INTRODUCTION

Cepaea nemoralis (Linnaeus, 1758) is one of the best known species of European land molluscs. It is also a classic subject for population genetic studies due to a number of heritable traits of the shell colouration (Murray 1975). Numerous publications, including monographs (Lamotte 1951, Schilder & Schilder 1953a, 1957), reviews and discussion papers (Jones et al. 1977, Clarke et al. 1978, Cameron 1997, Cook 1998, Sverlova 2004, Ożgo 2008, etc.), were devoted to its shell colour and banding polymorphism. Not long ago, C. nemoralis and another species of the same genus, Cepaea hortensis (O. F. Müller, 1774), were the focus of European citizen science thanks to the large project Evolution MegaLab (Silvertown et al. 2011, Cameron & Cook 2012, Cameron 2013).

The current range of C. nemoralis has expanded significantly due to human activity, both intentional (deliberate introductions of attractive snails) and more often accidental (usually together with seedlings of garden and ornamental plants). Already in the second half of the 19th century, this species was introduced to North America (Howe 1898, Alexander 1952), south-eastern Poland (Bąkowski 1880, Łomnicki 1899), and the Czech Republic (Peltanová et al. 2012). Almost immediately after the first discovery of C. nemoralis in the south-east of present Poland (Bąkowski 1880), an attempt was made to introduce it from there to Lviv (Łomnicki 1899), which was then also part of Poland, but is now the largest city in western Ukraine. However, this attempt was not successful (Gural-Sverlova et al. 2020). In the first half of the 20th century, C. nemoralis was mentioned only once for Lviv, when Urbański (1933) found several immature specimens in the Lychakiv cemetery. By the end of the 20th century, only one of the Lviv parks was inhabited by a declining population of C. nemoralis of unknown origin, with low abundance and limited phenotypic composition (Sverlova 2002a).

In other administrative regions of Ukraine, C. nemoralis was not reliably known until the beginning of the 21st century (Gural-Sverlova et al. 2024a: table 5). Some earlier mentions were most likely based on erroneous identifications of Caucasotachea vindobonensis (C. Pfeiffer, 1828), widespread in Ukraine (Sverlova 2006). In recent years, the number of known records of C. nemoralis in different parts of Ukraine has been rapidly increasing, both published (Gural-Sverlova et al. 2020, Balashov & Markova 2021, Gural-Sverlova & Lyzhechka 2021, Gural-Sverlova & Gural 2023, Martynov & Nikulina 2023) and those made by amateur naturalists especially via iNaturalist (2025) and UkrBIN (2025).

Similar trends have been observed in other parts of Eastern Europe (Gural-Sverlova et al. 2021a), which may be related to the activities of garden centres importing seedlings from other European countries (Gural-Sverlova & Gural 2025a), global warming and milder microclimates in urbanised areas. In particular, C. nemoralis is now already quite widespread in Minsk (Gural-Sverlova & Kruglova 2022), Moscow and the Moscow region (Egorov 2018, Gural-Sverlova & Egorov 2021, iNaturalist 2025). East of the Moscow region, C. nemoralis is found sporadically, according to published data, to Nizhny Novgorod (Mukhanov & Lisitsyn 2018), and according to observations in the database iNaturalist (2025), much further – to the Urals (Ufa) and even Central Asia (suburb of Almaty in Kazakhstan).

Far beyond its natural range, Eastern European populations of C. nemoralis are forced to adapt to a more continental climate, which is only partially mitigated by global climate change (in particular, warmer winters) and microclimatic features of settlements. This may affect their phenotypic composition (Gural-Sverlova & Gural 2021a). The combination of climatic selection and random population genetic processes (primarily the founder effect and genetic drift at the initial stages of the formation of new populations) can lead to a unique suite and/or ratio of inherited colouration traits, as was recently shown for western Ukrainian populations of the related species C. hortensis (Gural-Sverlova & Gural 2025b). This makes the phenotypic composition of Eastern European Cepaea populations particularly attractive for research. Given the youth of most populations of C. nemoralis in Eastern Europe, a detailed description of their polymorphism could provide an important basis for monitoring possible subsequent changes.

