Japanese rosy bitterling Rhodeus ocellatus kurumeus is an endangered cyprinid species in Japan, due to hybridization with its subspecies R. o. ocellatus introduced from mainland China. In order to collect information for conservation, the present distribution of R. o. kurumeus in Kyushu was studied using genetic and morphological markers. Of the 46 populations examined, mitochondrial DNA (mtDNA) of R. o. kurumeus was observed in 41 populations, among which that of R. o. ocellatus was also recognized in 13 populations. Although R. o. kurumeus was widely distributed in the northern-middle part of Kyushu, it is suggested that introgression by R. o. ocellatus has been allopatrically progressing in many habitats of R. o. kurumeus. The averaged number of pored lateral line scales positively regressed with the frequency of R. o. ocellatus mtDNA. It is evident that the number of pored lateral line scales is a more efficient morphological marker than the presence of a guanine layer along the anterior margin of the pelvic fins in the identification of R. o. kurumeus.
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