Abstract
At least 18 species of introduced marine sessile animal, including two newly discovered ones, the barnacle Balanus glandula and the ascidian Polyandrocarpa zorritensis, are known from Japanese waters. These include Polychaeta (2 species), Bryozoa (2), Gastropoda (1), Bivalvia (4), Cirripedia (6), and Ascidiacea (3). Dispersal routes and vectors of some of these species, and distributions of all the species in Japan, are reviewed based on the literature and original data. Dispersal routes have varies among species. In the case of sessile animals, fouling of ship hull accounts for introductions of foreign species arriving in Japanese waters; ballast water may be more important for dispersal within Japan. Introductions appear to have conspicuously increased during the two decades of the 1960s and 1970s, but fewer introductions have been reported in the last 20 years. The increase was probably because of the expansion ocean-going shipping coming to Japan, deterioration of water quality, and large-scale loss of natural coastline. The reason for the subsequent decrease are unclear, but likely causes for the change in the number of recorded introduced species are discussed.
