The Swedish Fauna of Freshwater Snails—An Overview of Zoogeography and Habitat Selection with Special Attention to Red-Listed Species

Abstract

The aim of this paper is to describe and analyse the present Swedish fauna of freshwater snails, based on material in museum collections, the literature, records in conservation reports and in databases. The Swedish fauna comprises 50 species (42 indigenous and 8 introduced). Moreover, 31 out of 50 species belong to the superorder Hygrophila. The species’ zoogeography, ecology and conservation status are highlighted. The species distributions can roughly be classified according to four different geographical groups of patterns. 23 species have their northern limit within or close to the limes norrlandicus, an important biogeographical transition zone in Central Sweden. The history of the exploration of the fauna and groups/species with taxonomical/determination problems are discussed. The six species, which are present in the Swedish National Red List, are discussed in detail. Main threats include the exploitation of Swedish rivers and the changing of insolation in small water bodies due to vegetation alteration, eutrophication and drainage. The threat status of the species in the national Swedish list and the general European list are compared and show a high degree of agreement.

1. Introduction

Freshwater snails occur on all continents (except Antarctica), and the total number of species has been estimated to be approximately 5370 [1]. They are found in all types of freshwater habitats and natural and man-made lakes, dams, ponds and watercourses of all sizes. Important ecological factors limiting the freshwater snail’s distribution include depth, shore profile, water movement, temperature, changes in water level, insolation, microclimate, transparency, available calcium, dissolved oxygen, pH, inter- and intraspecific competition, predators, spreading capacity and historical immigration routes (cf. details in [1,2,3,4,5,6,7,8,9]. Several species are benthic or living on the vegetation in water bodies and watercourses and feed on periphyton on artefacts in the water, pleuston from the water surface, debris and living plant material and play a significant role in the ecosystems as decomposers [3,6]. Also, many species are an important food source for predators such as fish [6]. Some species of freshwater snail species are important vectors of parasites in domestic animals, e.g., Galba truncatula (O. F. Müller, 1774) for the liver fluke Fasciola hepatica (Linnaeus, 1758), and in humans, e.g., species in the genera Biomphalaria and Bulinus for Scistosoma spp. (schistosomiasis) [10].

The active spread of freshwater snails is quite slow [11]. The spread over longer distances takes place passively by the help of other organisms, mainly birds and insects, to which the snails can incidentally be attached with their slime [11,12,13]. Species with an operculum can also attach to legs or feathers of the vectors [14]. Certain species are classified as invasive; they are found out of their native distribution range where they cause severe problems in ecosystem functions [15,16]. An example of such a species, also occurring in Northern Europe, is Potamopyrgus antipodarum (J. E. Gray, 1843) [17] (see introduced species below). The destruction and pollution of freshwater habitats, eutrophication, dam constructions altering the natural dynamics of watercourses and dehydration due to climatological changes are severe threats to several freshwater snail species [18,19].

The European (West Palearctic) fauna of freshwater snails is among the best known in the world, being studied and explored for more than two and a half centuries. The latest attempt to make a complete overview was provided by Glöer (2019–2022) [20]. Glöer lists 1534 species from the West Palearctic—defined as Europe to the Ural in the east, including Turkey, the Caucasus, Iran, the Near East and Africa north of the Sahara and the Macaronesia Islands. A large part of the fauna consists of stygobiotic species, which do not occur north of the southern limit of the latest (Weichselian) glaciation.

In Sweden, freshwater snails are present across various zoogeographic regions, from the boreal forests in the north to the more temperate zones in the south, with species distributions influenced by historical immigration routes and local ecological conditions. Some of these species are included in the Swedish Red List due to habitat degradation and environmental pressures.

The scope of this review is to describe and analyse the present Swedish fauna of freshwater snails, according to zoogeography and habitat selection of the species, based on all available material in museum collections, the literature, records in conservation reports and in databases, according to zoogeography, habitat selection and conservation status. Difficulties in taxonomy/species limitations/determination are highlighted. Red-listed species according to Swedish [21] and European [18] Red Lists are discussed. Red listing categories are given according to the international system (EX—extinct; CR—critically endangered; EN—endangered; VU—vulnerable; NT—near threatened and LC—least concern (not threatened)).

2. Material and Methods

2.1. The Geographical Area and Its Freshwater Habitats

Sweden constitutes the eastern part of the Scandinavian peninsula in Northern Europe (Figure 1). Geographical names referred to in the text are marked in Figure 2.

The area of Sweden is approximately 450,000 km², and it reaches approximately 1600 km in the north-south direction and approximately 500 km in the west-east direction. The northernmost part is situated north of the Arctic Circle. There is a 1600 km long border between Norway and Sweden through the Scandinavian Mountain ridge, with the eastern slopes on the Swedish side. The largest part of the mountain ridge and the vast hilly woodlands, covered by coniferous forests east of it, are dewatered by large rivers and lake systems, which debouch in the Gulf of Bothnia, the Bothnian Sea and the Baltic Sea. The greater majority of the fourteen largest rivers of Sweden were exploited for hydroelectricity in the 1950s–1970s. Only four of them (Torne älv, Pite älv, Kalix älv and Vindelälven) are completely protected against any kind of exploitation and have kept their natural hydrological dynamic.

