1. Department of Ocean Sciences, Tokyo University of Marine Science and Technology, Minato, Tokyo 108-8477, Japan
2. Faculty of Fisheries, Nagasaki University, Nagasaki 852-8521, Japan
3. Pearl Research Laboratory, K. Mikimoto & Co. Ltd., Osaki Hazako, Hajimajima-Cho, Shima, Mie 517-0403, Japan
4. National Research Institute of Fisheries Science, Fisheries Research and Education Agency, Kanazawa, Kanagawa 236-8648, Japan