Radix Montfort, 1810

Aksenova, Olga V., Vinarski, Maxim V., Itagaki, Tadashi, Ohari, Yuma, Oshida, Tatsuo, Kim, Sang Ki, Lee, Jin Hee, Kondakov, Alexander V., Khrebtova, Irina S., Soboleva, Alena A., Travina, Oksana V., Sokolova, Svetlana E., Palatov, Dmitry M., Bespalaya, Yulia V., Vikhrev, Ilya V., Gofarov, Mikhail Yu. & Bolotov, Ivan N., 2024, Taxonomy and trans-Beringian biogeography of the pond snails (Gastropoda: Lymnaeidae) of East Asia: an integrative view, Zoological Journal of the Linnean Society 201 (4), pp. 1-24 : 13-16

Genus Radix Montfort, 1810

Montfort 1810: 266.

Type species: Helix auricularius Linnaeus, 1758 (by monotypy).

Diagnosis: Shell variable in size and shape; varying from ear-shaped to high-conical. Shell of the largest species can reach 30–35 mm in height ( Fig. 5A–G,I,J). Prostate with a single internal fold. Praeputium sac-like; it is much wider than the penis sheath ( Fig. 4I–K). The ratio of the lengths of these structures varies within the taxon, but it is typically close to 1.0. The penial knot is absent. The spermathecal duct is long.

Species richness and taxonomic remarks: The genus contains two subgenera: Radix s.s. and Exsertiana Bourguignat, 1883 [type species Limnaeus natalensis var. exsertus Martens, 1866 = Radix natalensis (Krauss, 1848) ]. The latter subgenus is restricted to tropical regions of the Old World ( Vinarski et al. 2020). According to various authors, the genus includes two ( Jackiewicz 1998) or>40 ( Kruglov 2005) species. However, the validity of many nominal species of Radix has recently been rejected as a result of several integrative taxonomic studies ( Bolotov et al. 2014, Aksenova et al. 2016, 2017). The exact number of valid species of Radix remains unknown. This genus is represented in the study region by three species, which renders it one of the most species-rich lymnaeid genera of East Asia and Alaska. Of these three species, Radix auricularia (Linnaeus, 1758) has the widest geographical range, being found in most of Europe, northern and central Asia, northern Africa, and northern America ( Fig. 5A; Table 3 ). Radix plicatula is a Far Eastern species; its range extends to Vietnam and Taiwan in the south ( Fig. 5I, J). In Japan it has been recorded from Hokkaido Island to Okinawa Island ( Saito et al. 2021; see Supporting Information, Dataset S2). We have examined two lots of ‘syntypes’ of Limnaea okinawensis Ehrmann (nomen nudum inJacobi 1898: S. 85) kept in SMF. Unfortunately, Jacobi (1898), who provided a rather detailed description of the soft-body anatomy of this snail, did not illustrate the shell. Most probably, these specimens represent R. plicatula . Radix coreana appears to be a conchological form of R. plicatula from the Korean Peninsula ( Fig. 5B). It has also been recorded from the Russian Far East ( Bogatov and Zatravkin 1990, Vinarski et al. 2016). Two further nominal species can be considered probable synonyms of R. plicatula ( Table 2 ). Among these, Radix japonica (Jay, 1857) is the oldest available name. Its taxonomic identity, however, has remained obscure. Samples of Japanese lymnaeids that are labelled as ‘ Lymnaea japonica ’ or ‘ Radix japonica ’ in European zoological museums are rather heterogeneous. Some of them apparently belong to R. auricularia . The original description (Jay 1857) depicts the shell of this species as almost ovoid, with rounded whorls and a drop-like aperture. We could identify some specimens of Radix from Japan externally approaching the original picture (see Fig. 5D, E) and thus apply this species name to the R. plicatula clade revealed herein. However, the possibility that Radix japonica sensu Jay (1857) was, in fact, conspecific with R. auricularia cannot be ruled out. This makes Lymnaea japonica Jay, 1857 a taxon inquirendum, and no conclusive decision on its true identity is possible.

Radix hamadai , a species with a very peculiar conchology, was described from Japan (Ohta Prefecture, Kyushu Island). Despite its extreme external dissimilarity to typical representatives of R. plicatula , our results show that this taxon might represent an ecophenotypic morph of R. plicatula . Lymnaeid species with similar shells are known from other regions of Eurasia [i.e. Radix relicta from Lake Skadar, Balkans; Radix onychia ( Westerlund, 1883) from Lake Biwa, Japan]. A specimen, identified by Ohari et al. (2020) as R. auricularia with Ra03 haplotype, has peculiar conchological traits (see Ohari et al. 2020: fig. 2, E1–E2), which differentiate it clearly from the typical R. auricularia . Its shell shape and proportions are more similar to those of R. hamadai , and this specimen has been identified tentatively as Radix cf. hamadai during this study. However, the molecular data on the topotypic specimens of R. hamadai remain inaccessible, and we have no direct evidence that the studied specimen is conspecific with R. hamadai from Kyushu Island. Nonetheless, our results show that R. plicatula can develop a peculiar shell morphology resembling that of R. hamadai . The taxonomic status of the latter species needs additional research.

Distribution: Radix is widely distributed in Eurasia and Africa but absent from Australia and South America ( Aksenova et al. 2018, Vinarski et al. 2020). One species ( R. auricularia ) is native to Alaska, while its European lineage was introduced elsewhere in North America ( Burch 1989, Aksenova et al. 2018).