Distribution and current status of non-indigenous mollusc species in Lithuanian inland waters

We present a comprehensive overview of distribution data of non-indigenous mollusc species in Lithuanian inland waters. The data are based on our own observations obtained during 2007–2013 as well as on published and unpublished records from the last 80 years. Four nonindigenous species have been reported to occur in Lithuanian freshwaters: Dreissena polymorpha, Lithoglyphus naticoides, Potamopyrgus antipodarum a n d Physella acuta. The last species, however, has not been found in Lithuania since the 1950s. D. polymorpha is the most widespread non-indigenous mollusc species in Lithuania. It was observed in 159 out of 226 investigated localities. The other species are less common. L. naticoides was recorded in 43 sites, whereas P. antipodarum was found in 35 localities. In addition to the possible reappearance of P. acuta, four more mollusc species which are currently absent from Lithuanian freshwaters have been reported as non-indigenous from neighbouring countries and thus might potentially invade Lithuanian inland waters in the future.


Introduction
Lithuania is situated along the south-eastern shore of the Baltic Sea.The major part of its territory is covered by the Nemunas river basin and only the northern part falls into other river basins.The Nemunas River is connected to the central European invasion corridor linking up the Dnieper, Vistula, Oder, Elbe and Rhine rivers (Bij de Vaate et al. 2002) by the two canals of Oginsky and Augustow (Figure 1).Thus, the Nemunas River represents the northern branch of the central invasion corridor which provided a pathway for the expansion of a few Ponto-Caspian invaders, including molluscs, into Lithuanian waters (Arbačiauskas et al. 2011a).In addition, the Klaipėda port, located in the northern part of the Curonian Lagoon, has served as a gateway for species transferred by marine vectors (Leppäkoski et al. 2002).Other vectors such as deliberate or accidental introductions have also been involved in primary invasions by nonindigenous aquatic species (Arbačiauskas et al. 2011a).
Data about distribution of non-indigenous mollusc species in Lithuanian inland waters are scattered over many publications and other sources of information including "grey" literature.A critical overview and summary of this information could give a better understanding of patterns in spread and establishment of invasive mollusc species in this region.Thus, the purpose of this paper is to present a comprehensive overview of the distribution of non-indigenous mollusc species in Lithuanian inland waters based on published and unpublished species records and on our own sampling data, and to identify the most probable potential alien mollusc species.

Material and methods
Lithuania belongs to the Baltic Sea drainage basin.Its territory occupies over 64,800 km 2 , and 72% of its area belongs to the basin of the Nemunas River.Other main basins include the Lielupė River basin (8,939 km 2 ), Venta River basin (5,140 km 2 ), Dauguva River basin (1,857 km 2 ) and basins of several other rivers (including the Šventoji and Danė rivers) that empty directly into the sea or the Curonian Lagoon (surface area 1,584 km 2 , mean depth 3.8 m).In total, Lithuanian lakes occupy 913.6 km 2 .There are 2,850 lakes with a surface area larger than 0.005 km 2 and 3,150 small lakes (Gailiušis et al. 2001).
The distribution of non-indigenous mollusc species in Lithuanian inland waters was determined using both data from the literature and results of our surveys.The literature data also involve information from "grey" literature including scientific reports from the Fisheries Department (2000)(2001)(2002)(2003)(2004) and Lithuanian Environmental Protection Agency data of the State Surface Water Monitoring Program (2008Program ( -2011)).Our surveys, conducted during 2007-2013, include several different studies in 16 lakes and five outflowing rivers, and occasional investigations of various water bodies.Samples were collected using a standard dip net or a dredge.Additional information was obtained by visual inspection of bottom substrates.The number of sampling sites per water body and the number of samples collected from one site varied from one to several.
During our studies, geographic coordinates were measured at sampling sites.The majority of literature data had only approximate sampling site information.In such cases, where possible, the coordinates of the most probable sampling location according to descriptions were given, or reference was made to the coordinates of the geographic centre of the water body.