In Ukraine, the shell colour and banding polymorphism in C. nemoralis has so far been quantitatively studied almost exclusively in the west of the country (Gural-Sverlova et al. 2020, Gural-Sverlova & Lyzhechka 2021, Gural-Sverlova & Gural 2023, 2025a), mainly in Lviv and its immediate surroundings (Gural-Sverlova et al. 2021b, Gural-Sverlova & Kruglova 2022, Gural-Sverlova et al. 2024a). In recent years, however, we have accumulated quite a lot of data that has not yet been published. In eastern Ukraine, the phenotypic composition of one population of C. nemoralis from Donetsk has been described so far (Martynov & Nikulina 2023). Balashov & Markova (2021) list some phenotypes recorded in the central (Kyiv and surrounding areas) and southern (Odesa) parts of the country. In addition, a fairly large number of C. nemoralis photographs from different administrative regions of Ukraine have already been accumulated in citizen science databases (iNaturalist 2025, UkrBIN 2025).

In this paper we have aggregated the available quantitative and qualitative data on the shell polymorphism of C. nemoralis in Ukraine, and, as far as possible, attempted to assess the level of phenotypic diversity in recently formed populations of this species. In the future, this may become a good basis for monitoring possible changes in the phenotypic composition of C. nemoralis populations in urbanised areas of Ukraine.

MATERIAL AND METHODS

We analysed available data on the shell colouration in populations of C. nemoralis, found in different parts of Ukraine at the beginning of the 21st century:

In total, we aggregated and analysed qualitative data for more than 70 localities (usually settlements, less often their environs) from 17 administrative regions of Ukraine (Appendix 1). Quantitative data, primarily for the western part of the country, are presented in Appendix 2: seven regions, 28 localities, almost a hundred collection sites, more than 13 thousand specimens. Figure 1 shows the locations of all localities mentioned in Appendices 1 and 2.

Fig. 1

Localities of C. nemoralis in Ukraine (top) and its Lviv region (bottom), mentioned in Appendices 1 and 2 (qualitative and quantitative data on the phenotypic composition, in red) or only in Appendix 1 (qualitative data, in green)

Samples were usually collected at sites without significant anthropogenic barriers that would impede the movement of snails (Sverlova 2002b). Their size often did not exceed the diameter of the panmictic unit, which for C. nemoralis is estimated at 50–60 (Jones et al. 1977) or 100 m (Schnetter 1950). When searching for populations of C. nemoralis, we paid particular attention to sites with relatively young ornamental plantings, especially the currently popular thujas, junipers, and other conifers, as well as areas adjacent to garden centres, either operating or recently closed. Two typical habitats of C. nemoralis in Lviv are shown in Gural-Sverlova et al. (2021a: fig. 2).

In most cases, phenotypes were counted only for adult live snails or their empty shells with well-preserved colouration. The surface of empty shells was sometimes moistened with water, which made it possible to better distinguish their ground colour. If the related species C. hortensis was completely absent at the collection sites, and the abundance of C. nemoralis was low (Boiarka in the Kyiv region, Rava-Ruska in the Lviv region) or the population had an interesting colouration trait (spotted and split bands in Chortkiv, Ternopil region, see in Gural-Sverlova et al. 2021a: fig. 5), samples could additionally include immature specimens with a shell diameter of at least 1 cm. Since the bands on Cepaea shells do not appear immediately and not all at the same time, using younger specimens may lead to an incorrect determination of the banding type. Immature individuals of the two Cepaea species also cannot always be reliably distinguished. This is especially true of yellow unbanded shells common in C. hortensis.

The shell ground colour was classified as yellow (Y), pink (P), or brown (B). The group of yellow shells traditionally also included those with a white ground colour (Fig. 2, top), occurring in both Cepaea species (Gural-Sverlova & Gural 2022a). Unlike western Ukrainian populations of C. hortensis (Gural-Sverlova & Gural 2025b), white shells in C. nemoralis are found much less frequently here and are present in far from all populations (Gural-Sverlova et al. 2021b: 55). Therefore, we did not consider it appropriate to distinguish them as a separate group.

