Morphological development of eggs, larvae and juveniles of laboratory-reared Ryukyu-ayu Plecoglossus altivelis ryukyuensis

The development of embryos, larvae and juveniles of the Ryukyu-ayu, Plecoglossus altivelis ryukyuensis are described based on laboratory-reared specimens. The eggs were spherical, 0.98-1 .18 mm (mean 1 .06 mm) in diameter with an adherent membrane. The incubation period after fertilization was approximately 155 h at a water temperature of 19.7-22.0 deg C (mean 20.7 deg C). Newly hatched larvae were 5.0-5.9 mm (mean 5.5 mm) in body length (BL) with 59-62 myotomes. Within 5 days after hatching, the larvae had attained 6.4-8.2 mm (mean 7.8 mm) BL and had completely consumed their yolk. Notochord flexion began at 1 3.7 mm BL and was completed by 1 6.3 mm BL. The rudimental dorsal, anal, pelvic and adipose fins appeared at 14.7, 16.3, 21.8 and 21.8 mm BL, respectively. All fin rays reached the same fixed number of adult fish at about 28 mm BL. The comb-like teeth began to form at approximately 30 mm BL and were fully developed at about 40-50 mm BL. The proportions of P.a. ryukyuensis specimens, which were consistent with adult fish, occurred at approximately 40 mm BL.