Distribution and Bed Structure of the Two Intertidal Mussels, Septifer virgatus (Wiegmann) and Hormomya mutabilis (Gould)

Abstract

The distribution and bed structure of the two mytilid bivalves, Septifer virgatus and Hormomya mutabilis, were investigated on rocky intertidal shores at Shirahama, Wakayama Prefecture, Japan. S. virgatus inhabited more wave-exposed shores than H. mutabilis. On rock slopes receiving moderate wave action, both species formed contiguous mussel beds vertically, the former occurring from the upper to mid intertidal zones and the latter in the lower zone. Juvenile H. mutabilis were found abundantly in the mussel beds of both species, attaching to byssal threads or to the shell surface of other mussels. Very few S. virgatus of all sizes were found in the H. mutabilis bed. The amount of byssal thread material and the drag force required to detach mussels were both greater in H. mutabilis than in S. virgatus. Individual variation in shell shape was greater in H. mutabilis than in S. virgatus. A much larger amount of sediment accumulated in H. mutabilis beds than in S. virgatus beds. This may reflect the denser byssal threads network and more tightly packed individuals in the H. mutabilis beds. Factors determining the lower distribution limit of S. virgatus and the upper limit of H. mutabilis are discussed in relation to the differences in morphology and bed structure of the mussels.