Figs 2–10

Specimens of C. nemoralis from Lviv designated as yellow (2), pink (3), brown (4, 5), mid-banded (6–8), and three-banded (9, 10). For more details, see Material and Methods

In the group of pink shells, the ground colour varied from orange (Fig. 3, left) or pale greyish-pink to intense pink. We considered orange to be a combination of pink and yellow (maybe as a result of incomplete dominance of pink in heterozygotes?). Moreover, neither C. nemoralis nor C. hortensis ever produce orange shells in areas where pink ones are absent.

The third group included unbanded shells with a brown ground colour of varying intensity (Fig. 4), but most often dark, sometimes with a lilac or cherry tint (Fig. 5). Brown banded shells were absent.

For possible reasons for the large variation in shell colour within the groups described above, see Chowdhury et al. (2024).

According to the banding pattern, the shells were divided into four groups: unbanded, mid-banded, three-banded, and five-banded. In the group of unbanded shells, dark spiral bands were usually completely absent; only traces of them could occasionally be seen, more often near the aperture itself.

Shells with one distinct central (third) band were classified as mid-banded. Occasionally, the dark band was absent (a modification), but a lighter zone was visible on the shell periphery (Fig. 6), which is not the case for true (heritably) unbanded specimens. Such specimens, in particular, can be sometimes observed where there are no true unbanded shells, but mid-banded ones are often found (Gural-Sverlova et al. 2021b). Also included in this group were shells with one, or less often two, weak (blurred, rudimentary) bands below (Fig. 7) or above the shell periphery (Fig. 8).

In the three-banded group, the two upper bands are usually completely absent, occasionally appearing as traces or blurred. The three lower bands are distinct and may fuse together (Fig. 10), sometimes one of them is missing, weak, or appears on the shell somewhat late (Fig. 9).

Five-banded shells usually have five distinct spiral bands, discrete or fused. Sometimes one of the bands is missing (commonly the second or third from the top).

The combination of ground colour and banding gives nine variants of the shell colouration recorded in Ukraine. Below are their symbols in alphabetical and numerical order:

RESULTS

Today, both the greatest colouration diversity (Table 1) and the largest number of C. nemoralis observations (Fig. 1) have been recorded in western and central Ukraine. This primarily concerns Lviv and Kyiv with their immediate surroundings. All nine variants of the shell colouration, including the rare brown shells without bands, were also found in Ternopil (regional centre in western Ukraine) and near a large garden centre in Horodok, Lviv region. From the Khmelnytskyi region, adjacent to Ternopil one, some observations of C. nemoralis with a yellow five-banded shell are currently known, in particular from the botanical garden of Khmelnytskyi National University. Although in Khmelnytskyi itself, another phenotype (pink unbanded) was also found twice. Data for other parts of Ukraine (the east and especially the south) is so far very scarce.

Table 1

Phenotypes of C. nemoralis recorded in different regions of Ukraine (numbers indicate the number of settlements, for more details see Appendix 1)

In different settlements of Ukraine (Table 1), the most frequently observed specimens of C. nemoralis were those with yellow five-banded, pink unbanded, and yellow mid-banded shells, and somewhat less frequently those with pink five- and mid-banded shells. Other colouration variations, especially brown shells, can be considered rare or sporadically occurring. This, in general, coincided with the results of the qualitative (Table 2) and quantitative (Table 3) analysis of the phenotypic composition in western Ukrainian samples of C. nemoralis. The only exception was the rather frequent (more than 50%) absence of shells with one central band in small samples of this species (Table 2). On average, in the quantitatively analysed samples from western Ukraine, about 25% of specimens had a pink unbanded shell, and about 21% had a yellow shell with five bands (Table 3).

Table 2

Qualitative composition of phenotypes in C. nemoralis samples from western Ukraine (for the quantitative composition of samples, see Appendix 2)

Table 3

Variability of the phenotypic composition in C. nemoralis populations from western Ukraine and some other areas of Eastern Europe (for western Ukraine, only samples of 20 or more specimens were taken into account; if there were several such samples from one location, see Appendix 2, the average frequencies of phenotypes in them were used)

Samples from western Ukraine were usually polymorphic both in the shell colour and banding (Table 2). Even among small samples, this was true for more than half of them. As expected, the average number of phenotypes found increased with sample size, while the proportion of samples that are monomorphic for one or two traits decreased. In samples containing at least 20 specimens, monomorphism was most often manifested only in the presence/absence of bands due to the absence of unbanded shells. However, even in such cases, variability in ground colour and banding type (mid-banded, three-banded or five-banded) among banded shells almost always persisted.

