ULTRASTRUCTURE OF THE BYSSUS OF THE ZEBRA MUSSEL (DREISSENA-POLYMORPHA, MOLLUSCA, BIVALVIA)

The byssus attachment organ of zebra mussels (Dreissena polymorpha) was studied using scanning and transmission electron microscopy. We demonstrated that the byssus consisted of a root, a stem, threads, and attachment plaques. The root displayed two types of filaments, one with bilateral projections repeating every 60 nm and one without bilateral projections. Root filaments interdigitated with retractor muscle cells and inserted into the basal lamina of muscle cells. Filaments in different arrangements also were the dominant structural elements in the stem and threads. Stem filaments were clustered into tightly and loosely packed parallel bundles. Some of the tightly packed bundles displayed 350-nm periodic cross-bands. Longitudinally oriented thread filaments displayed a core averaging 7 nm and an electron-dense boundary. The stem, threads, and plaque were covered by a dense sheath. The plaque contained numerous vacuoles embedded in a filamentous mesh, and the attachment surface, except for the extreme edge, was electron-dense with numerous granular ''attachment spots'' applied to the inner face of the attachment surface. Bacteria were found frequently within the plaque vacuoles.