1National Research Institute of Aquaculture, Yokosuka, Kanagawa 238-0316, 2Tohoku National Fisheries Research Institute, Miyako, Iwate 027-0052, 3National Research Institute of Fisheries and Environment of Inland Sea, Onomichi, Hiroshima 722-0061, 4Hokkaido National Fisheries Research Institute, Kushiro, Hokkaido 085-0802, 5National Research Institute of Fisheries Science, Yokohama, Kanagawa 236-8648, 6Tsushima Branch of Nagasaki Prefectural Office, Tsushima, Nagasaki 817-8520, 7Aichi Prefectural Fisheries Experimental Station, Chita, Aichi 470-3412, 8Hokkaido Research Organization, Yoichi, Hokkaido 046-8555, 9Tamana Regional Promotion Bureau of Kumamoto-ken, Tamana, Kumamoto 865-0016, 10Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, 11Kyoto Prefectural Agriculture, Forestry and Fisheries Technology Center, Miyazu, Kyoto 626-0052, 12Mie Prefecture Fisheries Research Institute, Suzuka, Mie 510-0423, 13Chiba Prefectural Fisheries Experimental Station, Futtsu, Chiba 293-0042, Japan
Asymmetric shell marking (type A) in the Manila clam Ruditapes philippinarum is known to be a heritable dominant characteristic. We collected local samples from a wide range of the Japanese Archipelago and China, and found that the frequency of type A was heterogeneous among the samples. The frequency was significantly higher (14.5-28.1%) in samples collected in Hokkaido and the Kanto area (Tokyo Bay, Sagami Bay and Numazu) than those (0-9.9%) in the other samples collected in Tohoku and west of Hamana Lake including China. Non-native clam individuals had been released in the Banzu area of Tokyo Bay for nearly two decades, ceasing in 2007. The clam samples collected at Banzu in 2005 were subdivided into small (<20 mm in shell length) and large (≥25 mm) size groups, and a significantly higher frequency of type A was observed in the small group (22%) than in the large group (0%). On the other hand, clam samples collected at the same area in 2011-2012 presented a high frequency of type A (17.2-20.3%) regardless of the size. Samples collected from other areas of Tokyo Bay where no exogenous individuals had been released were observed to have maintained a high frequency of type A (17.9-26.4%). This simple phenotypic marker indicates that the Manila clam population in Japan is genetically structured and exogenous individuals introduced to Tokyo Bay may have had little effect on diminishing the frequency of type A.