Results
According to the published data, four nonindigenous mollusc species, the zebra mussel Dreissena polymorpha (Pallas, 1771), the gravel snail Lithoglyphus naticoides Pfeiffer, 1828, the acute bladder snail Physella acuta (Draparnaud, 1805) and the mud snail Potamopyrgus antipodarum (J.E.Gray, 1843) were recorded in Lithuanian waters (Table 1).It is considered that D. polymorpha appeared in Lithuanian waters during the early 1800s (Szidat 1926;Olenin et al. 1999).The first published records of the species were made in the western coast of the Curonian Lagoon in 1920s (Szidat 1926) and in the port of the Šventoji River during the 1930s (Šivickis 1934).

Species
Appearance  1).Georeferenced information on the distribution of D. polymorpha, L. naticoides and P. antipodarum in Lithuanian waters is presented in the supplementary material to this paper (Tables 1S-3S) and visualized in Figures 2-4.All collected information shows that D. polymorpha is the most widespread nonindigenous mollusc species in Lithuanian waters (Figure 2).This bivalve was recorded in 159 out of 226 study sites.It occurred in 43 lotic and 116 lentic sites (including the Curonian Lagoon).During our surveys, the species was detected in 9 lotic and 46 lentic sites, 39 of which were previously unpublished.It is worth noting that D. polymorpha was officially deliberately introduced into 36 lakes throughout Lithuania (Vaitonis et al. 1990).The data for 32 of these lakes were available or collected during our surveys.D. polymorpha was recorded in 30 of these lakes (see Table 1S) and only two lakes, lakes Karačiunka and Verniejus, were devoid of the species despite the introductions.
L. naticoides was mainly found in running waters: mostly in sites at the Nemunas River and also at some of its tributaries not far upstream from their confluences.The lentic sites where this species was detected are located in the Curonian Lagoon, and Kaunas and Elektrėnai water reservoirs (Figure 3).The species was also reported to occur in one lake, Lake Balsys (Gurskas 2008).Our surveys detected this species only in one previously unpublished site located in the Žeimena River.Altogether, L. naticoides occurred in 32 lotic and 11 lentic sites (Figure 3, Table 2S).
For more than 20 years, P. acuta had only been observed in the ponds of Kaunas botanical garden (Schlesh 1937, Schlesch and Krausp 1938, Šivickis 1960).During our survey in 2012, the ponds of Kaunas botanical garden (located at: N 54°52'14.01",E23°54'26.47")were sampled again, however, the species was not discovered at this site, nor has it been found at any other site studied to date.Thus, there are no data to confirm the recent presence of this snail in Lithuanian inland waters.
P. antipodarum was only known from the Curonian Lagoon and the Nemunas Delta for more than half a century (Šivickis 1960;Zettler et al. 2005;Zettler and Daunys 2007).However, this snail has been recently detected in four lakes, lakes Daugai, Dusia, Meteliai and Vilkokšnis, and the Verknė River (Butkus et al. 2012).During our later survey in 2012, it was also found in the Spernia River (outflowing from Lake Dusia), the Strėva River and in the closely linked lakes Spindžius and Spindžiukas.Finally, during the last survey in 2013, the species was recorded in two new localities, lakes Obelija and Plateliai.In total, the species occurred in 9 lotic and 26 lentic sites, 7 of which were previously unpublished (Figure 4, Table 3S).