In Southern Sweden, the landscape is markedly more influenced by the presence of man during the last several thousand years and has a more open character [22]. In the south, the landscape is dominated by four large lakes (Vänern, Vättern, Mälaren and Hjälmaren) and several smaller ones—the total number of Swedish lakes is estimated to be over 100,000, covering approximately 9% of the country’s area. The lakes are connected by systems of watercourses and, in some cases, also of man-built channels, debouching either eastwards, into the Baltic Sea, or westwards into Kattegat or Skagerrak (North Sea). The total length of the Swedish watercourses is approximated to be over 500,000 kms.

The limes norrlandicus is a marked and important biogeographical transition zone, running through middle Sweden, north of Lake Vänern in the west, eastwards with a northward prolongation in the east along the Baltic Sea, to approximately 62° N. It is frequently referred to as the northern limit of the oak (Quercus robur, Linnaeus, 1753). It also coincides with the northern limit of the bore-nemoeral+southern boreal zones, according to the Nordic Council of Ministers [23]. The limes norrlandicus is mainly a climatological transition zone. The annual mean temperature is 4–5 °C in the zone, and the length of the vegetation period is 160–170 days with a temperature over +5 °C [24]. It is also a geomorphological transition zone, as the large-scale wally landscape in the north changes into a flatter, smaller-scaled one in the south. In the zone, the continuous northern limit for several deciduous trees, other vascular plants and animal clusters [22,25,26]. Four of the large freshwater mussels have their northern limit in the limes [27]. The limes also constitute the southern continuous limit of many organisms (e.g., dwarf birch Betula nana Linnaeus, 1753) [22,25,26]. The landscape around and south of the great lakes is more smoothly hilly and includes large, flat areas of agricultural land.

The character of the lakes and watercourses is very diverse. A majority are naturally oligo-mesotrophic, but in some areas with basic bedrock and/or soils, the lakes are of natural eutrophic or alcalitrophic character. Large water bodies with naturally oligo-mesotrophic conditions were severely affected by acid precipitation during many decades of the 20th century. To slow down, and if possible, remedy these effects, large-scale liming was, and is still being, performed in many lakes and watercourses [28]. The southernmost part of Sweden (the province of Skåne and the coastal parts of the provinces of Blekinge and Halland) are open, with a mostly flat agricultural landscape. This area of Sweden has only a few smaller, shallow lakes and a few, mostly smaller, watercourses. The islands of Öland and Gotland in the Baltic Sea have limestone bedrock and are hence dry with a few small watercourses, tarns and ponds—often of temporary character and with very high alkalinity. There is a marked decrease in annual precipitation in the west-east direction; especially, Öland and Gotland have a drier climate than western Sweden [24]. In the east, the Baltic Sea, the Sea of Bothnia and the Gulf of Bothnia offer brackish habitats with a salinity of 1–14‰ (decreasing from south to north).

2.2. The Exploration of the Swedish Freshwater Snail Fauna

The Swedish fauna of freshwater snails, despite the large size of the country (Figure 1), must be considered well known. The first species records, according to modern taxonomy, go back to its founder, Carl Linnaeus (1707–1778), and the famous 10th edition of his Systema Naturae [29], in which he introduced the binary nomenclature. Linnaeus described 13 freshwater snail species from Sweden. The results of more aimed surveys on the mollusc fauna were published by the Swedish malacologist Carl Agardh Westerlund (1831–1908) in several publications—his first [30] and his last [31,32]. Westerlund must be considered a taxonomical splitter [33], and many of the taxa he described are now placed in synonymy.

Another important author who contributed to our knowledge of Swedish malacology was Bengt Hubendick (1916–2012) [34]. He was an extreme lumper—the very opposite of Westerlund, e.g., the paper on the world fauna of species in the Lymnaeidae [35]. In many cases the pendulum has today swung in the other direction (cf. Hubendick’s species concept in the genera Lymnaea, Stagnicola and Radix/Ampullaceana/Peregriana) with modern views based on molecular genetics [36]. Beside his taxonomical study Hubendick published a pioneering paper in which he connected the geographical distribution spread of freshwater snails in South and middle Sweden with the autecology/habitat selection of the species and environmental factors [37]. His freshwater snail fauna of Sweden [38] should also be mentioned.

A more modern oversight of the Swedish freshwater snails emerges in the checklist of species by von Proschwitz [39] and the complete fauna of Swedish land and freshwater snails [40]. Also, important information is found in several reports of environmental monitoring of the invertebrate bottom fauna, especially the compilation by Degerman et al. [41] and the reinvestigation of Hubendick’s snail localities from the 1940s in the 1990s [42].

2.3. Compilation of the Investigated Material

This compilation is built on museum material in the collections of the four natural history museums of Sweden: the Swedish Museum of Natural History (Stockholm), the Gothenburg Natural History Museum (Gothenburg), the Museum of Evolution (Uppsala) and Lund University Biological Museum (Lund). The material originates from the beginning of the 19th century until today. Important data are also found in the abovementioned environmental monitoring projects [41,42] and in Artportalen (The Swedish Species Observation System) at SLU Artdatabanken (SLU Swedish Species Information Centre). The same material also forms the ground for the Swedish Snail Fauna by von Proschwitz et al. [40].

2.4. Taxonomic, Nomenclatural and Determination Problems

In the latest decades, several taxonomic problems, mainly delimitation of genera and species, have been solved. The application of these results is a challenge, as it, in some cases of common species, requires time-consuming anatomical studies of the genitalia of huge museum materials.

