Morphological divergence of Pseudodiaptomus inopinus Burckhardt, 1913 (Copepoda: Calanoida) between the Japan Sea and Pacific coasts of western Japan

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Two morphological groups of Pseudodiaptomus inopinus were detected in western Japan. One was from the Japan Sea and Tsushima Strait coasts and the other from the Pacific coast including the west and east coasts of Kyushu Island. Differences in females were seen in the posterior processes of the genital operculum, shape of the genital flap, and medial spinules on the first exopodal segment of the fifth leg, and those in males were in the shape of the first exopodal segment of the fifth left leg, and position of the medial process on the third exopodal segment of the fifth right leg. Proportional length of the caudal rami also differed in both sexes. Among these morphologies, the length of the genital operculum process discretely separated each group, but variations in the other morphometric characters were overlapping. Geographic distributions of the two groups were separated with a boundary between two rivers on the northwest coast of Kyushu Island. These results indicate that P. inopinus in Japan is a species complex and specimens from the Japan Sea and Pacific coasts belong to different species.