Embryonic Development and Morphological Changes with Growth of the Larval and Juvenile Ayu Plecoglossus altivelis altivelis of Lake Ikeda in Southern Kyushu

Abstract

The embryonic development of Plecoglossus altivelis altivelis was described on the basis of eggs inseminated artificially on October 24, 1984 in Lake Ikeda. The eggs were spherical, 0.87-0.92mm in diameter. The incubationp eriod was about 230 hours at a water tempera-ture of 14.5-23.2°C (av. 20.1°C). The standard length newly hatched larvae had a small yolk sac (av. 5.3mm) and 61-62 myotomes. Within several days of hatching, the larvae attained 6.0-7.9mm in standard length and had completely consumed their yolk substances.
Morphological changes of the larvae accompanying their growth were observed on 1.263 specimens collected by the larval net (80cm in diameter, 0.353 in meshes), the surf zone net (1m×5m, 1mm) and the cast net (13mm). The larval fin fold gradually disappeared, and the rudimental dorsal, the anal, the ventral and the adipose fins were visible at 11.4, 12.8, 24.3, 44.5mm in standard length, respectively. The conical teeth appeared at 24.3mm and the comb-like teeth at 55mm. Larvae smaller than 25mm inhabited the offshore waters of the lake, while those larger than 27mm appeared in the surf zone of the lake shore. The vertebral centrums of the larvae ossified at a size of 27.3mm. It is suggested that the considerable morphological changes of the ayu occurred at sizes of 25-30mm and 50-55mm in standard length. The former corresponds to the end of the larval stage and the latter to the end of the juvenile stage, respectively.