Even if the main traits in the geographical distribution of the species are known, there are details to be cleared. Especially the following seven cases should be mentioned:

(1) Valvata-species group: The delimitation of certain species in the genus Valvata, and also the taxonomic rank of certain forms, have caused problems. Especially the status of Valvata ambigua Westerlund, 1873, which has been proved to fall into the conchological variation of Valvata piscinalis (O. F. Müller, 1774), and based on molecular genetic results, it must be regarded as a synonym [43]. Another problem is that juveniles of V. piscinalis often have been misidentified as the very rare Valvata macrostoma Mörch, 1864.

(2) Stagnicola-species group: In which Stagnicola palustris (O. F. Müller, 1774) s.l. is a complex with the following three species occurring in Sweden: S. palustris s.s., S. fuscus (C. Pfeiffer 1821) and S. corvus (Gmelin, 1791) [20,40]. It is often necessary to study anatomical characters to obtain a proper determination.

(3) Lymnaeidae: The delimitation of the species in the genera Radix/Peregriana/Ampullaceana is problematic. Shell characters are often extremely variable. In some cases, not even mantle pigmentation or anatomy present characters sufficient for a correct species determination. The species in question are Radix auricularia (Linnaeus, 1758), Peregriana labiata (Rossmässler 1835), Ampullaceana balthica (Linnaeus, 1758), and Ampullaceana ampla (W. Hartmann, 1821) [20,40,44,45]. More research on Swedish populations of these species, applying modern molecular genetic methods, is needed.

(4) Gyralus-species group: Gyralus albus (O. F. Müller, 1774), Gyraulus acronicus (A. Ferussac, 1807) and the recently acknowledged separate species G. stromi (Westerlund, 1881) [40,46] are, in many cases, difficult to separate based on shell characters only.

(5) Gyraulus parvus (Say, 1817)–Gyraulus laevis (Rossmässler, 1835) cannot be considered distinct species on molecular genetic grounds [47]. There are, however, differences between the indigenous European form (G. laevis) and the recently introduced North American form (G. parvus), both in morphology and not at least in ecology. They are therefore here treated as separate forms [40].

(6) Segmentina species group: A nomenclatural problem is the discovery that two sibling species have been hiding under the name Segmentina nitida (Müller, 1774) [48]. The name of the other taxon remains uncertain. Here, it is tentatively referred to as S. cf. clausulata (A. Férussac, 1807) (=S. distinguenda (Gredler, 1860), S. oelandica (Westerlund, 1885)). Both taxa occur in Sweden, the rarest being S. nitida s.s. [20,48].

(7) Confusion concerning use of specific names in the genus Viviparus: The name Viviparus fasciatus (O. F. Müller, 1774), which is a synonym of V. viviparus, has erroneously been used for both the Swedish species in alternative meanings by different authors. This has caused great confusion over the years, also concerning distribution patterns. Unfortunately, wrong attribution of the names for old records is still seen in some Swedish databases, e.g., the Swedish Species Observation System (Artportalen).

3. Results and Discussion

3.1. Taxonomic Overview

The present knowledge (February 2025) of the taxonomical composition of the Swedish freshwater snail fauna is presented in

Table 1. Taxonomic overview of the Swedish fauna of freshwater snails.

Group	Indigenous	Introduced	Total
Valavatoidea	4	-	4
Hygrophila	31	6	37
Neriotomorpha	1	-	1
Caenogastropoda	6	2	8
Total	42	8	50

. Of the total 50 taxa, 42 are indigenous. The superorder Hygrophila constitutes 31 species (74%), and the rest belong to different gill-breathing groups.

3.2. Zoogeographical Overview

The distribution of the species can roughly be classified into five different groups, which are presented as follows: 1. Species with a throughout distribution in Sweden; 2. Species with their ± continuous northern distribution limit in the limes norrlandicus zone; 3. Species with their ± continuous southern limit in the distribution in the limes norrlandicus zone; 4. Species with special geographically deviant distribution patterns; 5. Introduced species (Figure 3).

Examples of species in each group are given in Figure 4a–f. For detailed distribution maps for each species, see von Proschwitz et al. [40]. The Red List categories are given for Sweden [21] and for Europe [18], as well as habitat selection according to Figure 6.

Group 1 (Table 2 and Figure 3): Throughout distribution in Sweden. The group comprises nine species (18% of the total fauna). Most of them are common, rather common and abundant, and less specific in habitat selection: V. piscinalis, Lymnaea stagnalis (Linnaeus, 1758) (Figure 4a), Galba truncatula (O. F. Müller, 1774), S. palustris, S. fuscus, A. balthica and Bathyomphalus contortus (Linnaeus, 1758). The local frequency of the species, however, may vary considerably. Smaller distribution gaps may occur, e.g., the absence of L. stagnalis in an approximately 10 kms broad zone along the west coast.

In the south Swedish Highlands, there are areas with lower annual temperatures and shorter vegetation periods than the adjacent lower coastal areas in the west and east [24]. These areas are also naturally oligo-mesotrophic. Together these factors result in a relatively species-poor fauna, with many freshwater snail species being very rare or absent [37,38], e.g., V. piscinalis. Almost all species are absent in waters on higher levels in the mountain ridge, with the exception of G. truncatula. A special case of throughout distribution is the rare and red-listed G. laevis, which has few, but scattered, localities almost all over the country.

