ドンコの生活史

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Mogurnda obscura are the large eleotrid goby, about 15cm in length (Fig. 1: A, B). They are distributed in the western Japan, and are dwelling in rivers, brooks, lakes and swamps, and solitary in life (Fig. 2: A). Especially the river dwellers are found on sandy mud bottom of stagnant waters in middle and lower course of river. They are rather nocturnal and feeding on small fishes and crustaceans, especially small shrimps. They are active in warm seasons, especially in spawning season extending from April to June, and they have a tendency to become hibernant in winter. The male fish are larger than the female, and they are polygamy. The sex dimorphism evidently appears in the form of genital papilla. No nuptial coloration appears in both sexes. The number of the ripe ovarian eggs, 1.5 to 1.9mm in diameter, were counted 934 to 3, 559 in 5 specimens ranging 95 to 162mm in total length (Table 1). The nest preferences were widely given to many kind of substances, e. g., stone, wood, bamboo, grass root, etc. in various forms. The egg masses were found mainly on underside of the nest shelters, and they were guarded by the male parent until the egg hatched out (Figs. 2, 3). A pair of the ripe fish, reared in an aquarium, spawned an egg mass on the inner wall of the earthernware pipe given as a nest. The egg is ellipsoid in shape, 5mm in long axis and 2mm in short axis. It is demersal adhesive one with a large yolk (Figs. 2, B: 3). The embryo hatched out after it developing to juvenile form (Fig. 4: A, B). The incubation period was about 30 days at 16 to 19°C. The newly hatched larvae were 7.5mm in total length with separate ventral fins, and after the hatching they entered instantly into a bottom life (Fig. 4). The larvae as well as the ones of the relatives, M. (M.) adspersus and M. mogurnda, and shout, and grew in an aquarium (Fig. 1, C). It is expected that the spawning is induced smoothly by the setting of artifical nest on the bottom of the habitat, because the nest is wanting under the natural condition. The authors adopted earthernware pipes and pieces of bamboo, about 30cm in length and 7cm in diameter, for the artificial nest, and attained a desired effect.