有明海 シロチチブ

In the innermost part of Ariake Sound, which is situated on the coast of Kyushu, Japan, three gobioid fishes, Tridentiger undicervicus Tomiyama, Tridentiger trigonocephalus (Gill) and their allied Triaenopogon barbatus (Gunther) live a similar life in same muddy shallow. Tridentiger undicervicus is a plump gobioid fish some 50 mm in total length, with trilobed teeth-bands, hitherto collected only from Ariake Sound (Fig. 1). It inhabits the rather hard tidal flat of sandy mud (Fig. 2). The sexes are distinguished in this gobioid fish of over 25 mm in total length from the structure of urogenital papilla, as is already mentioned in Triaenopogon barbatus. The ripe ovaries contained two egg-groups, one the ripe eggs, 0.50～0.59 mm in diameter, and the another the unripe ones, 0.03～0.14 mm in diameter. The number of ripe, ovarian eggs in one individual was enumerated as 1,812 to 5,770 in seven individuals (Table 1). The ripe testis is a white, thick bandformed body with a pair of appendant organ ("Seminal vesicle": Weisel, G. F. 1949). The spawning occurred in ordinal habitat of the adult fish from the end of May to the end of September, mainly in July, also was observed in the spawning-ground of Triaenopogon barbatus together with the spawning of the latter. The spawned eggs were deposited in one layer on the cleaned inner surface of the slightly opened, empty shells of Ostrea rivularis Gould, Ostrea denselamellosa Lischke, Pinna pectinata japonica Reeve and Rapana thomasiana Grosse, lying on the tidal flat (Table 2). The number of the spawned eggs in one sheet was enumerated as 7.693 and 13099 in two cases. In three cases out of the observed over twenty egg masses , the mass consisted of two remarkably different egg-groups of developmental stages. The male parent fish with flat head was guarding the eggs in the nest (Fig. 2). The polygamous habit of large male fish was supposed from the observational data on the spawning habit, obtained from the natural spawning-ground. The spawned egg is elongate pyriform in shape, 1.15 mm in long axis and 0.45 mm in short axis, with a sheet of adhesive filaments at the basal end. It is smaller than the egg of Tridentiger trigonocephalus of similar form. The incubation period was about four days at the water-temperature ca. 25℃ (Fig. 3, The newly hatched larva is about 2.0 mm in total length with 24 (9 + 15) myotomes. The postlarvae, 10～15 mm in total length, lived a swimming-life , feeding on planktonic copepods, in the water-way of the estuary . The larvae were collected with the set-nets from June to September together with small shrimps and other fish larvae, including the larvae of Triaenopogon barbatus and Tridentiger trigonocephalus of similar size. After August, the juveniles, over 15 mm in total length were found already entering into the bottom-life everywhere the adult fish lives, together with the adult of Triaenopogon barbatus and Tridentiger trigonocephalus as well as their juveniles (Fig. 3, E～I). The gut-contents of juveniles and adult fish were mainly organic detritus. This gobioid fish seems to grow to 22～52 mm in one year, becoming mature in most individuals in this period, and 52～68 mm in two years, based upon the examination of size frequency and scale-phases of the collected specimens. The female minimum adult was 28 mm in total length. The life-span seems to be over three years. Tridentiger trigonocephalus (Gill) is also a plump gobioid fishes, synonymous with Tridentiger bifasciatus Steindachner, some 60 mm in total length. with trilobed teeth bands (Fig. 5, B～E). It is distributed in China, Korea, Eastern Siberia and Japan (Fig. 4). In the innermost part of Ariake Sound, it inhabits the vicinity of datum line of both habitat of Triaenopogon barbatus and Tridentiger undicervicus. The sexes are distinguished in this gobioid fishes over 40 mm in total length, based upon the structure of the urogenital papilla, as in Tridentiger undicervicus. The ripe ovaries contained two egg-groups of different features, the one of the ripe eggs, 0.50～0.60 mm in egg-diameter, and the another of the unripe ones, 0.03～0.14 mm in diameter. The ripe egg-group seems to be spawned at. one time, and the number of eggs enumerated as 1,248～9,700 in seven examined individuals (Table 3). The ripe testis is similar to that of T. undicervicus (Fig. 5, A). The spawning occurred in the same oyster-beds as in Triaenopogon barbatus, from April to June, mainly in May. The eggs were deposited in the empty shells of Ostrea rivularis Gould just as T. barbatus, and the male parent fish was also guarding the eggs in the nest, whereas the female left the nest after the spawning?act finished (Table 4). In this species also the polygamous habit of large male parent fish is supposed from the data obtained from the natural spawning-ground. The spawned egg is elongate pyriform in shape, measuring 1.4 mm in long axis and 0.6 mm in short axis with a large perivitelline space. The tip of the egg in somewhat pointed, and the basal end is provided with a bundle of adhesive filaments. The shape of this egg is quite same with those of Tridentiger obscurus (Temminck et Schlegel) and T. undicervicus (Fig. 6). The incubationperiod was about 8.5 days at the water-temperature ca. 20℃. The newly hatched larva is 2.4 mm in total length. The postlarvac, 7～15 mm in total length, were collected with set-nets set in the water-way of the estuaries from May to July. The early postlarvae supposely live a swimminglife there. The juveniles, over 15 mm in total length, have already entered into the bottom life in the same habitats as those of Triaenopogon barbatus and Tridentiger undicervicus. The larvae, over 10 mm in total length, of these three allied species can be distinguished with certainty from each other by examination the number of fin-rays and the size of eye. Based upon the size frequency and the scale-phases of collected specimens, this goby seems to grow 35～55 mm in total length in one year and 55～75 mm in two years. A few individuals seems to live over three years. The female minimum adult seems to be 38 mm in total length. The growth-type and the scale-phase seem to be similar in Triaenopogon barbatus and Tridentiger undicervicus, but slightly different in Tridentiger obscurus which is distributed widely in Middle and Southern Japan, but not in Ariake Sound.