No less than 23 (46%) of the Swedish freshwater snail species have continuous northern limits, which accumulate within the limes (Distribution group 3:

Table 3. List of species with their continuous northern distribution limit ± in the limes norrlandicus zone (Group 2). The habitat selection is marked according to classification in Figure 6.

Species	Remarks	Habitat	Red List Sweden	Red List Europe
Marstoniopsis insubrica (Küster, 1853)		Shore zones of lakes, in the space between stones. (B)	LC	LC
Bithynia tentaculata (Linnaeus, 1758)	Extension further northwards along the coastal area of the Bothnian Sea and the Gulf of Bothnia.	Lakes and ponds of eutrophic character, slowly flowing parts of watercourses. (A)	LC	LC
Bithynia leachii (Sheppard, 1823)		Vegetation-rich lakes and ponds of eutrophic character, slowly flowing parts of watercourses. (B)	LC	LC
Valvata cristata O. F. Müller, 1774	Extension further northwards along the coastal area of the Baltic Sea into the province of Jämtland.	Vegetation-rich lakes and ponds with soft bottoms. (B)	LC	LC
Valvata macrostoma Mörch, 1864	Extremely rare. A few isolated localities further northwards.	Smaller water bodies and ponds with rich vegetation, temporal ponds in woods. (D)	NT	NT
Stagnicola corvus (Gmelin, 1791)	Details in distribution and ecology are partly unclear due to previous unclear taxonomy.	Naturally eutrophic, vegetation-rich ponds and shore zones of lakes, also in slowly flowing parts of watercourses. (B)	LC	LC
Myxas glutinosa (O. F. Müller, 1774)	Extension further northwards along the coastal area of the Bothnian Sea and the Gulf of Bothnia.	Lakes, with hard water, mainly on hard bottoms. Also in brackish waters. (B)	LC	LC
Radix auricularia (Linnaeus, 1758)	Extension further northwards along the coastal area of the Bothnian Sea and the Gulf of Bothnia.	Naturally eutrophic lakes and ponds with hard water. Also in brackish waters. (B)	LC	LC
Ampullaceana ampla (W. Hartmann, 1821)	Details in distribution and ecology are partly unclear due to previous unclear taxonomy.	Naturally eutrophic and vegetation-rich lakes and ponds. (D)	LC	LC
Acroloxus lacustris (Linnaeus, 1758)		Natural eutrophic, vegetation-rich ponds, also in slowly flowing parts of watercourses. (D)	LC	LC
Physa fontinalis (Linnaeus, 1758)	A few isolated localities further northwards.	Vegetation-rich ponds and shore zones of lakes, also in slowly flowing parts of watercourses. (A)	LC	LC
Aplexa hypnorum (Linnaeus, 1758)	Rare. A few isolated localities further northwards.	Ponds (also of temporary character), stagnant water between tussocks in fens. (D)	LC	LC
Planorbis planorbis (Linnaeus, 1758)	A few isolated localities further northwards.	Vegetation-rich ponds and shore zones of lakes, also in slowly flowing parts of watercourses. (A)	LC	LC
Planorbis carinatus O. F. Müller, 1774	Rare in the west.	Natural eutrophic, vegetation-rich ponds, also in slowly flowing parts of watercourses (B).	LC	LC
Anisus leucostoma (Millet, 1813)		Ponds (also of temporary character), ditches, oxbow lakes. (D)	LC	LC
Anisus vortex (Linnaeus, 1758)	A few isolated localities further northwards.	Vegetation-rich ponds and shore zones of lakes, also in slowly flowing parts of smaller watercourses and ditches. (B)	LC	LC
Gyralus crista (Linnaeus, 1758)	Extension further northwards along the coastal area of the Baltic Sea and the Sea of Bothnia and westwards into the province of Jämtland.	Vegetation-rich, eutrophic ponds and ditches, also in shore zones of lakes. Also in brackish waters. (D)	LC	LC
Gyraulus albus (O. F. Müller, 1774)	A few isolated localities further northwards.	All kinds of freshwaters. Also in brackish waters. (A)	LC	LC
Gyraulus riparius (Westerlund, 1865)	Also in an isolated locality further northwards (in the province of Medelpad).	Shore zone of naturally meso-eutrophic lakes. (B)	LC	LC
Ancylus fluviatilis O. F. Müller, 1774	A few isolated localities further northwards.	Streaming, well-oxygenated watercourses with hard bottoms of rocks and stones, also in rocky, wave-exposed shores of big lakes. (C)	LC	LC
Planorbarius corneus (Linnaeus, 1758)	Eastern, introduced in a few localities in the west.	Natural eutrophic, vegetation-rich ponds and lakes, also in slowly flowing parts of watercourses. (B)	LC	LC
Segmentina cf. clausulata (J. Férussac, 1807)	See taxonomic remarks.	Natural eutrophic, vegetation-rich ponds, also in slowly flowing parts of smaller watercourses. (D)	LC	Not rated.
Hippeutis complanatus (Linnaeus, 1758)		Natural meso-eutrophic, vegetation-rich ponds, also in slowly flowing parts of smaller watercourses. (B)	LC	LC

, Figure 3 and Figure 5).

Table 2. List of species with a throughout distribution in Sweden (Group 1). The habitat selection is marked according to classification in Figure 6.

