The phylogenetic status of the alpine land snail Fruticicola biconica has remained questionable since it was described by Eder in 1917. Considered a microendemic species from mountain tops in Central Switzerland, the shell is specially adapted for life under stones. Herein, we show via molecular and anatomical investigations that F. biconica neither belongs to the land snail genus Trochulus, nor to any other genus within Trochulini, but rather warrants placement within the newly established genus Raeticella Kneubühler, Baggenstos & Neubert, 2022. Phylogenetic analyses reveal that R. biconica is clearly separated from Trochulus. These findings are supported by morphological investigations of the shell and genitalia.

Endemism, integrative taxonomy, LGM, mountains, nunataks, phylogeny, Switzerland, Trochulus

Discovered in the Bannalp, Nidwalden and known from only a few localities in the Central Swiss Alps, Fruticicola biconica was described by the Swiss zoologist Leo Eder in 1917.

Later, F. biconica, known as the Nidwaldner hairy snail, was moved to the widely used genus Trichia W. Hartmann, 1840 and circulated throughout the European literature under this designation (e.g., Kerney et al. 1983). The generic name, Trichia, was subsequently replaced by Trochulus Chemnitz, 1786 due to homonymy with Trichia De Haan, 1839 (Crustacea, Xanthidae).

Previous studies (Pfenninger et al. 2005; Dépraz et al. 2009; Duda et al. 2014; Kruckenhauser et al. 2014; Proćków et al. 2021) included T. biconicus individuals in their genetic analyses of Trochulus species. Pfenninger et al. (2005) and Dépraz et al. (2009) used the same sequence of T. biconicus collected at the type locality at Bannalp. This sequence clustered within the so far known Trochulus species and some newly identified lineages, which were not further described (fig. 2 in Pfenninger et al. 2005; fig. 1 in Dépraz et al. 2009). Most likely, Pfenninger et al. (2005) and Dépraz et al. (2009) used misidentified specimens in their phylogenetic studies, or some samples were mixed. Since these authors did not publish images of the investigated specimens, an unequivocal identification is not possible. Duda et al. (2014) and Kruckenhauser et al. (2014) found that T. biconicus, “T. oreinos oreinos” (A.J. Wagner, 1915), and “T. oreinos scheerpeltzi” (Mikula, 1957) form basal lineages in comparison to specimens of Trochulus s. str. The latter two taxa were elevated from subspecies to species level (Bamberger et al. 2020) and are today known to belong to the newly described genus Noricella (Neiber et al. 2017). Proćków et al. (2021) found the same phylogenetic pattern as Duda et al. (2014) and Kruckenhauser et al. (2014) and questioned the affiliation of biconicus to Trochulus. Already Turner et al. (1998) had disputed the phylogenetic position of T. biconicus. Until today, the phylogenetic position of T. biconicus within the Trochulini remained unclear. Hence, an integrative taxonomic approach is applied in this study to investigate the phylogenetic affiliation of T. biconicus.

Neiber et al. (2017) clarified the phylogenetic positions of some species within the Trochulini by establishing the new genus Noricella Neiber, Razkin & Hausdorf, 2017. In their study it was proven that N. oreinos and N. scheerpeltzi differed from the closest related genus Edentiella Poliński, 1929 in some apomorphic nucleotide substitutions and by morphological characters. Edentiella contains at least one longitudinal septum separating an additional lacuna in the penial papilla which is lacking in N. oreinos, in most Trochulus species, and in Petasina (Neiber et al. 2017). These authors also included some representatives of Trochulus but did not have specimens of R. biconica available. Turner et al. (1998) had already considered R. biconica to be only distantly related to Trochulus s. str. because of 1) the lack of periostracal hair even in juveniles, 2) a very long flagellum, and 3) only four instead of six or eight mucous glands. Hence, Turner (1991) suggested to move R. biconica into a subgenus of Trochulus. The questionable position of biconicus in Trochulus was recently re-addressed by Proćków et al. (2021). In our analysis, the calculated p-distance of R. biconica and the investigated Trochulus specimens comprises the highest values. The p-distance of R. biconica and Noricella species is lower than for Trochulus, which means that Raeticella is genetically closer, based on COI, to Noricella than to Trochulus. Even Ichnusotricha, which belongs to the tribe of Ganulini is genetically more similar to Raeticella than Trochulus is to Raeticella.

