Alien Marine Crustaceans of Japan: A Preliminary Assessment

Abstract

Although invasions may have commenced in the 1500s, our record of invasions of alien marine and estuarine species in Japan begins largely in the 1930s. We expand the previous inventory of 10 alien species to 31 alien and cryptogenic species, underscoring that this, too, may be a striking underestimate. Most of the marine crustacean invasions into Japan have occurred along the Pacific coast; a number of alien crustaceans form abundant populations in urbanized bays near international ports. The specific geographic sources of most invasions are not known; studies have clarified the origins of the barnacle Balanus glandula (from America), but hybridization or low genetic divergence has inhibited clarification of the exact source of the crab Carcinus and the barnacle Megabalanus coccopoma. The biogeographic origins of the alien crustacean fauna in Japan include the North Atlantic and North and South Pacific oceans. We cannot distinguish between ship fouling and ballast water as vectors for most species; this said, it is probable that ship fouling has been a major contributor to the arrival of alien barnacles. No species are yet known to have been introduced solely by ballast water, but this may be an artifact of the lack of collections and identification of potential ballast-only taxa (planktonic copepods, cladocerans, and mysids). Other vectors include importations from China and Korea of shellfish for stocking, crabs for farming, and of live bait; all of these may lead to the introduction of novel genetic stocks and of associated species. There are few studies that have examined the ecological and economic impact of alien crustaceans. The barnacle Amphibalanus amphitrite appears to be competitively superior to native barnacles and has a negative impact on their densities. Alien crabs inhabit communities established by alien barnacles and mussels (which serve as their prey); these “alien communities” occur, for example, in the inner areas of Tokyo Bay. The European crab Carcinus appears to be declining in some regions while the American crab Pyromaia is increasing. The economic cost to the power industry, shipping, aquaculture and fisheries for clearing the biofouling associated with alien barnacles and other alien fouling organisms is probably severely underestimated. Although the Invasive Alien Species Act was passed in Japan in 2005, it did not refer to alien marine organisms. Consequently, preventing the introduction of alien marine species into Japan, or their subsequent dispersal along its coastline, is proving difficult to enforce.