Discussion
Among the four non-indigenous mollusc species reported from Lithuanian inland waters, one species supposedly has not survived in the region.Originally from North America (Taylor 2003), the snail P. acuta had been accidentally introduced to Lithuania, probably through the aquarium trade, and since the 1930s it had only been reported from the ponds of Kaunas botanical    3S.
garden (Schlesh 1937;Schlesch and Krausp 1938;Šivickis 1960).However, P. acuta has not been found there since the 1950s, and there are no data that it has ever been observed in any other site in Lithuania.The five ponds of Kaunas botanical garden are small (< 0.005 km 2 ) interconnected ponds with natural thermal regime.
The most likely reason for the disappearance of the locally established P. acuta is the longterm application of herbicides for aquatic weed control (V.Mildažienė pers.communication).This might have affected P. acuta through depletion of oxygen or food resources, or a combination of several processes resulting from the macrophyte decay after the herbicide application.In fact, no other aquatic mollusc species beside the snail Planorbis planorbis (Linnaeus, 1758) was found when all of these ponds were surveyed in 2012.
The other three non-indigenous species, D. polymorpha, L. naticoides and P. antipodarum, have successfully established and currently inhabit Lithuanian inland waters.D. polymorpha and L. naticoides, originating from the Ponto-Caspian region, probably invaded the inland waters of Lithuania with the first invasion wave at the end of the 18 th century or in the early 1800s (Arbačiauskas et al. 2011a).Inland shipping between the Nemunas and Dnieper river basins is considered to be the main vector of the primary invasion (Arbačiauskas et al. 2011a).The snail of New Zealand origin, P. antipodarum, was first recorded in the Curonian Lagoon (Gasiūnas 1959) suggesting marine shipping (ballast waters or transported materials of freshwater origin) as the primary invasion vector (Global Invasive Species Database 2013).
D. polymorpha became established in Lithuanian inland waters during the last couple of centuries.After its establishment in the Nemunas River and the Curonian Lagoon, it has spread into many lakes and rivers.High rate of reproduction and two distinct life stages, the free swimming larvae and the sessile adult, enabled the fast colonisation of new habitats (Bobeldyk et al. 2005;Burlakova et al. 2005).Additional to the primary invasion vector, accidental introductions (by fishing boats) and deliberate translocations between lakes have accelerated the spread of the species (Arbačiauskas et al. 2011a).The aim of these deliberate translocations was to improve the feeding conditions for commercially important fish species, such as the roach, which readily consumes these mussels (Arbačiauskas et al. 2011a).
Although introduced populations of D. polymorpha have become established in the majority of lakes, several of them escaped the invasion.Despite these deliberate introductions, the species has not established and is today absent in lakes Karačiunka (0.126 km 2 ) and Verniejus (0.959 km 2 ).These lakes are very different from each other in various characteristics, therefore the reasons of their unsuitability for D. polymorpha remain unclear.The smallest lentic water body in Lithuania where D. polymorpha is known to be established is Lake Spindžiukas with a surface area of 0.106 km 2 .The mean area of the five smallest lakes harbouring the species is 0.143 ± 0.09 km 2 .Generally, the size and/or trophic status of the water body are thought to be the main environmental variables influencing the establishment of D. polymorpha, which prefers large mesotrophic lakes (Karatayev et al. 2003).This suggests that water oxygen saturation, pH, calcium content and food particle amount/size may affect species establishment.Concerning the presence of the species in lotic systems, it is associated with source populations in lentic environments.Lentic systems supply donor populations to outflowing rivers, where self-sustaining populations are absent (Lucy et al. 2008).The gradual decrease in density from the upper to the lower reaches of a river is generally observed (Butkus 2011).
In contrast to D. polymorpha, the gravel snail L. naticoides mainly inhabits lotic waters.This species was usually observed in large rivers, water reservoirs and the Curonian Lagoon.Therefore, the record of the snail in Lake Balsys reported by Gurskas (2008) seems unusual and warrants verification.
L. naticoides was frequently observed in low densities.However, in the Nemunas River, especially in the lower reaches, this snail was among the most dominant species in the macroinvertebrate assemblage, indicating that under favourable environmental conditions it may attain high densities (Arbačiauskas et al. 2011b).
