Benthic adult individuals and planktonic larvae of the barnacle Balanus rostratus were collected monthly from April, 2003, to March, 2005, in Shizugawa Bay, Miyagi Prefecture, Japan. The adult barnacles, with a maximum base diameter of 20-60mm, exhibited an increase in gonad somatic index from October to December, and breeding individuals appeared in December and January. The occurrence of nauplius larvae of B. rostratus was restricted to winter months, from January to March, and this was followed by the recruitment of small juveniles of 1-4mm in maximum base diameter to the rocks. Under natural conditions, slow growth was observed in newly settled individuals in spring, with the mean base diameter only increasing to 7mm by December-January; however, relatively high growth rates were observed in young barnacles transplanted to plastic plates and further cultured in Shizugawa Bay. The mean maximum base diameter of these cultured barnacles increased from 6.7±1.0mm in June, 2004, to 17.7±2.6mm in December, 2004, and to 26.2±2.7mm in April, 2005. The variation in growth rates between wild and cultured specimens of B. rostratus may be attributable to differences in environmental conditions or size-dependent mortality. Under natural conditions, the number of newly settled barnacles decreased drastically after May and they almost disappeared until January during the experimental period. The cause of this decrease may be due to post-settlement mortality biased towards larger individuals (e.g., size-selective predation), which may result in underestimation of the growth rate of young barnacles in the natural population. During the two-year investigation, few individuals recruited into size classes over of 10mm in maximum base diameter because of the low survival rate. Although B. rostratus is regarded as a long-lived species with a high survival rate among its large individuals (>20mm in maximum base diameter), the high mortality of young barnacles has the potential to lead the population to extinction. Thus, the intermittent occurrence of a dominant year class may be a strategy for sustaining the population.

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The sessile organism community structure at nine sites on the intertidal coast of Tokyo Bay (excluding the bay-mouth region) was investigated seasonally from July 2004 to May 2005 with semi-quantitative measurements of each species. Cluster analysis conducted according to the dissimilarity of community structure classified the study sites into Clusters X and Y. Cluster Y was further divided into two subgroups, Clusters Y1 and Y2. Cluster X consisted of four sites in the northwestern part of the bay, while Cluster Y1 was composed of two sites in Chiba City and Cluster Y2 was made up of three sites relatively close to the mouth of the bay. Nine species, including the Pacific oyster Crassostrea gigas, the native barnacle Fistulobalanus albicostatus, and the introduced mussel Xenostrobus securis, distinguished Cluster X. Six species, including the green alga Ulva sp., the sea anemone Haliplanella lineata, and the serpulid polychaete Hydroides ezoensis, represented Cluster Y1, and six species, including the red alga Caulacanthus ustulatus, the chiton Acanthochitona achates, and the limpet Lottia tenuisculpta, characterized Cluster Y2. We suggest that the characteristic species of Cluster X indicate low salinity and high eutrophication, while those of Cluster Y2 indicate relatively higher salinity and lower eutrophication.

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The structure of sessile organism communities sampled from 24 lighted buoys in Tokyo Bay in 2004 and 2005 was investigated, with semi-quantitative measurements of each species. Two introduced mussels, Mytilus galloprovincialis and Perna viridis, dominated most of the 24 samples. However, cluster analysis conducted according to the dissimilarity of species composition among the 24 samples classified the sessile organism community in Tokyo Bay into two sub-communities. One was formed in the inner half of the bay and was represented by the introduced mussel Xenostrobus securis and the introduced petricolid Petricola sp. cf. lithophaga. The other was formed in the outer half of the bay and was characterized by the native barnacles Megabalanus rosa and Balanus trigonus. A comparison of the community structure found in the present study with that in a previous investigation from 1973 to 1975 showed that the community had strengthened its “introduced-species-dominated” character over the past three decades. This change was mainly caused by the proliferation and range expansion of the three bivalve species, Perna viridis, Xenostrobus securis, and Petricola sp. cf. lithophaga, all of which were introduced into the bay during the 1980s. Additionally, embryos of the introduced barnacle Amphibalanus variegatus, which was recently recorded in Tokyo Bay, were found in an adult sample; this indicates its successful reproduction in the bay.

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