The ratio of phenotypes in samples was very variable (Table 3). Any of the phenotypes, even the rarest one, could locally be the prevailing colouration variant. But most often it was a pink unbanded shell, especially where the proportion of the predominant phenotype was 50% or more (Table 4). On the other hand, any of the common phenotypes could be completely absent in some areas. Thus, of the 60 sites used for the quantitative analysis of the phenotypic composition in western Ukrainian populations of C. nemoralis (Table 3), the above-mentioned P-0 phenotype was prevailing at 22 sites and absent at eight.

Table 4

Percentage of sites where one of the phenotypes was prevailing (the total number of sites and the minimum size of the samples used in the calculations are similar to Table 3)

On average, banded shells occurred at higher frequency than unbanded ones, multi-banded shells than mid-banded ones, and five-banded shells than three-banded ones (Table 3). However, the average number of light phenotypes, which we consider to include not only unbanded but also mid-banded shells with yellow and pink ground colours slightly exceeded that of darker ones (multi-banded and brown shells) (Table 3). Pink unbanded shells were present in more samples than yellow unbanded ones (Table 2).

DISCUSSION

The first reports of C. nemoralis findings in many settlements of Ukraine appeared quite recently, often in the last few years (Gural-Sverlova et al. 2024a, iNaturalist 2025, UkrBIN 2025). However, it is already safe to assume that we are witnessing the beginning of a rather rapid transformation of this species into a common component of urban fauna, at least in the western and central parts of the country. Somewhat earlier, a similar process was observed for another western invader, Arion vulgaris Moquin-Tandon, 1855 (Balashov et al. 2018), which is still the most widespread in western and central Ukraine (Gural-Sverlova et al. 2024b: fig. 19). Numerous garden centres, which often import seedlings of garden and ornamental plants from abroad, could have played a decisive role in both the initial penetration of both species into different parts of Ukraine and in their subsequent dispersal (Gural-Sverlova & Gural 2025a). Already a century ago, Boettger (1926) noted that both C. nemoralis and C. hortensis are easily spread along with garden plants, which even then made it problematic to determine the exact boundaries of their natural ranges.

Nowadays, seedlings are usually sold in containers with soil or a special potting mix. The overgrown roots together with the soil form a dense lump, which is transferred undamaged from the container to the planting site. This way, the root system of the seedling remains intact, and the seedling takes root better. On the other hand, this creates very favourable conditions for the unintentional dispersal of a number of land mollusc species. These could be eggs laid in the ground, juveniles and even adults using containers as shelters. It is no coincidence that recently formed populations of both C. nemoralis and the related species C. hortensis (secondary introductions of the latter) in western Ukraine are usually found at sites with young ornamental plantings, especially conifers, or near them (Gural-Sverlova et al. 2021a, Gural-Sverlova & Gural 2025b). These can be both public places and areas with private houses. We previously observed a similar pattern in the initial stages of the colonisation of Lviv by A. vulgaris (Gural-Sverlova et al. 2024b). However, due to its high locomotor activity, this species soon spread over almost all suitable urban habitats, becoming ubiquitous.

Cepaea species can be brought to garden centres repeatedly from different foreign sources, further increasing the phenotypic diversity of the populations formed there. Large batches of “infected” seedlings may also be delivered there at once. It is not without reason, that the greatest number of inherited variants of the shell colouration in both C. nemoralis and C. hortensis was discovered by us near two of the largest garden centres in the Lviv region: Plants Club in Pidbirtsi and Elit Flora in Horodok (Gural-Sverlova & Gural 2022a, 2025a). Thus, large populations of Cepaea can arise in garden centres and adjacent areas, facilitating not only their further dispersal along with the products sold, but also the maintenance of a sufficiently high level of phenotypic diversity in recently colonised regions.

