:

publisher<Synopsis>Opsariichthys uncirostris uncirostris, Hasu in Japanese common name is a peculiar piscivorus cyprinid among Japanese freshwater fishes. The larval development of O. u. uncirostris is clarified to elucidate the drastic change; the developmental series is divided into nine stages on the basis of morphological, functional and behavioral traits. The larval development of Hasu was compartmentalized by formation process of fins and the notochord. Hatching was observed on the third day after fertilization (6.2±0.70 mm TL: Total Length). The pre-larva at first day after hatching (DAH1), expressed pectoral fin, then opened eyes, and started moving over the bottom (7.0±0.38 mm TL). Mouth opened at DAH3 (6.7±0.40 mm TL). The flexure of the notochord was completed at DAH8 (7.9±0.19 mm TL). Feeding had started at the same time when reached to the post-larva, but it was frequently observed to fail in feeding. At DAH70 (11.8±2.29 mm TL), all the fin rays reached the fixed number. As the characters related to swimming developed drastically at DAH 0-8, those period was thought to be important for Hasu in the developmental process