ボウズハゼ

Sicydium japonicum Tanaka is a fresh water goby, distributed in the South-Easten Japan, the Riu-Kiu Islands and the Northern Formosa (Fig.1). The authors studied its habits in the rivers of Miyazaki Prefecture, Kyushu. It lives under the stone of the mid-stream, feeding on Diatoms and Cyanophata growing on the stone-surface of the river-bed (Fig.2). The spawing-season seems to extend from early July to early September in the studied district. The number of ovarian eggs of a fish, 105 mm in total length, was 224,960. The breeding-rooms were found under the stones of the bed in the torrential mid-stream. The spawned eggs were attached to the under surface of the stone in masses. The spawned eggs were attached to the under surface of the stone in masses. The male parent fish was observed to stay in the room until the eggs hached (Fig.3). The egg is spherical, very small, 0.45 mm in diameter, with a bundle of adhesive filaments (Fig.4, A-D). The incubation-period seemed somewhat shorter than 2 days at the temperature about 25℃. The newly hatched larva is 1.5mm in total length with the particular body-form differing from the larvae of many other gobies in that it rather resembles to the newly hatched larvae from small pelagic eggs. It has not yet developed the pectoral fin, air-bladder nor melanophore on the eye-ball, with the mouth and anus not yet formed (Fig.4, E). The larva absorbed the yolk in 4 days after hatched, and attained 1.9 mm in total length. By this stage, the air-bladder, pectoral fin and attained 1.9 mm in total length. By this stage, the air-bladder, pectoral fin and melanophores on the eye-ball had appeared, the mouth and anus were formed (Fig.4, F).The hatched larva seems to descend into the sea with the flowing water, and spend its first winter in the coastal water of the sea (Fig.4,I). It grows to about 32mm in total length in the next spring, and approaches the river-mouth, feeding on plenctonic copepods (Fig.2, E: Fig.4, G). The juveniles, about 33 mm in total length, were found ascending the river in aggregation. They enter into the independent life in the mid-stream (Fig.4, H). It grows to 70 mm in total length in about 2 years and becomes mature.