Invasion of Japanese shores by the NE Pacific barnacle Balanus glandula and its ecological and biogeographical impact

A common littoral barnacle, Balanus glandula Darwin, 1854 (Crustacea, Cirripedia) from the west coast of North America has become established in the mid-littoral zone of harbors on the Pacific side of northern Honshu, Japan. The new immigrant occupies space in the lower half of the Chthamalus spp. and Balanus albicostatus zones, and in the upper fringe of the Semibalanus cariosus zone. However, the species has not been detected in harbors along the Sea of Japan, south of the Ozika Peninsula, northern Honshu, and to the north and east of Kushiro, Hokkaido. Considering its present distribution, shell sizes, and abundance, it is most likely that it was introduced into 3 separate port areas in NE Honshu, Japan, during the last 20 to 40 yr, probably by anthropogenic activities. It was apparently also introduced to the La Plata estuary, Argentina, during the same period. B. glandula is becoming a dominant littoral zone species, especially in embayments, largely at the expense of the tropical endemic B. albicostatus in the upper littoral fringe. Taking into account the biogeographical distribution of common Japanese barnacles in relation to water temperature, the future distribution of the newly introduced species is considered.