Structural Peculiarities of the Byssal Apparatus and Byssal Groove of the Foot in the Mediterranean Mussel (Mytilus galloprovincialis, Bivalvia, Mytilidae) from the Zhitkov Bay of the Sea of Japan

The peculiarities of morphological structure of the byssal apparatus, byssal threads, and byssal groove of the foot were studied in the commercial species of the Mediterranean mussel (Mytilus galloprovincialis) from the Sea of Japan. It was demonstrated that the byssal apparatus consists of a root, stem, and byssal threads that are elliptical in shape in cross-section. Each byssal thread consists of a corrugated, wide proximal part (which is located immediately behind the cuff and accounts for one-third of its length) and a relatively elastic, narrow distal part (accounts for two-thirds of the thread length), terminating with an oval attachment disk at the distal end. In each byssal thread, three types of byssal prepolymerized collagens (P, D, and NG) with a different block copolymer structure are contained at its different parts. The surface of the byssal threads is tuberous along its entire length. Two reinforcing cords are located on the attachment disk surface. The edges of the attachment disks are semitransparent. The byssal groove of the foot is arranged more primitively than in the studied Mytilidae species. It is noteworthy that the distal fossa is absent at the distal end of the byssal groove of the foot. The peculiarities of morphological structure of the byssus and attachment disks in M. galloprovincialis are explained by the peculiarities of the secretory organ structure.