Abstract
Abstract Individuals of the brackish water ostracod species Ishizakiella ryukyuensis are found sympatrically with those of the closely related species I. miurensis at the Kii Peninsula, western Japan, and possess a projection on the male copulatory duct. By contrast, individuals of this species recovered from the Nansei Islands, southern Japan, do not have such a projection. Molecular phylogenetic trees based on mitochondrial COI sequences consistently indicated that the populations at the Nansei Islands formed a paraphyletic group outside of a cluster consisting of the population at the Kii Peninsula. Thus, the development of the projection in the male copulatory organ is considered as a derived character for the individuals at the Kii Peninsula, and this feature may have been brought about by the sympatric distribution and a process of character displacement.
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