MOLLUSCS OF THE MIDDLE AND LOWER ODRA : THE ROLE OF THE RIVER IN THE EXPANSION OF ALIEN SPECIES IN POLAND

The malacofauna of the middle and lower Odra (Oder River), a large river degraded by human activities, is still rich and diversified. Fifty five mollusc species were recorded from 31 studied localities: 32 Gastropoda and 23 Bivalvia. The highest number of species (47) inhabited the areas between groynes, whereas the fewest species (20) were found in the river current outside groynes. As many as 14.5% of the recorded species were alien to the Polish fauna: Lithoglyphus naticoides, Potamopyrgus antipodarum, Physella acuta, Ferrissia fragilis, Menetus dilatatus, Corbicula fluminalis, C. fluminea, and Dreissena polymorpha. The presence of so many alien species shows that the Odra, together with the system of canals (which through the Spree and the Havel connect it to the drainage basin of the Elbe), is a major route along which alien molluscs migrate to Poland.


INTRODUCTION
The molluscan fauna of the Odra (Oder River), like those of the other large Polish rivers, has not been sufficiently studied.This is reflected, for instance, by the absence of monographic studies on the whole molluscan fauna inhabiting the largest rivers that would take into account the habitats of the current, the river margin, and water bodies situated in the flood plains.So far, the most extensive data on the occurrence of benthic macrofauna in the upper, middle, and lower courses of the Odra and its tributaries have been provided by the surveys of the International Commission for the Protection of the Odra against Pollution (SCHÖLL et al. 2003).That study features 26 snail species and 14 bivalve species detected along the whole river course, but unfortunately the localities of particular species (names of towns or villages) are not mentioned.More or less comprehensive contributions to the knowledge about molluscs of selected stretches of the Odra and its valley have been made by LEHMANN (1873), BOETTGER (1926), JAECKEL (1955), TETENS & ZEISSLER (1964), HASTRICH (1994), BRINKMANN et al. (1997), SCHMID (1999), JANICKI (2002), DOMAGA£A et al. (2003DOMAGA£A et al. ( , 2004)), £ABÊCKA et al. (2005), MÜLLER et al. (2007), WAWRZYNIAK-WYDROWSKA (2007), WILKE (2007) and ZETTLER (2012).
In 2009-2011, research was carried out within the project "Groynes as a factor shaping anthropogenic habitats in rivers: an analysis of the complex of benthos, plankton and periphyton in isolated still-water bodies between groynes as exemplified by the middle and lower course of the Odra River".It focused on the specific, anthropogenic river habitat with a slow current, between groynes protecting the river banks.Activities of the research team were coordinated by the second author of this paper.The major aim of the project was to evaluate the areas between groynes, referred to as groyne fields, in this large lowland river as habitats contributing to an increase in biological biodiversity.Among benthic organisms, molluscs constituted ca.25% of the total number of collected specimens (1.7% Bivalvia and 23% Gastropoda).The rich malacological material of 335 snail and bivalve samples provided a reliable basis for publishing the research results.The examined malacofauna of the middle and lower Odra con-tained a large proportion of alien species, suggesting a crucial role of the water route in their expansion over the territory of Poland.This assumption is supported by the presence of the Odra-Spree Canal and the Odra-Havel Canal, connecting the river system of the Elbe to those of the Odra and Vistula (via the Bydgoszcz Canal).

