1. Department of Biology, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
2. Institute for Freshwater Biology, Nagano University, 1088 Komaki, Ueda, Nagano 386-0031, Japan
3. Division of Mathematical Sciences, Tokyo Woman's Christian University, 2-6-1 Zempukuji, Suginami-ku, Tokyo 167-8585, Japan