. Treatment with 5-HT had no significant effect on the sperm kinematics of either manually removed or spawned spermatozoa. These data suggest that D. polymorpha spermatozoa have a similar morphology to other bivalve sperm, spawning results in a significant enhancement in sperm kinematics, and 5-HT does not directly alter sperm motion.