STUDY AREA
The Odra, the second largest Polish river, constitutes one of the six largest river systems of Europe.It is 854.3 km long (741.9km situated at least partly within Poland), and its drainage basin encompasses 118,861 km 2 (106,821 km 2 in Poland).The average annual flow, according to a water gauge in Hohensaaten-Finow, amounts to ca. 17 milliard m 3 , which corresponds to the average flow rate of 540 m 3 s -1 (DUBICKI & B£ACHUTA 1999).The character of the Odra is determined by extensive regulatory works conducted along its course and the fact that it has been connected by canals to the other river systems of Europe.Since the beginning of the 18th century, stone groynes have been constructed in order to move the main current away from the river banks; altogether, ca.10,000 such groynes have been built (RAST et al. 2000).Between the groynes, especially the longest ones, groyne fields have formed, characterised by slow water flow and in some places even the absence of any flow.In such areas, organic and mineral matter accumulates.In the course of the regulation works, as a result of shortening and straightening of the river course, many former meanders became oxbow lakes.Some of them are still permanently connected to the main course of the river.Groyne fields and oxbow lakes represent lentic environments in the river system of the Odra.
The banks of the studied stretch of the Odra are protected by groynes along almost all of its length (Fig. 2).Because of considerable length and height of the groynes, large isolated still-water bodies -groyne fields -form between them.Where the current is weak, crevices between the stone blocks as well as their surfaces become covered by sand or silt sediments, whereas in places exposed to a stronger current, the blocks remain bare.
In the groyne fields (Fig. 3), silt sediment prevails; littoral vegetation is dominated by reed canarygrass (Phalaris arundinacea), and elodeid communities also frequently develop there.In the groyne fields, especially at the bases of groynes, the current is slow, and sand and silt fractions prevail in the bottom sediments.The areas in front of groynes, especially at their tips, are characterised by heavy turbulence and the bottom composed of gravel and sand.Conditions in groyne fields somewhat resemble those found in lake littoral.Lentic habitats develop at the bases of groynes, with the bottom composed of fine-grained sediments, rich in organic matter, similar to the littoral of a eutrophic lake.The central, deepest parts of the groyne fields resemble lake sublittoral.They are characterised by sandy sediments, often with traces of deoxidation, and the presence of large boulders and shell debris (Fig. 3).
The studied oxbow lakes (Fig. 4) are permanently connected to the Odra and most of them retain the shape of former meanders.Their length varies from 520 to 5,000 m, whereas their width ranges between 20 and 630 m.In the oxbows sand and silt sediments prevail, characterised by high organic matter content.Sedges (Carex spp.) dominate in the littoral zone, accompanied by elodeids, nymphaeids and helophytes, and also meadow vegetation when the water level is high.

MATERIAL AND METHODS
Benthos samples were collected from 31 localities distributed along the middle and lower courses of the Odra (Fig. 1, Table 1).Molluscs were collected from groyne fields (g, 15 sampling sites); from the current at the tips of the groynes (s, 15); control sites (c, 4); and oxbow lakes (o, 12).The control sites were situated in the area of the Odra without groynes, but close to the research sites, so that they reflected the character of the river.The material was collected with a triangular drag net, side length of 25 cm, maximum handle length 4 m, which allowed penetration of sediments between stone blocks and collection of samples from greater depths.In the field, silt was rinsed off the samples placed on the 50-µm net, and larger debris (e.g.pieces of rock, timber and leaves) was removed on a sieve with mesh size of 5 mm (larger macrobenthic organisms were collected by hand at that stage).Sand and gravel sediments were subjected to sedimentation by stirring the sediment in a water-filled container and decanting from the surface of the sediment the organisms whose specific weight was smaller than the weight of the sediment.The procedure was repeated many times in order to remove mineral fraction.Then the samples were fractioned on a sieve with mesh size of 3.0 mm into macro-and meiobenthos and preserved in 98% ethanol.In the laboratory, molluscs were collected from samples prepared in this way.

