Bull. Jpn. Soc. Fish. Oceanogr. 73(3), Page 161-171, 2009   Distribution of planktonic larvae and spats of the pen shell Atrina spp. in Ariake Bay, southern Japan for years 2003-2005 Kengo Suzuki1†, Tatsuya Yurimoto1, Yasunori Watanabe2, Yuichi Koshiishi3, Katsunori Kimoto1, Mikihide Yoshida4, Hiroshi Fujisaki5, Akihiko Fujii6, Hiroshi Nasu7* and Yukio Maeno1 1 Seikai National Fisheries Research Institute, Fisheries Research Agency, 1551-8 Taira, Nagasaki, Nagasaki 851-2213, Japan 2 National Fisheries Research Institute of Fisheries and Environment of Inland Sea, Fisheries Research Agency, 2-17-5 Maruishi, Hatsukaichi, Hiroshima 739-0452, Japan 3 National Research Institute of Fisheries Science, Fisheries Research Agency, Fukuura, Kanazawa, Yokohama, Kanagawa 236-8648, Japan 4 Ariake-Sea Laboratory, Fukuoka Prefectural Fisheries and Marine Technology Research Center, Yoshitomi, Yanagawa, Fukuoka 832-0055, Japan 5 Saga Prefectural Ariake Fisheries Research and Development Center, Nagata, Ashikari, Ogi, Saga 849-0313, Japan 6 Nagasaki Prefectural Institute of Fisheries, 1551-4 Taira, Nagasaki, Nagasaki 851-2213, Japan 7 Kumamto Prefectural Fisheies Research Center, 2450-2 Oyanonaka, Kamiamakusa, Kumamoto 869-3603, Japan * Kumamoto Prefectural Government, Tokyo Office †e-mail: skengo@affrc.go.jp The spatial distribution and abundance of planktonic larvae and spats of pen shells (Atrina lischikeana and Atrina pectinata) were investigated in Ariake Bay, southern Japan, from 2003 to 2005, to determine the recruitment stage of spats in relation to larval supply and spat survival after settlement. The distribution type was an aggregated distribution for both the larval and spat stages. However, spatial patterns of occurrence for larvae and spats were different. The planktonic larvae were broadly distributed in Ariake Bay, particularly in the middle of the bay during high summer. The spats were observed from the northern part to the eastern coast of Ariake Bay. In particular, spats over 25 mm in shell length were mainly obtained from the eastern coast of the bay. The umbo-stage larvae were frequently observed at the middle or the bottom layer of the water column in depths of more than 5 m, and full-grown larvae were obtained only from the bottom layer. The survival of spats was related to several parameters concerning sediment, especially concerning low values of acid volatile sulfide (AVS). The broad distribution of larvae in Ariake Bay was due to dispersion with a tidal residual current and a long planktonic period of more than one month. On the contrary, the narrow distribution of spats along the eastern coast of Ariake Bay may be affected by the bottom sediment conditions. These results suggest that spat recruitment is definitive in the post settlement stage, at 25 mm in shell length, and recruitment also depends on the ambient sea floor environment rather than larval supply in Ariake Bay. Key words: pen shell, Atrina spp., larvae, spats, spatial distribution, sediment conditions