Sperm sphere in unionid mussels (Bivalvia: Unionidae)

Abstract

We observed spermatozoa of five freshwater Japanese unionid mussel species under a light microscope. Males discharged spermatozoa as spherical masses. Spermatozoa embedded their heads into a spherical, colorless body with their tails extended around the sphere. We termed this spherical mass of sperm as a "sperm sphere". Just after release the sperm spheres rotated rapidly by synchronous movement of the sperm tails. By the next day, the sperm spheres increased in size and some spermatozoa had detached from the sperm spheres. Spermatozoa embedded in the sperm sphere were active at least forty-eight hours after being discharged. Single spermatozoa in a balanced salt solution were also active until forty-eight hr after extraction from the gonads, while they lost their motility within a few minutes in freshwater. Spermatozoa may maintain their motility for a long time in the form of a sperm sphere and the use of sperm spheres may allow the fertilization of eggs discharged into the gill chambers of females far away from the male.