RESULTS
Fifty five mollusc species were identified in the studied material (32 Gastropoda and 23 Bivalvia) (Table 1).Species alien to Poland, as well as species protected in Poland, constituted a considerable part of the malacofauna.Alien species were represented by the snails Lithoglyphus naticoides, Potamopyrgus antipodarum, Physella acuta, Ferrissia fragilis, Menetus dilatatus, and the bivalves Corbicula fluminalis, C. fluminea, and Dreissena polymorpha.Protected species included Borysthenia naticina (Gastropoda) as well as Anodonta cygnea, Sphaerium rivicola, and S. solidum (Bivalvia).Alien mollusc species constituted 14.5% of all detected species, but in terms of density they accounted for as much as 80% of the total catch.
The most common species in the studied stretch of the Odra, present in the highest percentage of mollusc-containing samples (n = 335) and found in the highest number of sites (out of the total of 31), included: 1. Potamopyrgus antipodarum -49.1% of samples, 21 sites 2. Viviparus viviparus -39.8%,21 Most of these species occurred along the whole studied part of the river.Bithynia tentaculata f. producta and Menetus dilatatus had never been reported from the Odra before.
Almost equal numbers of mollusc species were detected in the oxbow lakes (o) and control sites (c) situated outside the area with groynes: 36 and 35, respectively (Table 1).Stagnophilous snails, such as P. acuta, P. planorbis, and V. piscinalis, dominated in the oxbows; each of these species constituted ca.10% of the total number of collected snails.Interestingly, P. antipodarum constituted merely 4% of the total.In the control zone, P. antipodarum and V. viviparus dominated, each of them accounting for 17% of the total number of detected snails.Both reophilous (L.naticoides, Ancylus fluviatilis, S. rivicola) and stagnophilous species (e.g.P. planorbis) were found in the control zone.
The presence of Lithoglyphus naticoides (Fig. 5) among alien species is worth mentioning, as it represents the Ponto-Caspian element in the Polish fauna.It was found in six localities along the stretches of the river between Nowa Sól and Urad.The distance from the Odra-Spree Canal (marked by the earlier presence of that snail species) was 13 km downriver and 123 km upriver (Table 1).The species was most frequent in groyne fields, where its density ranged from 1 to over 500 indiv.m - .Potamopyrgus antipodarum (Fig. 6), an invasive species originating from New Zealand, is presently the most common and most abundant snail species in the Odra.Its mass occurrence was recorded along the whole river course, where it inhabited groyne fields as well as oxbow lakes and the river current near the tips of groynes.However, in the groyne fields its relative abundance was the highest, namely 80%, and in the oxbow lakes it was the lowest, amounting to only 4%.In many locations the abundance of P. antipodarum was considerable, reaching up to 6,800 indiv.m - , while the average density amounted to 260 indiv.m - .The highest density of that snail species was found in the material collected in the upper part of the studied stretch of the river, between OEcinawa and Po³êcko.P. antipodarum was found on sandy bottom, bottom composed of sand and silt, on pieces of rock, and on aquatic plants.
Physella acuta (Fig. 7), a snail species introduced in Europe probably from North America, was found along the whole course of the Odra and inhabited all the studied parts of the river (Table 1).However, its relative abundance was the highest in the oxbow lakes (11%) and control sites (4%).Adult specimens were characterised by significant shell sizes (up to 15.5 mm high), and the spire/aperture height ratio was, on average, 1:2.05 (from 1:1.63 to 2:2.4).Such proportions are typical of the bladder snail identified as P. heterostropha (GLÖER 2002).However, since the specific rank of this taxon has recently been questioned (see Discussion), it is here classified as P. acuta.
Ferrissia fragilis, the species most often referred to under synonymous names F. clessiniana (Jickeli, 1882) and F. wautieri (Mirolli, 1960), also represents the North American element in the Polish fauna.A single specimen found in groyne fields in R¹pice, near the mouth of the Odra-Spree Canal, represented the so-called septal form, with a horizontal septum which is believed to develop under adverse conditions, e.g.low water level, oxygen deficit, etc. (RICHARDOT 1976).F. fragilis is known from a number of locations scattered over Greater Poland, Mazovian Lowland and Upper Silesia (STRZELEC 2011), where it occurs as an ancylid form devoid of septum.The only other record of the septal form in Poland is a flooded subsidence basin surrounded by forest, situated near Zabrze (SPYRA 2008).
Menetus dilatatus (Fig. 8A-C) is another North American introduction.In North America it occurs along the Atlantic coast, from Nova Scotia to Florida, ranging west to the Mexican Plateau, Texas, Oklahoma, and central and northern California (JOKINEN 1992).Previously, in Poland it had been found only in the Konin Lakes, fed by heated effluent water discharged by a power plant (BERGER & DZIÊCZKOWSKI 1977).A single specimen of this rare freshwater snail was found in the groyne fields in R¹pice, where it occurred together with F. fragilis.
Borysthenia naticina (Fig. 9), a protected species, one of the rarest in the Polish malacofauna, was observed only in G³ogów, where six adult specimens were found in two samples collected from the groyne fields.The find confirmed the results of a recent research conducted by ZETTLER ( 2012), who recorded B. naticina near the town of Stützkow, in the lower course of the river.The coincident observations indicate that the Odra is still a refugium of this interesting and poorly studied species.
Another noteworthy mollusc is the rare form of Bithynia tentaculata f. producta, found near the groynes in G³ogów, Cigacice and Krosno Odrzañskie.The form is characterised by larger shell dimensions than the typical B. tentaculata and a narrowing of the body whorl.The largest specimen was 13.2 mm high and 7.4 mm wide.As for smaller bivalves, the pill-clam Pisidium crassum is worth mentioning; encountered mainly in Polish lakes (PIECHOCKI 1989), it was found in the groyne fields in Po³êcko and S³ubice.fluminalis was also recorded (Table 1).Most of the collected specimens were juvenile; due to insufficiently developed taxonomic features they could not be reliably identified and thus it was impossible to trace possible differences in distribution of the two species along the river course.The considerable number of  Dreissena polymorpha, an invasive species native to the Ponto-Caspian area, is presently widespread across Poland and Europe, and since 1985 also in North America (STAÑCZYKOWSKA & LEWANDOWSKI 2011).In the middle and lower Odra it was one of the frequent molluscs, but in the collected samples it was not abundant.This could be associated with the sampling method, which focused on loose sediments (sand, silt) and not on hard surfaces to which the mussel attaches itself with byssus threads.D. polymorpha was recorded in the groyne fields, oxbow lakes, and at the tips of groynes.Its recordes from the environs of Wroc³aw (Uraz, Prê¿yce, OEcinawa) extended the range of the species southward in comparison to recently published data (STAÑCZYKOWSKA & LEWANDOWSKI 2011).
Bivalves of the genus Sphaerium, S. rivicola (Fig. 12) and S. solidum (Fig. 13), believed to be endangered with extinction in Poland (VU and EN categories; DYDUCH-FALNIOWSKA & ZAJ¥C 2002), were relatively abundant and frequent in the studied stretch of the Odra.S. rivicola was recorded in 14 sites along almost the whole river course, whereas S. solidum was encountered in nine localities (Table 1).Both species inhabited the groyne fields and control sites.A single occurrence of S. rivicola was recorded in an oxbow lake.However, both species were most frequent in the groyne fields.
The swan mussel (Anodonta cygnea) was found only in the sand and silt sediment in the groyne field in Nowa Sól.Habitats of that type, distributed along the river course, are characteristic of the occurrence of A. cygnea, and the species is likely to be found in a greater number of localities.Other protected mussels -Pseudanodonta complanata and Unio crassus -undetected in our research, still occur in the Odra.This is suggested by the data published by GLÖER & MEIER-BROOK (2003: distribution map, p. 74), SCHÖLL et al. (2003), ZAJ¥C (2004), andZETTLER (1997: distribution map, p. 215).

