The amphioxus, Branchiostoma belcheri, is currently listed in the registry of “Endangered Animals of Japanese Marine and Fresh Water Organisms” issued by the Japan Fisheries Resource Conservation Association. We surveyed a new population of this species in an area near the Irago Channel and the Enshu-Nada Sea in Aichi Prefecture, Japan. This population was originally discovered by a research group of the Aichi Fisheries Research Institute when they were collecting Japanese sand lances (Ammodytes personatus) in this area. Every month from June to November of 1995 with the exception of October, we collected amphioxi and sand lances in an area of about 1 km2 to study the population profile of the amphioxus and the relation between these two animals. The newly discovered population of the amphioxus was suspected to be larger than any other Japanese populations of this species. Animals of this population could be divided into two groups according to body size distribution: those of approximately 20 mm long on the average (termed small) and those of approximately 50 mm long on the average (termed large). No small amphioxus was found in June; 10% or fewer of the animals were of small size in July and August; and about 40% of the animals were of small size in September and November. The breeding season was estimated to last from June (or earlier) to August based on observation of the gonadal size. Amphioxi share a habitat with sand lances, which stay in the sand during the summer dormant period. The newly discovered amphioxus population may provide an important source of material for modern biological researches on Cephalochordata in Japan, although special care for protection of this population should be taken.
