Morphologies and population genetic structures of the eight-barbel loach of the genus Lefua on southern Sakhalin

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Coldwater, primary freshwater, fish such as the eight-barbel loach, Lefua nikkonis, are thought to have colonised Hokkaido from the continental Far East via Sakhalin Island during the Late Pleistocene. Lefua populations have been reported on southern Sakhalin, but detailed morphological and population structure analyses have not yet been completed. This information is important for reconstructing the colonisation history of L. nikkonis to Hokkaido. In this study, morphological analysis revealed that L. nikkonis and two continental congeners, Lefua pleskei and Lefua costata, are distinguishable from each other, and Lefua collected from southern Sakhalin is morphologically more similar to L. nikkonis. Random forest analysis, a machine learning classification method, classified all Sakhalin individuals as L. nikkonis. Haplotype analysis of mitochondrial DNA sequences revealed that all but one Sakhalin haplotype are shared with L. nikkonis and none is shared with the two continental congeners, which supports the hypothesis that Sakhalin Lefua are L. nikkonis. However, none of the Sakhalin haplotypes was distributed on northern Hokkaido. This discontinuous distribution of haplotypes across Sakhalin and Hokkaido suggests that the Sakhalin Lefua populations are not native. Some of the Sakhalin haplotypes were found only on Hokkaido’s Ishikari River system or the Tokachi River system, suggesting that they originated from these regions. Because previous field surveys reported wild Lefua only from northwestern Sakhalin, we concluded that native Lefua on southern Sakhalin may have gone extinct after they colonised Hokkaido in the Middle Pleistocene.