DISCUSSION
In spite of the common opinion that pollution and regulation of the river-bed have an adverse effect on benthic organisms, the malacofauna of the Odra -a heavily degraded river -is still rich and diversified.The total of species recorded in the Odra (including those recorded by other authors and undetected in this study) is 67 (40 Gastropoda,27 Bivalvia), which constitutes 76.1% of the total freshwater malacofauna of Poland (PIECHOCKI & SULIKOWSKA-DROZD 2008).Species complementing the list of 55 taxa in Table 1 include Bithynia troscheli, Marstoniopsis scholtzi, Valvata macrostoma, Stagnicola corvus, Myxas glutinosa, Aplexa hypnorum, Anisus vor ticulus, Gyraulus riparius, Unio crassus, Pseudanodonta complanata, Sinanodonta woodiana and Pisidium obtusale.It ought to be remembered, however, that the research carried out in 2009-2011 did not encompass the whole course of the Odra.
We did not take into account three species occurring in the Odra according to JEACKEL (1955) andSCHÖLL et al. (2003): Radix ovata, R. peregra and Physella heterostropha.According to GLÖER (2002) R. ovata is a junior synonym of R. balthica, while R. peregra -a synonym of R. labiata -does not occur in the river, and P. heterostropha is a synonym of Physella acuta (see below).The occurrence in the Odra of such species as Stagnicola fuscus and S. turricula, listed by BOETTGER (1926) as forms of S. palustris, and Omphiscola glabra recorded by SCHÖLL et al. (2003), is dubious.According to BARGUES et al. (2006), S. turricula is identical with S. palustris, whereas the remaining two species do not occur in Poland (JACKIEWICZ 1998).
Our research has shown that groyne fields provide an exceedingly favourable habitat for molluscs.This is probably associated with the presence of various kinds of bottom (silty, sandy, rocky, and vegetated) in a small area, so that species of various ecological requirements can exist in close proximity.An additional favourable factor is probably that of diversified water flow velocity, making it possible for reophilous species to exist alongside stagnophilous ones.
The strikingly high proportion of alien species (13.4%, including Sinanodonta woodiana, not recorded in this study) indicates that the Odra River system is of crucial significance in the expansion of these species over the territory of Poland.A major role in the process of dispersal of new arrivals is certainly played by the canals: the Odra-Spree Canal and the Odra-Havel Canal, connecting the drainage basins of the Odra and the Elbe.The highest number of alien species was detected in places where these canals entered the Odra, including, among other species, M. dilatatus (the second record in Poland), F. fragilis, P. For many years, P. acuta was regarded as Mediterranean, but recent research has shown that the species, presently cosmopolitan, started its expansion from North America and continued spreading during the 19th and 20th centuries.In Europe it first appeared in Mediterranean countries and then colonised other areas.The analyses of alloenzymes and mitochondrial DNA have shown that bladder snails, earlier classified as P. acuta, P. heterostropha and P. integra, are conspecific (WETHINGTON & LYDEARD 2007).
Potamopyrgus antipodarum, at present one of the most common freshwater snails in Poland, probably colonised our country through the drainage basin of the Odra and the canals connecting the Odra to the Elbe.This is confirmed by the presence of this species in the system of Elbe-Havel-Spree water connections already at the time of World War I (JAECKEL 1955), its pioneer locations in Lake Trl¹g (catchment area of the Noteae River, 1933) (URBAÑSKI 1938), the Odra and its tributaries (JAECKEL 1955) The presence of alien molluscs and mass occurrence of invasive species do not seem to have an unequivocally negative effect on the native malacofauna.This fact is reflected by both high species diversity of snails and bivalves and high abundance of such native molluscs as V. viviparus, B. tentaculata or V. piscinalis.The out-competing of Physella fontinalis by P. acuta may be an exception.Both species have similar ecological requirements and inhabit eutrophic waters with luxuriant vegetation, in locations where the bottom is covered with sand and silt.P. acuta was recorded in 16 locations, mainly in groyne fields, and P. fontinalis in 13; the latter species was more frequent in oxbow lakes.The two species co-occurred in only eight sites.
The river-bed of the Odra, diversified habitats of the areas between groynes, and numerous oxbow lakes provide favourable conditions for the occurrence of molluscs with various ecological requirements.This is confirmed, among other things, by the presence in the Odra of protected reophilous species, e.g.B. naticina, S. rivicola, and S. solidum, which have become extinct in many other Polish rivers.