Species	Remarks	Habitat	Red List Sweden	Red List Europe
Valvata piscinalis (O. F. Müller, 1774)		Lakes and slowly flowing waters. Also in brackish waters. (A)	LC	LC
Lymnaea stagnalis (Linnaeus, 1758)	Rare in an approximately 10 km wide zone along the west coast.	Lakes, smaller water bodies and slowly flowing waters. Also in brackish waters (B)	LC	LC
Galba truncatula (O. F. Müller, 1774)	Host species of Fasciola hepatica (Linnaeus, 1758).	Prefers smaller waters, ponds and ditches. Amphibious. Also in wet meadows and fens. (D)	LC	LC
Stagnicola palustris (O. F. Müller, 1774)	Details in distribution and ecology are partly unclear due to previous unclear taxonomy.	Shore zones of lakes, small water bodies and slow-running parts of watercourses. Also in brackish waters. (B)	LC	LC
Stagnicola fuscus (C. Pfeiffer, 1821)	Details in distribution and ecology are partly unclear due to previous unclear taxonomy.	Shore zones of lakes, small water bodies and slow-running parts of watercourses. Also in temporary waters. (B)	LC	LC
Ampullaceana balthica (Linnaeus, 1758)		All kinds of freshwaters. Also in brackish waters. Most common freshwater snail in Sweden. (A)	LC	LC
Peregriana labiata (Rossmässler, 1835)	Details in distribution and ecology are partly unclear due to previous unclear taxonomy.	Oligo-mesotrophic smaller lakes and tarns and forest and fen ponds. (D)	LC	LC
Bathyomphalus contortus (Linnaeus, 1758)		Lakes, ponds and watercourses. Also in brackish waters. (A)	LC	LC
Gyraulus laevis (Alder, 1838)	Extremely rare. See taxonomical remarks.	Lakes, smaller waters and slowly flowing parts of watercourses. (B)	NT	LC

Table 2. List of species with a throughout distribution in Sweden (Group 1). The habitat selection is marked according to classification in Figure 6.

Species	Remarks	Habitat	Red List Sweden	Red List Europe
Valvata piscinalis (O. F. Müller, 1774)		Lakes and slowly flowing waters. Also in brackish waters. (A)	LC	LC
Lymnaea stagnalis (Linnaeus, 1758)	Rare in an approximately 10 km wide zone along the west coast.	Lakes, smaller water bodies and slowly flowing waters. Also in brackish waters (B)	LC	LC
Galba truncatula (O. F. Müller, 1774)	Host species of Fasciola hepatica (Linnaeus, 1758).	Prefers smaller waters, ponds and ditches. Amphibious. Also in wet meadows and fens. (D)	LC	LC
Stagnicola palustris (O. F. Müller, 1774)	Details in distribution and ecology are partly unclear due to previous unclear taxonomy.	Shore zones of lakes, small water bodies and slow-running parts of watercourses. Also in brackish waters. (B)	LC	LC
Stagnicola fuscus (C. Pfeiffer, 1821)	Details in distribution and ecology are partly unclear due to previous unclear taxonomy.	Shore zones of lakes, small water bodies and slow-running parts of watercourses. Also in temporary waters. (B)	LC	LC
Ampullaceana balthica (Linnaeus, 1758)		All kinds of freshwaters. Also in brackish waters. Most common freshwater snail in Sweden. (A)	LC	LC
Peregriana labiata (Rossmässler, 1835)	Details in distribution and ecology are partly unclear due to previous unclear taxonomy.	Oligo-mesotrophic smaller lakes and tarns and forest and fen ponds. (D)	LC	LC
Bathyomphalus contortus (Linnaeus, 1758)		Lakes, ponds and watercourses. Also in brackish waters. (A)	LC	LC
Gyraulus laevis (Alder, 1838)	Extremely rare. See taxonomical remarks.	Lakes, smaller waters and slowly flowing parts of watercourses. (B)	NT	LC

Group 2 (Table 3, Figure 3): Continuous northern limit ± in the limes norrlandicus zone. For many of the species, the continuous northern distribution limit lies well in the limes zone, e.g, S. corvus, Acroloxus lacustris (Linnaeus, 1758) (Figure 4b) and Anisus leucostoma (Millet, 1813), or have a few scattered localities north of it, e.g., Physa fontinalis (Linnaeus, 1758) and Planorbis planorbis (Linnaeus, 1758). Some species have a continuation of the distribution northwards along the Baltic Sea and the Sea of Bothnia, e.g., Bithynia tentaculata (Linnaeus, 1758) and Acroloxus lacustris (Linnaeus, 1758), and some even to the northern end of the Gulf of Bothnia, e.g., R. auricularia (Figure 4c), Gyralus albus (O. F. Müller, 1774), and/or westwards along the river Indalsälven into the calcareous area in the province of Jämtland—Valvata cristata O. F. Müller, 1774, Gyraulus crista (Linnaeus, 1758). As among the species in distribution group 1, there are several that have a distribution gap in the two naturally oligo-mesotrophic and species-poor areas in the central part of the South Swedish Highlands [37,38], e.g., Bithymia leachii (Sheppard, 1823), S. corvus, Aplexa hypnorum (Linnaeus, 1758) and Anisus vortex (Linnaeus, 1758). It could be mentioned that several decades of liming in lakes and watercourses in this area [28], in some cases, resulted in a higher pH of the waters than before the acidification set in, which in turn made it possible for some freshwater snails to spread into new areas (cf. distribution maps in [42]). There are also more subtle sub patterns, e.g., some species are almost absent or very rare in the west, e.g., Planorbarius corneus (Linnaeus, 1758) and Planorbis carinatus O. F. Müller, 1774. The ecology of the species differs within distribution group 2, but many species are more fastidious, preferring natural meso-eutrophic waters (e.g., V. macrostoma, A. ampla, S. corvus, and S. cf. clausulata).

