Luchuphaedusa (Oophaedusa) ophidoon (Pilsbry, 1905), a clausiliid snail endemic to Shimo-Koshiki Island, has been considered as a single species. However, the present analysis on morphological variation of L. (O.) ophidoon has revealed that this species consists of three morphological distinct forms : Northern large form (NL), Southern large form (SL), and small form (Sm). These forms differ from each other in several conchological and anatomical characters. NL inhabits the northern region of the island and is distributed allopatrically with other forms. On the other hand, SL inhabits the southern region. Sm is endemic to a small cape, Sebi-zaki. At Sebi-zaki, SL and Sm come into parapatric contact forming a narrow hybrid-zone. Although the results of a crossing experiment and field observations suggested nonassortative mating in the hybrid-zone, presumed F_1 hybrid exhibiting intermediate morphology was extremely rare and morphological differences between SL and Sm was clear in the hybrid-zone. In addition, introgression across the hybrid-zone was not detected. SL and Sm maintain distinct gene pools, despite hybridization. SL and Sm are therefore distinct species isolated reproductively. Strong selection against hybrids must be operating in the hybrid-zone and might prevent intergradation. NL would be the syster taxon of Sm. Whether NL is conspecific with the other forms remains to be determined, although NL is clearly diverged morphologically and evolutionarily from the other forms. Therefore, L. (O.) ophidoon is a species complex in which reproductive isolation has been developed in part. Based on the phylogenetical and biogeographical evidence, genetic differentiation, involving speciation, in L. (O.) ophidoon species complex has occurred within the small island of 11 km in the maximum length. L. (O.) ophidoon species complex provides excellent material for studying speciation process in land snails.
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