THE FIRST RECORD OF THE ASIAN CLAM CORBICULA FLUMINEA ( BIVALVIA : VENEROIDA : CORBICULIDAE ) IN THE UPPER VISTULA ( SOUTH POLAND )

The Asian bivalve Corbicula fluminea was first reported from Poland in 2003, from the Odra (Oder) River. It was found in the lower and later in the middle course of the river. In May 2011 this invasive species was found for the first time in the Vistula River, in Cracow, which indicates an extension of its distribution range in Europe. The Vistula is the largest river in the Baltic Sea catchment area. The invasion of C. fluminea may threaten native species and natural habitats in the Vistula and its tributaries.


INTRODUCTION
Corbicula fluminea (O.F. Müller, 1774) is a medium-sized clam whose shell may be up to 65 mm long, but normally does not exceed 25 mm.The shell is thick-walled, oval in its lower part and markedly triangular near the umbo, which is located more or less centrally.Its shape is variable and depends on the animal's habitat and age.The shell surface is covered with many growth ridges (concentric sulcations) and striae.Its colour is either light (yellowish-olive or yellow-brown) or dark brown (STAÑCZYKOWSKA & KO£ODZIEJCZYK 2011).
The first record of C. fluminea in Poland dates from 2003 (DOMAGA£A et al. 2004).The species was then found in the lower stretch of the Odra, in a canal discharging cooling water from the Lower Odra Power Plant near Szczecin.As a result of further surveys, covering the entire river, C. fluminea was found in both the middle and the lower sections of the Odra (WAWRZYNIAK-WYDROWSKA 2007).
Since little is known about the biology of the species, it is impossible to clearly identify the factors facilitating its fast adaptation to the conditions of the colonised areas.It is expected that new breeding sites of the invasive clam may appear, as it keeps extending its geographical range.
This paper presents data on a new Polish record of this species, the first one in the Vistula River, greatly extending its distribution range.

MATERIAL AND METHODS
Observations were carried out between May and October 2011 near the Wawel hill in Cracow, on the right bank of the Vistula River (Bulwar Poleski) (UTM DA24; 50°03'10''N, 19°55'53''E; »213 m a.s.l.).The material was collected by hand from a patch of river sand (a stretch 40 m long and 2 m broad).Shells protruding from the sand were collected for about 10 minutes.In October, samples were collected with a net from the river bed to the depth of about 40 cm.
The collected material was cleaned from sediments and labelled.Live specimens were preserved in 75% ethanol solution.The height and length of the shells were measured with a vernier calliper.
Species identification was based on shell shape, sculpture, and colour as well as hinge structure, according to GLÖER & MEIER-BROOK's (1994) key.Professor ANDRZEJ PIECHOCKI (University of £ódŸ) and Dr ANNA M. £ABÊCKA (Jagiellonian University) confirmed the identification.

RESULTS
Empty shells of C. fluminea (Fig. 1) were discovered in the study site on 29 May 2011, when 20 paired and 35 single valves were collected.The largest shell was 22.5 mm long and 20.6 mm wide.Field research was repeated on 18 June, 24 September, and 23 October 2011.Each time, several dozen shells in various condition were collected; most of them were single valves of various size (not larger than 25 mm).On some shells a line typical of temporary growth arrest was noticed.Some shells had a preser ved ligament, and some had a complete endostracum.
Live specimens of C. fluminea were found on 23 October 2011.Three live bivalves were collected from the river bed at the depth of about 40 cm.Their shells were yellow (lighter than the shells found on the bank) and their length did not exceed 12 mm.

DISCUSSION
It is difficult to determine precisely the circumstances under which C. fluminea appeared in the Vistula.Data from the Cracow Vistula Basin Management [Zarz¹d Zlewni Wis³y Krakowskiej] indicate that engineering works were carried out in the river stretch under study in March and April 2009.A dredger then cleared the river bed, collecting sand.Some sand was moved towards the banks (R. JEZIOROWSKI, personal information).If the material collected in May 2011 was there as a result of the works, it should be assumed that the clam had been in the river at least since 2008.
Some shells have a line marking temporary growth arrest, which may indicate their second year of life (STAÑCZYKOWSKA & KO£ODZIEJCZYK 2011).If it was a pioneering generation, then their presence in the Vistula should be dated to 2008.The presence of young specimens reflects reproductive abilities of the population living in the river.Shell sizes suggest that the first generation of the invasive clam appeared in the Vistula not later than in 2008 and that the species successfully overwintered there in the natural environment.
In May 2010, high water levels were recorded in the Vistula in Cracow, which may have had an impact on the distribution of the species.Since C. fluminea reproduces hermaphroditically and its development is marked by the presence of the pediveliger larva (STAÑCZYKOWSKA & KO£ODZIEJCZYK 2011), the flood wave may have moved the clam population down the river.Therefore, it seems necessary to investigate the site of C. fluminea in the Vistula near the Wawel hill as well as the distribution of the species along the river and its tributaries.
In view of the Vistula catchment area, the presence of the species in the river is significant for its further invasion in Europe.Within a few years since its first record in Poland, the species has extended its range to include this largest river in the Baltic Sea catchment area.If the Asian clam finds the local conditions favourable -and the material collected shows that it does -a faster expansion of the species further into Poland can be expected.
The expansion of C. fluminea is predicted to have a negative impact on the biological diversity of surface waters in the Vistula basin.STAÑCZYKOWSKA & KO£ODZIEJCZYK (2011) report that a single specimen releases from 320 to 387 larvae per day, i.e. approximately 35,000-70,000 larvae per season.Competition for food and the high fertility of the species pose a danger primarily to other bivalves, in particular those of the families Sphaeriidae and Unionidae (STAÑCZYKOWSKA & KO£ODZIEJCZYK 2011).Apart from further monitoring and measures preventing new introductions, there is no way to limit the population size of this invasive species.It is notable that C. fluminea in Poland in 2011 was included in an official list of alien species of plants and animals that may endanger native species or natural habitats if introduced to the natural environment (ROZPORZ¥DZENIE 2011).Under Art. 120 section 2 of the Polish Nature Conservation Act (USTAWA 2004), the importing, keeping, breeding and marketing of such species require the consent of the Director-General for Environmental Protection.In view of the above, measures should be taken to increase awareness among the aquarists, who are most probably responsible for the introduction of C. fluminea in the Vistula.
Beside the closely related and very similar clam Corbicula fluminalis (O.F. Müller, 1774) and the Chinese pond mussel Sinanodonta woodiana (Lea, 1834), C. fluminea is a recent addition to the malacofauna of Poland (PIECHOCKI 2008).The speed with which the species is spreading is unknown, and human activity (deliberate or accidental) seems to be a decisive factor behind the spread.Observations in the middle course of the river Rhine suggest that the clam spreads at a rate of 85-115 km a year (STAÑCZYKOW-SKA & KO£ODZIEJCZYK 2011).The results presented here show that the species has entered the Vistula, thus extending its range to the largest river of the Baltic Sea catchment area.
The first record of Corbicula fluminea in Vistula