When landscaping public and private areas, the phenotypic diversity of newly formed Cepaea populations, in addition to random factors (founder effect), obviously depends on the number of planted seedlings and the degree of their “infection” with Cepaea. It is not surprising that at those sites of Lviv and its immediate environs where we recorded a recent co-introduction of two Cepaea species, the phenotypic diversity of C. nemoralis was usually higher (Gural-Sverlova et al. 2024a). This may be due to the larger number of founder individuals. Where only C. nemoralis was introduced relatively recently, we often noted the absence of phenotypes less common in the study area: brown, three-banded, yellow unbanded shells, see Results. Moreover, a clearly pronounced predominance of one variant of the shell colouration was also more often observed there. Most commonly it was a pink unbanded or yellow mid-banded shell.

Secondary introductions of C. hortensis, coinciding in time with the beginning of the rapid dispersal of C. nemoralis over urbanised areas, have already led to the appearance in western Ukraine of many inherited colouration traits that were absent in the descendants of the primary introduction (second half of the 20th century). Among them there are both the colouration forms common for C. hortensis (yellow banded and pink shells), and rarer ones (brown shells) as well as traits that are sporadically found even in the natural range of this species (dark shell lip, banded shell with the absence of the second and fourth bands) (Gural-Sverlova & Gural 2023, 2025b). In another part of Eastern Europe, far distant from western Ukraine (Moscow region of Russia), where the first known populations of C. hortensis were discovered just over 10 years ago (Egorov 2015), shells with a dark lip were also recorded in one of them almost immediately (Egorov 2018: fig. 5F; Gural-Sverlova & Gural 2021b: fig. 5A). In the Moscow region, a dark lip was present only in some pink unbanded shells; other phenotypes (yellow unbanded, yellow banded, pink banded) had a light lip typical of C. hortensis, sometimes with only a slight pinkness in the columellar area (Gural-Sverlova & Gural 2021b). In western Ukraine, where the dark lip in C. hortensis has so far been recorded only from Lviv and the Lviv region, it is usually found in all pink shells in the population (both banded and unbanded), and somewhat less frequently also in all brown or some yellow shells (Gural-Sverlova & Gural 2025b). Thus, although C. hortensis has only recently appeared in many areas of Eastern Europe, two cases of independent importation of carriers of a rare allele causing atypical lip colouration have already been discovered there.

Unlike C. hortensis, both in Ukraine, with its best-studied western part, and in other areas of Eastern Europe, almost no colouration traits have been recorded in C. nemoralis that could be considered rare or sporadically occurring in the natural range. Even brown banded shells are almost completely absent here (Gural-Sverlova et al. 2021a: table 1). The only exception is an interesting population of C. nemoralis in the Ternopil region of Ukraine (Chortkiv), where many banded shells had the uneven pigmentation of bands, making them appear spotty, almost intermittent (interrupta form), sometimes longitudinally split (Gural-Sverlova & Lyzhechka 2021, Gural-Sverlova et al. 2021a: fig. 5). Such band colouration is inherited in a linked manner with the shell ground colour (Murray 1975). According to Schilder & Schilder (1957: map 77), there are two centres in Europe where this trait is most common and where it could have arisen independently: in the north (Denmark, northern Germany, see also Schlesch 1952, Schilder & Schilder 1953b) and the south (northern Italy and southern France up to the Pyrenees). In the first case, spotted bands are more often present in pink shells than in yellow ones. In the second case, rather the opposite trend is observed (Schilder & Schilder 1957: 165). In a large sample from Chortkiv that we examined in 2020 (Gural-Sverlova & Lyzhechka 2021), unevenly coloured (spotted) bands were present in about 18% of yellow and about 41% of pink banded shells.

In Ukrainian populations, as in other Eastern European populations of C. nemoralis, there is usually no variability in the lip colouration. However, at one site in Lviv we found three adult specimens with lighter coloured bands and lip: from a light pink lip combined with brownish-pink bands to hyalozonate bands, a pale shell and a white lip with a barely noticeable pinkish tint (Gural-Sverlova et al. 2021a: fig. 6). In another Lviv population, one juvenile with hyalozonate bands was discovered (Gural-Sverlova et al. 2021b). Although such colouration forms are heritable in C. nemoralis (Murray 1975), it is possible that these single individuals were modifications. For comparison: in some populations of C. hortensis with signs of secondary introductions observed in the Lviv region (Gural-Sverlova & Gural 2025b), hyalozonate (completely colourless and transparent) or more often just weaker pigmented bands can be present in a fairly large number of banded specimens. According to Schilder & Schilder (1957), a white lip in combination with a well-expressed ground colour and dark bands, which cannot be considered a manifestation of albinism, is found in C. nemoralis only in limited areas, but within them it can be common. Such shells are not yet known from Eastern Europe.