The shell morphology of R. biconica differs from all known Trochulus species by having a flat shell with a low spire. The last whorl is bluntly keeled. Adults are always hairless (Proćków 2009). In this regard, it is most like the shells of the two Noricella species (Duda et al. 2011, 2014), but the anatomy of the genital organs of these species is different. Both Noricella species have four pairs of mucous glands, compared to two pairs in R. biconica. Noricella oreinos possesses an additional fold and bulge in the penial papilla, which seems to be unique to this species (Duda et al. 2014). The section of the penial papilla in R. biconica shows similar internal features as in T. caelatus (Proćków 2009), T. striolatus (Proćków 2009; Duda et al. 2014; Proćków et al. 2021), and T. suberectus (Proćków 2009). Raeticella biconica does not possess periostracal hairs, neither as a juvenile nor as an adult. This, however, is considered a typical feature for Trochulus species (Proćków 2009).

Hewitt (2004) observed that many taxa in temperate refugial regions in Europe and North America show relatively deep DNA divergence, indicating their presence over several ice ages and suggesting a mode of speciation by repeated allopatry. On the one hand, this possibly explains the deep split between Raeticella and Trochulus and shifts the splitting event of these groups to the Pliocene. On the other hand, we observed a low genetic diversity within our analysed populations. So, this species probably underwent a bottleneck event during the Pleistocene and the Last Glacial Maximum (LGM). Some isolated populations obviously survived this icy period. The LGM lasted about 30–19 ka in the Alps. During that period, this area was covered by massive ice sheets, and the glaciers reached out to the forelands of both, the northern and southern side of the main alpine chains. However, mountain tops above more than 2000 m were not covered by ice during the LGM. The recession of the glaciers from their maximum extent started around 24 ka (see Ivy-Ochs 2015). We hypothesize that the original distribution area of R. biconica was much larger, but only a few individuals survived on neighbouring nunataks (glacial islands) during the LGM. A similar scenario is assumed for the evolution of the two Noricella species (Duda et al. 2011, 2014; Kruckenhauser et al. 2014). Gittenberger et al. (2004) also hypothesized the survival of Arianta arbustorum alpicola (A. Férussac, 1821) on nunataks. A similarly fragmented distribution pattern can be observed in the eastern alpine mollusc species Cylindrus obtusus (Draparnaud, 1805) (Schileyko 2012: 95, fig. 2). Schileyko argued that the missing fossil record for this species proves that it was formed at the end of the Würm glaciation approximately 10–12 ka ago. As a species adapted to cold environmental conditions, this species was then assumed to be forced to follow the retreating snow and ice fields, which subsequently lead to habitat fragmentation. This assumption requires an ancestor from interglacials (which is also not found in the fossil record), and has to explain the rapid transformation of an Ariantine species from a globular or even depressed shell to a turriform shell. This is most unlikely. Based on COI sequences, Cadahía et al. (2014) estimated 1.5–12 mya for the split between Arianta and Cylindrus. So, we assume that Raeticella gen. nov., like the monotypic genus Cylindrus, evolved much earlier and survived the Pleistocene by chance on nunatak mountain tops.

The current distribution pattern does not necessarily and strictly reflect the “survivor” populations. ARNAL (2018) found a limited gene flow between the “isolated” populations of R. biconica. This shows that dispersal is not completely impossible, but, due to the high-altitude adaptation of the species, it is rather limited to other, hitherto unpopulated high alpine areas. Possible vectors may be large pasturing animals like sheep and goats, but also ibex, chamois, or birds.

