Seven species belonging to Radula subg. Radula are accepted for Australasia. Radula oreopsis M.A.M.Renner is described as new, while R. kurzii, R. multiflora, R. reflexa and R. sharpii are excluded from the region. Molecular and morphological data provide evidence suggesting that the broad species concepts recently applied to subg. Radula in Australia and across the Pacific are not useful. Many subtle yet consistent differences in size and shape, and in micromorphological and anatomical characters potentially inform species circumscription. However, most differences between species are virtually impossible to apprehend independent of molecular data corroborating their significance. Herbarium-based studies and the interpretation of type material are therefore challenging. However, the molecular phylogeny based on three chloroplast markers unites a morphologically heterogeneous array of individuals from across Australasia and the Pacific into a single fully supported clade containing individuals corresponding to the type of R. javanica as well as individuals from Australia, New Zealand and Fiji attributed by various workers to R. erigens, R. javanica, R. multiflora and R. reflexa. There is a general lack of congruence between morphological and molecular groups across the phylogeny. Morphologically similar individuals are resolved in different clades where they are more closely related to morphologically dissimilar species, which may hint at morphological convergence. Morphologically different individuals are nested within each other. The unique cell ornamentation in R. oreopsis, but not in other individuals (here attributed to other species) within the same clade is one example hinting at rapid morphological evolution. The dispersed nature of land within island archipelagos means spatial isolation could contribute to origin and maintenance of species diversity across the Pacific. Every habitat may be effectively peripherally isolated by dispersal limitation. If rates of dispersal and divergence are equivalent across the region, the Pacific and bounding lands including the Wet Tropics Bioregion could maintain species paraphyly in perpetuity.
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