1. National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba 305-8569, Japan, National Food Safety and Toxicology Center, Department of Zoology, Institute for Environmental Toxicology, Michigan State University, East Lansing, Michigan 48824, and Tokyo University of Fisheries, 4-5-7 Konan, Minato-ku, Tokyo 108, Japan