Group 3 (

Table 4. List of species with their continuous southern distribution limit ± in the limes norrlandicus zone (Group 3). The habitat selection is marked according to classification in Figure 6.

Species	Remarks	Habitat	Red List Sweden	Red List Europe
Gyraulus acronicus (A. Férussac, 1807)	Occurs also in the inlands of the south.	Lakes and tarns of oligo-mesotrophic character, also in large-middle-sized watercourses. (B)	LC	DD
Gyraulus stromi (Westerlund, 1881)	Details in distribution are unclear due to confusion with G. acronicus.	Lakes and tarns of oligo-mesotrophic character, also in large-middle-sized watercourses. (B)	LC	Not rated

and Figure 3): Only the following two species (4%) have the limes norrlandicus as their continuous southern distribution limit: Gyraulus acronicus (A. Férussac, 1807) and the only recently as a distinct species recognised, Gyraulus stromi (Westerlund, 1881) (Figure 4d). The limit is strict for G. stromi. Gyraulus acronicus is rare or shows large distribution gaps in the middle Swedish area around the great lakes but has rather many isolated occurrences in the South Swedish Highlands, where the climate is harsher with longer winters. These occurrences coincide also rather well with the naturally oligotrophic areas of the Highlands [37:maps 19,35,38].

Group 4 (

Table 5. List of species with special, geographically deviant distribution patterns (Group 4). The habitat selection is marked according to classification in Figure 6.

Species	Remarks	Habitat	Red List Sweden	Red List Europe
Theodoxus fluviatilis (Linnaeus, 1758)	Southern-eastern Sweden, with an extension northward in the coastal area of the Baltic Sea, the Sea of Bothnia and the Gulf of Bothnia.	Lakes and watercourses with hard bottoms. Also in brackish waters. (B)	LC	LC
Viviparus viviparus (Linnaeus, 1758)	Eastern Middle Sweden.	Vegetation-rich shore zones of lakes and watercourses. Also in brackish waters. (B)	LC	LC
Viviparus contectus (Millet, 1813)	Southern Sweden.	Vegetation-rich shore zones of lakes and watercourses. (B)	LC	LC
Bithynia transsilvanica (Bilelz, 1853)	Extremely rare, two localities in middle Sweden.	Vegetation-rich, slowly flowing watercourses. (C)	VU	VU
Valvata sibirica Middendorff, 1851	Inland of northernmost Sweden.	Shallow, vegetation-rich lakes and tarns, oxbow lakes, also in slowly flowing watercourses. (B)	NT	Not rated
Omphicsola glabra (O. F. Müller, 1774)	Pronounced western-suboceanic with markedly decreasing frequency eastwards.	Small ponds, woodland puddles, often of temporary character, ditches and slow-flowing smaller watercourses. (D)	NT	NT
Anisus vorticulus (Trochel, 1834)	Extremely rare, only found in the southernmost part (Skåne). Possibly extinct.	Ponds, ditches and slow-flowing watercourses, also in vegetation-rich shore zones of lakes. (D)	DD	NT
Segmentina nitida (O. F. Müller, 1774) s.s.	Extremely rare, only found in the southernmost part (Skåne). Earlier not separated from S. cf. clausulata (cf. above).	Naturally eutrophic, vegetation-rich shore zones of lakes and watercourses. (B)	LC (but not rated s.s). Suggested VU.	Not rated (s.s).

and Figure 3): of species with distributions that cannot be attributed to any of the former groups. Theodoxus fluviatilis (Linnaeus, 1758) is a common species in the brackish water along the east coast of the Baltic Sea, the Sea of Bothnia and the southern part of the Gulf of Bothnia (cf. Table 7). It is also fairly common in the great lakes, Mälaren and Hjälmaren in eastern Sweden and some lakes connected with them, as well as in inland waters in the southernmost province, Skåne.

Viviparus viviparus (Linnaeus, 1758) (Figure 4e) and Viviparus contectus (Millet, 1813) have distinct distribution areas, which earlier were geographically isolated from each other. Viviparus viviparus in eastern Middle Sweden and V. contectus in the southernmost part of the country. This separation has partly been broken in later decades, and both species have expanded into new areas, partly by the help of man, e.g., V. contectus at Edsfjärden, province Ångermanland, north of the natural distribution area of V. viviparus. The latter species has also expanded westwards, through the Göta Canal system into the large lakes Vättern and Vänern [49].

Valvata sibirica Middendorff, 1851 (Figure 4f), occurs in a broad belt through the inlands of the northern part of Sweden, down to 62°21 in the province of Jämtland. It is the westernmost outpost (with localities also in northernmost Norway and Finland) of this Siberian species [5,50,51].

Omphiscola glabra (O. F. Müller, 1774) (Figure 7d) has a western-suboceanic distribution, occupying a narrow zone along the Swedish south and west coast (including the southernmost province, Skåne) with a few scattered localities east and northwards in Middle Sweden [40,52]. The isolated northernmost locality was detected in 2023 and lies in the province of Dalarna [53].

