Anatomical and genetic data reveal that Rivularia Heude, 1890 belongs to Viviparinae (Gastropoda: Viviparidae)

Within Viviparidae, the systematic affinities of several genera are uncertain, with respect to one another, but also as to their subfamily attribution. From the genera that have been assigned unambiguously to one of the three subfamilies, a clear biogeographic pattern emerges that highlights a wide geographic separation of Bellamyinae from Viviparinae and Lioplacinae. Here we study Rivularia Heude, 1890, a genus previously assigned to Bellamyinae and largely restricted to the Hunan Province of China. We find that, although the taxon falls within the geographic range of Bellamyinae, comparative anatomical and genetic data reveal that it is a viviparine, tentatively the sister group of European Viviparus. Anatomical support includes the morphology of the radula and salivary glands, the presence of the testis in close association with the digestive gland in the viscera, rather than in the pallial cavity (Bellamyinae) and in females, the occurrence of the albumen gland and capsule gland ventral rather than posterior to the brood pouch, and proximal, rather than distal, to the last U-turn in the female reproductive tract. Greater differences are found when compared with Lioplacinae. Our finding represents the first record of natural co-occurrence of Viviparinae and Bellamyinae. Vicariance is the most likely cause for the strongly disjunct occurrence of Viviparinae in the Palearctic.