ハゼクチの水槽内産卵, 卵発生と仔稚魚について

抄録

Synechogobius hasta (Temminck et Schlegel) is one of the largest gobioid fish, attaining to about 50 cm in total length, and is abundantly distributed in Ariake Sound which is situated on the west coast of Kyushu.The fish lives in the muddy shallow region of the sound and makes a burrow in the mud for spawning.From 1969 to 1973, spawning of the fish was observed in the aquarium, and spawning behavior, embryonic development, and morphology of larvae and juveniles were studied.
The adult of the fish has following sexual characters (Fig.2).The female has a genital papilla with a rounded end, whereas the male has one with a pointed end.The male has enlarged lips and developed jaws, so that the head is nearly U-shaped in dorsal and ventral view, whereas the head of the female is nearly V-shaped.The male is bright brownish dorsally and yellow ventrally and colors get brighter along with sexual excitement.The female is always dull brown dorsally and white ventrally.
Pair of mature fish, administered some gonadotropic hormone injections, were shut in an earthen pipe or an opaque vinyl chloride pipe, 8-9 cm in diameter and 60 cm in length, closed with net at both ends.The pipe was set in the aerated aquarium (Fig.4).
Spawnings occurred in March and always in the morning.At the spawning, parent fish, being upside down in the pipe, laid 15, 000 to 51, 000 eggs in one layer on the upper wall of the pipe during 2 to 4 hours (Fig.5).The fish laid all eggs at a time.The parents ate a considerable number of their own eggs.
The degree of egg adhesion to the pipe wall varies by egg mass, seemingly dependent on egg quality such as fertility and vitality.
Eggs were almost spherical in shape just after spawning.They got larger and club-like in the early developmental stages, measuring 4.9-6.6 mm (5.5 mm) in longer axis and 1.0-1.2 mm (1.1 mm) in shorter axis.
Development of egg is shown in Table 2 and Fig.6.Hatching took place about 15 days after spawning at 13.0 18.9°C in water-temperature.
The egg in the early stage had many small oil globules in the yolk, measuring 0.02-0.08 mm in diameter, which finally united into one before hatching.
In many of egg masses, there were often observed agrippa eggs.In normal egg, the embryo formed primarily on the lateral position of the egg.It shifted the position to near the top of the egg according to the rotation of embryo and yolk in the progress of blastopore closure.After the rotation of embryo, its tail prolonged towards the basal end of the egg.In abnormal egg, the rotation of embryo and yolk failed to occur, so that the embryo kept on the lateral position of the egg was provided with the tail near the top of the egg and the tail prolonged towards the opposite direction to the normal one (Figs.7 and 8).
It was also often observed that the embryo fell out of the egg before the development was completed.These abnormally hatched larvae died in some days after hatching.Abnormal hatching tended to take place in the egg masses with many agrippa eggs.
Just hatched normal larva, measuring 6.1 mm in total length, had a little yolk, the rudiment of the caudal fin, cupulae of neuromasts, and 36 myomeres (12+24).The larva boremelanophores on the ventral ridge of breast and tail, air-bladder, and dorsal and ventral parts of the intestine.
In 14-18 days after hatching, most of individuals attained the total length of about 12 mm and were provided with cup-like pelvic fin.And shortly after that, they entered into the bottom life.Beginning of scale formation and completion of nostril were in the stages of about 10 mm and 17 mm in total length respectively.