Fig. 1 .
Fig. 1.The studied stretch of the Odra River with research sites

Fig. 7 .
Fig. 7. Physella acuta, a cosmopolitan species of North American origin (Photo: B. WAWRZYNIAK-WYDROWSKA) young specimens is a proof of Corbicula's expansion over the drainage basin of the Odra.According to KO£ODZIEJCZYK & £ABÊCKA (2011), C. fluminalis is an Asian species, whose native range encompasses northern Iran, Afghanistan, a considerable part of Mesopotamia (Iraq, Syria, Kuwait, Turkey), South Caucasus, India, and Kashmir, whereas C. fluminea originates from drainage basin of the Ussuri River, the south-eastern part of China and Korea (STAÑCZYKOWSKA & KO£ODZIEJCZYK 2011).

Fig. 13 .
Fig. 13.Sphaerium solidum, a protected species, one of the rarest Polish bivalves, still relatively frequent and abundant in the Odra River (Photo: B. WAWRZYNIAK-WYDROWSKA) Fig. 11.Corbicula fluminalis, an invasive species originating from Asia (Photo: B. WAWRZYNIAK-WYDROWSKA) acuta, C. fluminea, and C. fluminalis.The above-mentioned species were also recorded in the drainage ba-sin of the Elbe (FALKNER et al. 2001, ARLT 2005, MÜLLER et al. 2005).They represent the North American element in the European fauna.The catchment area of the Odra and the canals connecting it with the Elbe and Vistula river systems were also of great importance for the westward spread of Ponto-Caspian species.This is shown by the recent distribution of D. polymorpa and L. naticoides in Poland and Germany (GLÖER & MEIER-BROOK 2003: distribution maps, PIECHOCKI 2004, KO£ODZIEJCZYK 2011, STAÑCZYKOWSKA & LEWANDOWSKI 2011).The numerous localities of L. naticoides situated between Nowa Sól and Urad suggest that the middle Odra constitutes a refugium of this species, once invasive and presently receding, which confirms the tentative view of KO£ODZIEJCZYK (2011) on the current status of this Ponto-Caspian mollusc in the Polish waters.
, and its expansion to the Upper Silesia (through the Gliwice Canal) (STRZELEC 2011) as well as its present mass occurrence along the whole course of the Odra.The recent invasion of the Asian bivalves of the genus Corbicula has a similar and equally drastic character.Research conducted in 1998-2001 did not reveal the presence of Corbicula species in the Odra (SCHÖLL et al. 2003).They were first recorded in 2003 (C.fluminea) and 2004 (C.fluminalis) in heated waters of the Lower Odra Canal near Czarnów and Gryfino (DOMAGA£A et al. 2004, £ABÊCKA et al. 2005).In 2006 and 2007, a number of localities of C. fluminea were discovered in the upper, middle, and lower course of the river, where the species was represented both by live specimens and by empty shells (MÜLLER et al. 2007, WAWRZYNIAK-WYDROWSKA 2007, WILKE 2007).However, the cited authors did not encounter C. fluminalis in the course of field research.Studies of the fauna of groyne-associated habitats indicate that both bivalve species quickly expand their range along the Odra, and at the same time highlight the importance of the Havel-Odra and Spree-Odra canals as immigration routes of invasive species Poland.It is worth mentioning that C. fluminea has recently been recorded in the Vistula in Cracow (MAAEKIEWICZ 2013).