The ultrastructure of the sperm and motile spermatozeugmata released from the freshwater mussel Anodonta grandis (Mollusca, Bivalvia, Unionidae)

Abstract

Mature sperm are released from Anodonta grandis as multiple spherical spermatozeugmata, which are complexes measuring 40–50 μm and containing up to 2800 sperm when fully populated. The 1.5 × 4.0 μm cylindrical sperm head is anchored in the spherical spermatozeugma periphery, with the 35 μm long flagella projecting perpendicular to the surface. Rotational and progressive motility is achieved by an apparently asynchronous but sequential flagellar beat. An acellular 80-nm lamina around the spherical spermatozeugma encircles a central fluid-filled sphere called the globe. Each sperm head is embedded in a chamber formed by extensions of the 80-nm lamina and in contact with the sperm plasmalemma in a flagellar collar posterior to the 5 mitochondria at the base of the sperm nucleus. A proximal centriole located in a fossa in the nucleus base is connected to a distal centriole by a dense amorphous matrix. The distal centriole is locked to the flagellar collar via 9 striated, bifurcated pericentriolar processes. The anterior part of the sperm contains an acrosome-like region containing 0.1 μm diameter membrane-bound vesicles. The spermatozeugma complexes may survive up to 24 h and may serve to transport sperm to the gill chamber of the female mussel.