Figure 6. Overview (%) of the habitat selection of the indigenous Swedish species of freshwater snails (42). Absolute numbers in parentheses. The species included in each group (A–D) are indicated in Table 2, Table 3, Table 4 and Table 5.

Figure 6. Overview (%) of the habitat selection of the indigenous Swedish species of freshwater snails (42). Absolute numbers in parentheses. The species included in each group (A–D) are indicated in Table 2, Table 3, Table 4 and Table 5.

Bithynia transsilvanica (Bilelz, 1853) (Figure 7c), Anisus vorticulus (Trochel, 1834) (Figure 7e), and Segmentina nitida (O. F. Müller, 1774) s.s. are extremely rare species, only found a few times in Sweden. Bithynia transsilvanica has been found in two localities (streams) in middle Sweden [54], the last time in 2004. Anisus vorticulus and S. nitida s.s. are only found in the southern part of the southernmost province, Skåne, with a small reservation for the latter species, as all museum material has not been revised yet. The latest record of A. vorticulus was in 2008 [55]. Special efforts to re-find have been unsuccessful. B. transsilvanica, A. vorticulus and S. nitida s.s. occur in northern Germany [20,56], and it seems probable that the Swedish localities are outposts on the absolute northern border of the species’ distribution. In such areas, an ongoing process with continuous colonisations and extinctions is not uncommon [5,8,57].

Group 5: introduced species that have established reproducing populations in Sweden (

Table 6. List of introduced species of Sweden (Group 5).

Species	First Record	Habitat
Melanoides tuberculata (O. F. Müller, 1774)	1982	Only artificially heated waters.
Potamopyrgus antipodarum (J. E. Gray, 1843)	1920s [58]	Established and spread. Fresh and brackish waters.
Pseudosuccinea columella (Say, 1817)	1943 [60,61]	Only artificially heated waters.
Physella acuta (Draparnaud, 1805)	1895 [61,62]	Established outdoors.
Gyraulus parvus (Say, 1817)	1999 [63]	Established outdoors. See taxonomical remarks.
Gyraulus chinensis (Dunker, 1848)	2001 [61]	Only artificially heated waters.
Ferrissia californica B. Walker, 1903	1943 [64]	Established outdoors.
Planorbella duryi (Wetherby, 1879)	1994 [60]	Only artificially heated waters.

, Figure 3). The species Melanoides tuberculata (O. F. Müller, 1774), Pseudosuccinea columella (Say, 1817), Gyraulus chinensis (Dunker, 1848) and Planorbella duryi (Wetherby, 1879) have so far only been found indoors (mainly in hothouses). Physella acuta (Draparnaud, 1805) has spread in limited areas in southern Sweden, whereas Gyraulus parvus (Say, 1817) and Ferrissia californica B. Walker, 1903, are only known from a few localities in the city areas of Stockholm and Gothenburg [40]. The New Zealand species P. antipodarum was documented from the Baltic Sea in the 1920s [58] and from the freshwater Lake Mälaren in the 1950s [40]. It has expanded northwards in the Baltic Sea, the Sea of Bothnia and the Gulf of Bothnia and shows a high degree of naturalisation (personal observation). P. antipodarum has also spread into freshwater habitats in southernmost and eastern Sweden and has been spotted in a few localities in the mouths of some watercourses in the western part of the country, e.g., in the Gothenburg area ([59], personal observation). Through the Göta Canal system, P. antipodarum has reached Lake Vättern and Lake Vänern [59]. As it reproduces parthenogenetically, the species can easily establish new populations [58].

3.3. Habitat Selection and Salinity Tolerance

The major habitat selection of the species is presented in Figure 6. For more details see Table 2, Table 3, Table 4 and Table 5 and text in [40,51]. Several species with a broad ecological spectrum (17%) also have a wide distribution (cf. Table 2, Table 3 and Table 4), and more than half (52%) occur in lakes and tarns but also in slowly flowing watercourses. Moreover, 26% prefer smaller waters like ponds and ditches (26%). The preferences are not fixed, except for a few species, and species occurring in smaller water may also occur in vegetation-rich, sheltered shore parts of lakes, tarns and even slowly flowing parts of watercourses. The number of species bound to more rapidly flowing waters is few, the best example being A. fluviatilis, which requires strongly oxygenated waters. However, the species is also found on wave-exposed rock shores of large lakes. Ancylus fluviatilis is an abundantly occurring index fossil for the Ancylus lake, a freshwater stage (approximately 10,700–9800 cal. yr. BP) in the postglacial development of the present Baltic Sea [65].

The majority of the Swedish freshwater snails can also occur in the very mouth area of rivers and streams which debouch into the Baltic Sea, the Sea of Bothnia and the Bay of Bothnia (data from distribution maps in [59], personal observation). At least 12 species (

Table 7. Species tolerance (‰ salinity) in brackish waters (Baltic Sea, Sea of Bothnia and Gulf of Bothnia) ordered after increasing tolerance.

