Alien Species Introduced with Imported Clams: the Clam-eating Moon Snail Euspira fortunei and Other Unintentionally Introduced Species

Abstract

The clam Ruditapes philippinarum is a commercially important fishery resource in Japan. Because the amount of production has been decreasing gradually in Japan, the clam has been imported from China and Korea in recent years to provide seed for aquaculture and also for recreational shellfish gathering. The object of this study is to confirm whether alien species are being introduced together with the clam and to obtain information on their distribution in Japanese waters. Three species of mollusk-eating moon snail, Euspira fortunei, Glossaulax didyma, and Glossaulax reiniana, were collected from sacks filled with R. philippinarum imported from China. Seven other gastropod species, nine species of bivalves including Meretrix pethechialis, the purse crab Phiiyra pisum, and a brachiopod, Lingula unguis , were also collected from the sacks. Almost all the unintentionally introduced animals were living and were directly released with commercially introduced clams into Mangoku-ura Inlet, Miyagi Prefecture, by a fishermen's cooperative. Although the snail Euspira fortunei is not indigenous to the eastern and northern coasts of Japan , relatively large populations of it occur in the clam production areas of Lake Hamana (Shizuoka), the estuary of the Obitsu River (Chiba), and Mangoku-ura Inlet. I believe that a population of this invasive snail has already been established at least in Mangoku-ura Inlet and is becoming a new, strong predator of the clam stocks. Prevention of further spread and estimation of the rate of predation are important to the clam culture. Among the 22 alien species recorded in this survey , seven were also found in the native communities. The effect of the alien population on the preexisting population is also important problem to be solved.