Abstract
The vertical distribution and life cycle of two isopod crustaceans, Dynoides dentisinus Shen and Cirolana harfoldi japonica Thielemann, were studied at Shirahama, Wakayama Prefecture, central Japan, where two mytilids Septifer virgatus (Wiegmann) and Hormomya mutabilis (Gould) formed vertically contiguous mussel beds. The abundance of the isopods tended to increase downshore within the S. virgatus bed in the upper and middle intertidal zones but decreased abruptly within the H. mutabilis bed in the lower intertidal zone. Very few individuals were found within the H. mutabilis bed throughout the year. The size structure of both species of isopods did not differ greatly among five shore levels within the S. virgatus bed. Multiple linear regression analyses against density of D. dentisinus revealed that the amount of sediment within the mussel beds tended to be negatively correlated while the density and volume of mussels both tended to be positively correlated. Both species reproduced twice a year. However, seasonal changes in size structure suggested that the longevity of the two generations born during the year differed between the species : spring-autumn and autumn-spring generations for D. dentisinus, and spring-next spring and autumn-next autumn generations for C. h. japonica. The association between isopods and mussels is discussed with a focus on the presence or absence of sediment within mussel beds.