Species	Limit Salinity Tolerance ‰
Myxas glutinosa (O. F. Müller, 1774)	<3 [66]
Gyraulus albus (O. F. Müller, 1774)	<3 ([66], personal observation)
Gyralus crista (Linnaeus, 1758)	<3 [66]
Valvata piscinalis (O. F. Müller, 1774)	<4 [66]
Radix auricularia (Linnaeus, 1758)	<6 ([66], personal observation)
Bathyomphalus contortus (Linnaeus, 1758)	<6 [66]
Viviparus viviparus (Linnaeus, 1758)	<6 [59,66]
Lymnaea stagnalis (Linnaeus, 1758)	<7 [66]
Stagnicola palustris (O. F. Müller, 1774)	<8 [66]
Ampullaceana balthica (Linnaeus, 1758)	<14 ([66], personal observation)
Potamopyrgus antipodarum (J. E. Gray, 1843)	<17 [66]
Theodoxus fluviatilis (Linnaeus, 1758)	<19 ([66], personal observation)

) have extended their occurrence also into brackish habitats. The number of species increases as the salinity decreases in the direction northwards as follows: Baltic Sea—Sea of Bothnia—Bay of Bothnia. The species T. fluviatilis and P. antipodarum) have the majority of their Swedish occurrences in brackish waters. Data in Table 7 from [37,40] personal observation.

3.4. Red-Listed Species, Threats and Conservation

According to the latest Swedish Red List [21], 6 species (14%) of the total stock of 42 indigenous freshwater snail species are red-listed (Figure 7 and

Table 8. Red-listed Swedish freshwater snail species. Categories for Sweden and Europe.

Species	Red List Category Swedish National Level	Red List Category European Level
Bithynia transsilvanica (Bilelz, 1853)	VU	LC
Valvata sibirica Middendorff, 1851	NT	Not rated
Valvata macrostoma Mörch, 1864	NT	NT
Omphiscola glabra (O. F. Müller, 1774)	NT	NT
Gyraulus laevis (Rossmässler, 1835)	NT	LC
Anisus vorticulus (Trochel, 1834)	DD	NT

).

Three of the six species on the Swedish list (7%) are red-listed also on the European level. Valvata sibirica has not been rated, and B. transsilvanica and G. laervis are considered not threatened (LC) in the European evaluation. One species, G. laevis, is listed as DD on the European level but NT in Sweden. Generally, the categories given in the European red-listing [18] are in good accordance with the Swedish National Red List [21]. Moreover, V. sibirica, which has not been evaluated on the European level—B. transsilvanica and G. laevis have been rated higher in Sweden, and one–A. vorticulus—as DD. S. nitida s.s. should be added to the new national Red List 2026. The proposed category is VU due to the presumed limited distribution area and the few localities in Sweden.

Bithynia transsilvanica (Figure 7c) and A. vorticulus (Figure 7e) may be considered possibly extinct, despite repeated investigation in the species formerly known localities and in their vicinity, as well as any other recently searched locality in Sweden.

Valvata sibirica (Figure 7b) is known from about 100 localities. However, the species has disappeared from several of its former localities due to the exploitation of the rivers for hydroelectricity [50]. For the species protection, the north Swedish rivers should remain under national protection. Also, it is essential to protect small tributaries, ponds, and oxbow lakes, which are connected to the larger rivers, as V. sibirica mainly occurs in these habitats [50,51]. Furthermore, forestry and alterations in the riparian zone threaten the species directly by damaging the habitats and indirectly by changing the water temperature through increased insolation. Long-term climate warming is a possible future threat to V. sibirica.

Valvata macrostoma (Figure 7a) is very rare, and only a few records have been made after 1950 [51,67]. It occurs in naturally eutrophic smaller water bodies, such as tarns and ponds of different kinds, both in woodlands and in open landscapes, but often they are shaded by trees or bushes. Valvata macrostoma has also, in a few cases, been found in vegetation-rich shore zones of naturally eutrophic lakes. The major threats are eutrophication, drainage, choking up and filling up of ponds, and cutting of shading shore vegetation, which might change the water temperature. These smaller water bodies are highly interesting objects for conservation of freshwater molluscs, as they may harbour several other rare and threatened freshwater snails, such as O. glabra, A. hypnorum, A. vorticulus, S. nitida s. s., S. cf. clausulata, and the rare mussel Euglesa pseudosphaerium (Ehrmann, 1933).

Omphiscola glabra (Figure 7d) mainly inhabits smaller water bodies, such as ponds of different kinds. It prefers meso-naturally eutrophic waters. It may occur in temporary woodland puddles as well as in oxbow lakes, slowly flowing small streams, and small freshwater pools close to the sea in the province of Bohuslän on the west coast. Threats and conservation measures for its habitats are the same as for V. macrostoma. A large part of the species’ remaining localities is situated in Northern Europe [52].

The few Swedish records of G. laevis (Figure 7f) are from shallow shore zones of large and middle-sized lakes, ponds and slow-flowing parts of streams. The habitats are all near the shore, shaded and rich in vegetation. The sites are mesotrophic—naturally eutrophic, but not eutrophicated. Alteration of the habitats, cutting of shore vegetation and eutrophication constitute major threats.

Beside possible exploitation of the large rivers in the north of Sweden, the most evident threats concern specialised species living in smaller watercourses and ponds in the southern parts of the country. This area is to a greater extent cultivated and more densely populated and hence is under stronger exploitation pressure. In South and Middle Sweden, future conservation measures should be aimed at protecting small water habitats. A long-term threat may also be the ongoing changes of the climate in a warmer direction. For more details on threats against and conservation of freshwater molluscs in general, see [18,19,68].