The Bionomics and Life History of the Gobioid Fish, Rhinogobius giurinus (RUTTER)

Rhinogobius giurinus RUTTER is a small gobioid fish, some 70mm in total length (Pl., ⅩⅥ Figs. 1, 2). In Japan it is restrictely distributed to the south-western districts. It inhabits in the lower part of river and estuary, and lives a individual life on the bottom of shallow. It is easily distingushed from the another gobioid fish, Rhinogobius similis, which inhabits in the same habitat by the form ofventral fin (Text-fig. 1 J, K). It is aboundant in Lake Ikeda and Pond Unagi which are situated in the southern Kyushu under landlocked condition. It fed on aquatic insects, small fishes, shells and crustaceans as well as organic detritus. In the littoral zone of Lake Ikeda it exceedingly fed on eggs of "Koayu", a landlocked form of Plecoglossus altivelis (Table 1). The spawning-season seemed to extend from the beginning of July to the end of October in River Muromi, Fukuoka City. The sex-dimorphism developed in the form of genital papilla as well as soft dorsal fin. The spawning occerred in the shallw where the adult fish always lived (Pl. ⅩⅥ, Fig. 3). The fertilized eggs were deposited to the flat bottom of stone settled on river bottom in one layer mass. The number of deposited eggs in a mass was enumeratd as 1,723, 1,736 and 2,061 in 3 cases, and number of ripe ovarian eggs was enumarated as 1,118～22,515 in 7 specimens (Table 2). The male parent fish stayed in the breeding-room constructed under the stone, and guarded over the egg-mass until they hatched out. The incubation-period of egg extended about 4 days at temperature about 25℃. The egg has shape of club with a shallow depression encircling near the tip. It is 2.4mm in long axis and 0.45mm in short axis, and have a bundle of adhesive threads at the basal end (Text-fig. 1, A～D). The newly hatched larva was 2.6mm in total length, and found its way into sea (Textfig. 1, E). The larval fish seemed to live a swimming-life in coastal zone for about 2 months (Text-fig. 1, F). The juvenile and young, over 17mm in total length, went up stream in the lower part of river during autumn and early winter (Text-fig. 1, G, H). The seasonal change in size-frequency of collected specimens, over 300 individuals, shows that some of the fish grow to over 33mm and become mature in a year, whereas the other become mature in 2 years