The most recent invader, the snail P. antipodarum, was present only in the Curonian Lagoon for more than half a century.It was recorded in the Nemunas Delta in 2004 (Zettler et al. 2005), but was not observed in other inland water bodies at that time (Gumuliauskaitė 2007;Arbačiauskas et al. 2011a).It was only in 2011, that this invasive snail species was recorded for the first time in several lakes and a river located in the southern part of Lithuania (Butkus et al. 2012).Later in 2012, P. antipodarum was discovered in four new localities, in two lakes and two rivers.Finally, in 2013, the snail was found in two further sites.This expansion pattern suggests that in addition to marine shipping (the vector of primary invasion into the Curonian Lagoon), secondary dispersal of this species by waterfowl, water recreation activities and subsequent localised natural dispersal, may be involved in the species spread across Lithuanian inland waters (Butkus et al. 2012).
Usually invasive species, mostly due to their high abundance and/or biomass, are associated with detrimental impacts on native communities, may alter ecosystem structure or functions and subsequently cause economic damage (Pimentel et al. 2005;Genovesi 2007).Although there are well-documented evidences of negative impact of D. polymorpha and P. antipodarum worldwide (Ojaveer et al. 2002;Hall et al. 2003;Hall et al. 2006;Alonso and Castro-Diez 2008;Riley et al. 2008;Karatayev et al. 2010 and references therein), more evidence is still needed in Lithuania.It was revealed that high abundances of non-indigenous molluscs may cause a substantial biological contamination and affect conventional metrics on ecological quality of surface waters (Arbačiauskas et al. 2011b;Šidagytė et al. 2013).The impacts of D. polymorpha on biodiversity, which appeared to be positive, and the budget of suspended material have been investigated in the Curonian Lagoon (Daunys et al. 2006;Zaiko et al. 2009).The overwhelming dominance of P. antipodarum in a benthic assemblage was observed soon after the species invasion into Lake Dusia (Butkus et al. 2012).Some investigations showed that in addition to the European "blacklist" mollusc species D. polymorpha and P. antipodarum (Panov et al. 2009), the snail L. naticoides can also be designated as a high-impact species (Arbačiauskas et al. 2011b).This snail has also been associated with transmission of fish parasites (Mastitsky 2007;Stanevičiūtė et al. 2008).However, to date there are no documented negative economic impacts of invasive molluscs in Lithuanian waters.
Spreading mollusc species that are currently absent in Lithuania but have become introduced to neighbouring countries and thus can potentially come to Lithuanian inland waters are listed in Table 2.The Nemunas River, the northern branch of the central European invasion corridor, and the Baltic Sea coastal waters may be the major pathways for newcomers.Among the most probable events is the reappearance of P. acuta, previously locally established in the Kaunas  (Mitchell and Brandt 2005;Son 2007;Andrzejewski et al. 2013).However, several studies suggest that temperature does not necessarily play a significant role in determining the occurrence of S. woodiana (Corsi 2007;Douda et al. 2012).
In conclusion, this study suggests that the distribution of the earliest invaders D. polymorpha and L. naticoides in Lithuanian inland waters has stabilised and significant further spread is unlikely.In contrast, the further expansion of the last newcomer P. antipodarum across freshwaters is in fact inevitable.Finally, appearances of new non-indigenous mollusc species are highly probable, whether from source populations in the adjacent countries or due to such vectors as ornamental and live food trade.

Figure 1 .
Figure 1.Central European invasion corridor, connecting the basins of the Black and Baltic Seas, and its northern branch crossing Lithuania.Numbers indicate the Oginsky (1) and Augustow (2) canals (modified from Bij de Vaate et al. (2002)).

Figure 2 .
Figure 2. Distribution of the zebra mussel Dreissena polymorpha in Lithuanian inland waters.Circles and triangles indicate localities in lotic and lentic waters, respectively.Colours indicate source of data: from literature (red), current study (green).Numbers correspond to site numbers in Table1S.

Figure 3 .
Figure 3. Distribution of the gravel snail Lithoglyphus naticoides in Lithuanian inland waters.Circles and triangles indicate localities in lotic and lentic waters, respectively.Colours indicate source of data: from literature (red), current study (green).Numbers correspond to site numbers in Table2S.

Figure 4 .
Figure 4. Distribution of the mud snail Potamopyrgus antipodarum in Lithuanian inland waters.Circles and triangles indicate localities in lotic and lentic waters, respectively.Colours indicate source of data: from literature (red), current study (green).Numbers correspond to site numbers in Table3S.