天竜川上流三峰川で採捕されたアマゴとヤマメとの間種 Occurrence of the Natural Hybrid between Oncorhynchus masou and Oncorhynchus rhodurus in the Upper Stream of Tenriu R.
It was affirmed previously by means of continuous breeding experiments carried on by the former Director of the Fisheries Experiment Station of Gifu Prefecture(1924-1927) that the hybrid between the sea-run type of Oncorhynchus masou and river-type of Oncorhynchus rhodurus is fertile, while the back-cross between F_1 and the latter is entirely sterile instead. The most clear specific difference between the two river-types of the above mentioned Oncorhynchus are (1) the colouration of the round spots scattered above and below of the lateral line and (2) arrangement of the circuli in the somatic scales. In the artiffcially crossed hybrid black spots of Oncorhynchus masou reveals as an apparently dominant character while crimson spots of Oncorhynchus rhodurus subsides as recessive character while the concentric circular circuli of Oncorhynchus rhodurus becomes dominant to the broken external circuli of Oncorhynchus masou. Whether such cross exists in nature or not was a question among ichthyologists long time since. Quite recently, however, dubiuous specimens supposed to be the river from of Oncorhynchus masou have been obtaind in the upper stream of Tenriu River, mingling with 'Amago' the river-type of Oncorhynchus rhodurus. Specimens obtained by Mr. Takeo Iwasaki of Ina City has been forwarded very kindly for examination. The writer came to conclusion that the specimens exposed is really the hybrid between two species of Oncorhyuchus although intrudence of Oncorhynchus masou into Tenriu tributaries is not solved yet. In the present paper is given the description of naturally produced hybrid between Oncorhynchus masou and Oncorhynchus rhodunus, given the new name Oncorhynchus hybridus sp. nov. (Japanese name 'Hiraki-masu'). Appreciations are due to Mr. Takeo Iwasaki by whose efforts such interesting specimens came to the writer's hand.
動物学雑誌 68(7), 259-262, 1959-07