Two species of the genus Phoxinus, P. lagowskii and P. oxycephalus are distinct species that can be distinguished by morphology and distribution, but juveniles are difficult to identify by morphology. We documented the growth process of Phoxinus from juveniles and examined the length at which it became distinguishable, and conducted subsequent surveys. We conducted RFLP analysis of mitochondrial cytochrome-b and morphological analysis of the appearance of 275 individuals of Phoxinus sp. collected from four rivers including the Ochiai River in Higashikurume City, Tokyo, where both species are considered to coexist. In the Ochiai River, we found hybrids between P. lagowskii and P. oxycephalus, and that both species have been present for many years. The distribution of these fishes in the Ochiai River suggested that there may be areas in the Ochiai River where both species can coexist and areas where hybrids of both species are formed.

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The Kanto region was previously inhabited only by Phoxinus lagowskii. However, we have already observed P. oxycephalus-like fishes and individuals that appear to be hybrids of both species in the Ochiai River in Tokyo, suggesting the possibility of an invasion of P. oxycephalus. In this study, we analyzed maternally inherited mitochondrial cytochrome-b RFLPs and external morphology using 154 individuals collected from the Ochiai, Yokose, and Nariki rivers in the Kanto region, four rivers in Okayama Prefecture where P. oxycephalus are dominant, and two rivers in Mie and Nara prefectures, which are at the distributional boundary of the two species. The results showed the possibility of gene transfer from the distribution boundary area to the Ochiai River. Hybridization and coexistence of the two species were also observed in other rivers. The fact that the introduction of domestic invasive species has been observed in many rivers indicates the possibility of accidental and anthropogenic migration by releasing juvenile fish, and the disturbance of the ecosystem by domestic invasive species may be occurring in each river.

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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[in Japanese]

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