THE BYSSAL APPARATUS IN THE PACIFIC MUSSEL (MYTILUS TROSSULUS, BIVALVIA, MYTILIDAE), FROM THE SEA OF JAPAN

摘要

In the commercial Pacific mussel species (Mytilus trossulus) from the Sea of Japan, features of the morphological structure of the byssal apparatus, byssal threads and byssal groove of the foot are studied. Steps in the process of byssal thread formation and characters of the morphological structure of the byssal groove related to this process are described in Myt. trossulus. The byssal apparatus is shown to consist of a root, a stem, and byssal threads, the latter in cross-section being ellipsoid in shape. A byssal thread consists of a corrugated proximal part located immediately behind the cuff and is 1/3 of its length, and a relatively smooth and whippy distal part taking 2/3 of the thread length and ending with an attachment disk at the distal end. On the surface of the attachment disk, three reinforcing cords are situated. The morphological features observed in these structures are discussed in terms of the peculiar distribution of mytilids in marine coastal zones, a successful habitat of Myt. trossulus being rocky shores with active wave activities.