We evaluated the morphological, distributional, and genetic characteristics of the freshwater sculpin, Cottus pollux, in Kyushu Island, Japan. Based on pectoral fin ray number, the sculpins inhabiting 30 rivers were divided into two types [modes 13 (M13) and 15 (M15)]. We evaluated four environmental parameters: length of main river (L-MR), average gradient of main river (G-MR), distance from sampling site to river mouth (D-SM), and gradient around sampling site (G-S), and compared the two types. L-MR, D-SM, and G-S were significantly larger for the rivers that contained M13 fish than for those containing M15 fish. M13 individuals were distributed in the upstream areas of large river systems, suggesting a fluvial life history, whereas M15 individuals were distributed in the downstream areas of small river systems, indicating an amphidromous life history. We conducted phylogenetic analysis based on mitochondrial 12S rRNA [788 base pair (bp)] and control regions (386 bp). Mitochondrial DNA analysis showed that both M13 and M15 groups were genetically of C. pollux middle-egg type (ME). Our findings proposed the hypothesis that C. pollux ME in Kyushu Island exhibits dimorphism in both morphological and distributional traits. Additionally, haplotype distribution indicated that the fluvial M13 populations had higher genetic specificity in each river in contrast to the existence of one genetic group of amphidromous M15 individuals in Kyushu Island.
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