A characteristic feature of the western Ukrainian populations of C. hortensis is the very high frequency of unbanded specimens, which we tend to interpret as a possible consequence of climatic selection in a more continental climate (Gural-Sverlova & Gural 2025b). In our samples of C. nemoralis from the same area, on average only about a third of individuals had unbanded shells (Table 3). Even if we additionally take into account shells with one central band, the total number of light phenotypes (Y-0, Y-1, P-0, and P-1) in C. nemoralis samples from western Ukraine is, on average, approximately 1.5 times lower than that of unbanded shells in C. hortensis. In other areas of Eastern Europe compared in Table 3, it exceeded 70% only in Minsk (Belarus), mainly due to the mid-banded shells frequently found there, and despite the low proportion of unbanded ones.

We cannot yet say what causes the differences in the proportion of light phenotypes in the two Cepaea species in western Ukraine. All the populations of C. nemoralis that we studied are still very young, while the descendants of the primary introduction of C. hortensis to western Ukraine had already widely spread in Lviv and the Lviv region before the beginning of the 21st century. It is possible that the influence of climatic selection may become more pronounced in western Ukrainian populations of C. nemoralis over time. Only long-term monitoring at model sites can confirm or refute this. It is also possible that C. nemoralis populations are more polymorphic, so that even strong climatic selection does not lead (or does not lead as quickly) to such a pronounced prevailing of any one phenotype or trait in them. Although even now, at sites where one phenotype clearly predominates (50% or more, see Table 4), it is more often light (usually pink unbanded, less often yellow with a central band), and not dark (yellow or pink with five bands). Based on the monograph by Schilder & Schilder (1957: table 13), who summarised the data on the phenotypic composition of two Cepaea species from different parts of their ranges, unbanded shells are found in C. nemoralis, in general, noticeably less frequently than in C. hortensis.

In the three best-studied areas of Eastern Europe (western Ukraine and especially Lviv with its immediate surroundings, the Moscow region in Russia, and Minsk in Belarus), presented in Table 3, not only the average level of phenotypic diversity (Gural-Sverlova & Kruglova 2022: table 3), but also the average ratio of different inherited traits (different variants of the shell ground colour and banding) and the phenotypes formed by their combination are quite similar. We had previously found statistically significant differences for only a few of them. Compared to western Ukraine, in the Moscow region yellow shells were significantly less common, and pink shells with one central band were significantly more common (Gural-Sverlova & Gural 2021a: table 2). In Minsk, the main differences from Lviv were related to the rare occurrence of pink unbanded shells and a high proportion of mid-banded ones (Gural-Sverlova & Kruglova 2022: table 3). In all cases, the variability of the phenotypic composition of populations within each studied area was very higher than the differences between areas.

Recently, the possible relationship between the level of phenotypic variability in introduced and/or urban populations of both Cepaea species and the time when they colonised certain areas has been analysed (Cameron et al. 2009, 2014, Cameron & von Proschwitz 2020, Gheoca et al. 2019, etc.). It has been suggested that a high level of variability, assessed by the inbreeding coefficient Fst, is characteristic of recently populated areas, both within the natural range and outside it (Cameron et al. 2009). However, low Fst values even in recently colonised areas may also be caused by common origin and/or strong climatic selection outside natural ranges (Cameron & von Proschwitz 2020, Gural-Sverlova & Gural 2022b). Populations of C. nemoralis from different regions of Eastern Europe show high Fst values (Table 5), which is quite expected given their youth and the high probability of heterogeneous origin (from different European countries through different garden centres or other routes).

Table 5

Values of the inbreeding coefficient (Fst) in some European areas, according to literature data and our research (the frequencies of mid-banded shells were calculated from the number of banded ones)