Present structure of sessile organism communities on lighted buoys in Tokyo Bay

The structure of sessile organism communities sampled from 24 lighted buoys in Tokyo Bay (Japan) 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.