A new population of the amphioxus (Branchiostoma belcheri) in the Enshu-Nada Sea in Japan

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 ne w population of this species in an area near the Irago Channel and the Ensh u-Nada Sea in Aichi Prefecture, Japan. This population was originally disco vered by a research group of the Aichi Fisheries Research Institute when th ey 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 km(2) 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 t his population could be divided into two groups according to body size dist ribution: those of approximately 20 mm long on the average (termed small) a nd those of approximately 50 mm long on the average (termed large). No smal l amphioxus was found in June; 10% or fewer of the animals were of small si ze in July and August; and about 40% of the animals were of small size in S eptember and November. The breeding season was estimated to last from June (or earlier) to August based on observation of the gonadal size. Amphioxi s hare a habitat with sand lances, which stay in the sand during the summer d ormant period. The newly discovered amphioxus population may provide an imp ortant source of material for modern biological researches on Cephalochorda ta in Japan, although special care for protection of this population should be taken.