Microhabitat structure, represented by the presence or absence of sessile organisms, may affect the composition of mobile communities. A barnacle, Chthamalus challengeri, forms dense patches of different sizes on an intertidal boulder shore in Amakusa, Japan, where mobile, herbivorous gastropods and chitons are the most dominant. To study the effects of the barnacle patches on the density and size structure of several herbivorous molluscs, we carried out quantitative quadrat sampling both inside and outside barnacle patches in July, 1994. Molluscan community composition and the size structure of dominant species inside barnacle patches were apparently different from those outside. The small gastropods Littorina brevicula and Peasiella inffracostata occurred only in the barnacle patches. The mean sizes of Nipponacmea teramachii, Monodonta labio, Patelloida pygmaea, Nipponacmea nigrans, and Lunella coronata were smaller inside than outside the patches, but Acanthochiton defilippi showed the reverse trend. Densities of A. defilippi, L. coronata, and the predatory gastropod Thais clavigera were higher inside the patches than outside, while the densities of M. labio, P. pygmaea, and N. nigrans were lower inside than outside. The influences of barnacles on herbivorous gastropods on the boulder shore are discussed in the context of some hypotheses proposed in recent studies of rocky shore community ecology.

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The annual cycle of the testicular structure of Actinia equina in Japanese waters was investigated by histological observation. This is the first study concerning the reproductive biology of this species in Japan. Specimens of A. equina were collected once a month from two intertidal rocky shores in eastern Sagami Bay, from January, 1991, to June, 1991, and from April, 1993, to January, 1994, respectively. Most of the specimens were male or non-sexual individuals and there were few females with mature oocytes. Spermatogenesis starts from late fall to early spring and the testicular cysts mature in mid-summer. This indicates that the spawning season of A. equina is mid-summer and sexual reproduction may occur in the summer. Although young are found in the enterons of adults of both sexes all year round, the source of these young remains unknown.

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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.

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Three species of Tetraclita, T. squamosa, T. formosana and T. japonica, live in the intertidal zone in Japan. The phylogenetic relationships among them (formerly considered as different subspecies of T. squamosa) were examined in a previous study of Yamaguchi (1987) based not only on comparisons of morphology and geographic distribution, but also on isozyme electrophoresis. In the present study, their interrelationships are examined using parts of the nucleotide sequences of a mitochondrial gene for cytochome oxidase subunit I (COI). In concordance with the results by isozyme electrophoresis, T. formosana and T. japonica are shown to be the closest among the three species. The ancestor of T. formosana and T. japonica, themselves being derived recently, diverged earlier from the last common ancestor of the three species.

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