In alpine environments, microendemic species with a relict distribution pattern may occur, which were much more widespread in earlier times. They are now restricted to a very small area due to changes in environmental condition (Turner 1991; Cook 2008; Veron et al. 2019). The distribution area of R. biconica is currently known to encompass 150 isolated sites on both sides of the Engelberger valley, all situated between 1890 and 2575 m of altitude (Baggenstos 2010).

The habitat of R. biconica is very special, and only few other snail species are known to survive in this harsh environment (Eder 1917; Baggenstos 2010). Apart from the occurrence of limestone scree, the snails very much depend on small-scale relief. Slope edges or hilltops, ridges and summits as well as rocky heads and rocky steps are more likely to be colonised by the snail than slope hollows and slope foothills. The highest density of R. biconica is reached in areas with more than 50% of rocky scree (Baggenstos 2010). All these sites are covered with snow for a relatively short time in winter. With its flat shell, R. biconica is perfectly adapted to live under or between stones (Figs 12, 13). Flatness was interpreted as an adaptation to the cold climate at the top of the mountains and may protect the animals from predators (Baur 1987). When it gets too hot, the snails retreat into the ground. The individuals are mainly active during night (Baggenstos 2010). Almost all known R. biconica habitats are blue grass meadows. These are alpine grasslands rich in flowers with a great diversity and a remarkably high proportion of Leguminosae. The prominent structural elements are Sesleria caerulea and Carex sempervirens. The soil cover is relatively thin, interspersed with gravel and stones and dries out quickly (Delarze et al. 2008). Wigger (2007) observed that R. biconica mainly feeds on decaying leaves of blue grass (Sesleria caerulea). The landscape of these meadows is strongly influenced by extensive pasturing and hiking tourism. Pasture animals like sheep, goats, and cows can modify the position of large stones and thus create new micro habitats for the snails. However, stronger interventions, such as the removal of stones or a climate-related transfer of the rubble-rich sites into closed meadows or woodland formations would cause the snail to disappear (Turner 1991).

Figure 12. 

Typical habitat of Raeticella biconica. This photograph was taken on 25.05.2009 on Chaiserstuel (46.8762°N, 8.4671°E, 2263 m) by Markus Baggenstos.

This stenoecious species is prone to extinction because of climate change. Over the last 100 years temperatures have increased by about 0.12–0.20 °C per decade in the Swiss Alps and the snow seasons have shortened (Kohler et al. 2014). Raeticella biconica already reached the summits of the mountains in their vicinity, and there is no more alternative for avoiding unsuitable climate conditions. Considering that global warming is ongoing, R. biconica may well become extinct in just a few years.

Figure 13. 

Close-up of Raeticella biconica crawling on the underside of a stone. This photograph was taken on 09.09.2009 on Chaiserstuel (46.8762°N, 8.4671°E, 2263 m) by Markus Baggenstos.

Long known morphological characteristics in conjunction with our genetic analyses show that R. biconica should be assigned to a new genus. Morphologically, the investigated individuals of R. biconica strongly resemble N. oreinos (Duda et al. 2011). But the genetic analyses of several different species from all genera within Trochulini reveal that R. biconica does not belong to any currently known genus. Therefore, a new monotypic genus within Trochulini is introduced.

We are indebted to Małgorzata Proćków for providing tissue samples of E. edentula and some Trochulus specimens, to Ted von Proschwitz for providing T. hispidus samples from the Swedish type locality, to Adrienne Jochum for the linguistic revision, to Tom Burri for paleoecological insights, to the Swiss Federal Office of Environment (FOEN) for financial support (contract no. 110010344 / 8T30/00.5147.PZ/0006), and to the cantonal authorities of Nidwalden (Felix Omlin), Obwalden (Andreas Bacher), and Uri (Georges Eich) for providing sampling permits.