Rapid expansion of the New Zealand mud snail Potamopyrgus antipodarum (Gray, 1843) in the Azov-Black Sea Region

This is a brief review of present invasion history of the New Zealand mud snail (Potamopyrgus antipodarum) within the AzovBlack Sea Region. The most recent locations of P. antipodarum expansion within the Azov-Black Sea Region (lower parts of the Danube, Dniester, and Don basins, small steppe rivers and streams) are described.

Potamopyrgus antipodarum (Gray, 1843) was introduced into Europe from New Zealand at the end of the 19th century. It is the most widespread non-indigenous mollusc in the world; its invasive range includes many countries of Europe, Asia, North America, and Australia: in Europe, only Iceland and some eastern countries (Albania, Bulgaria, and former Yugoslavia) have escaped invasion (Ponder 1988;Cianfanelli et al. 2007;Levri et al. 2007).
The first Potamopyrgus antipodarum in the Azov-Black Sea Region was found in 1951, simultaneously in two estuarian systems: Razim Lagoon (Grossu 1951) and the Dnieper-Bug Liman (Markovskij 1954). Later, in the twentieth century it was found in some marine and mesohaline localities: the Bug and Berezan estuaries, lagoonal Budakskij Liman (Anistratenko 1998;Chukhchin 1984); however, there was no rapid spread or formation of large colonies until recently. Currently, it is expanding extremely rapidly in both fresh and oligohaline waters. Between 2003 and 2007, P. antipodarum was found in many new sites along the coastal line of the Black Sea and Sea of Azov.
Molecular genetic studies on Potamopyrgus from some Western European habitats, demonstrated the existence of two strains, both genetically and morphologically different to each other (Jacobsen and Forbes 1997;Städler et al. 2005). They are assumed to have invaded from isolated habitats on the North Island of New Zealand at different periods of time. We reported two distinct lines of Potamopyrgus (Figure 1) from the Azov-Black Sea Region (Son 2007a;Son et al. 2008).
A revision of the genus Potamopyrgus (Stimpson, 1865) in the native range resulted in the description of four new species out of a total of eight and showed otherwise cryptic species of Potamopyrgus, which occur in rivers, lakes and estuaries (Haase 2008).
As conformity between New Zealand Potamopyrgus spp. and European clones descripted as P. jenkinsi (Smith, 1889), P. alexenkoae (Anistra-tenko in Anistratenko and Stadnichenko  1995), and P. polistchuki (Anistratenko 1991) is not yet established, we use P. antipodarum as the temporary name. This paper is based on material collected by the author. Original material is deposited in the mollusk collection of the Odessa Branch Institute of Biology of the Southern Seas (OB IBSS). All collected material was fixed in 70% ethanol. River basins of the Azov and Black sea coasts were investigated annually from 1999 to 2008. Molluscs were sampled using a net or quadrat frame (25x25 cm) on sand and mud and by hand from hard substrates.
From 2003 to 2008, P. antipodarum were found in new sites within the Azov-Black Sea Region (Table 1).
Studies on biotopical distribution of P. antipodarum in the Azov-Black Sea Region demonstrate that this species prefers ecotones or human-made habitats with low native species richness, where it occurs in high abundance. Records of P. antipodarum in non-disturbed streams demonstrate difference in a number of the species in streams and in reservoirs and river channel itself near the mouth of these streams. So, the numbers in the channels of the Don River and Dniester Estuary are lower than those in the streams running into them. Only in the Fontanka River was the species found in high abundance in the main channel. In other non-disturbed habitats, where P. antipodarum was found, it was present in low abundance with the exception of the Stentsovsko-Zhebriyanski Plavni wetland, where a number of the snails were observed in marshes and spring pools alternately connected with the sea and with the floodplain (Son 2007b).
In all basins of steep rivers, prone to drought, P. antipodarum was found in common man-made habitats in which there is an acceleration of current: aqueducts, artificial rapids, support of bridges, etc. These rheophilic biotopes in small stream were also noted as refugia for other alien species (Son 2007c; Son 2007a), but for P. antipodarum these biotopes are the most peculiar. As a rule, stream flow is artificially sped up in these habitats because upstream of the sampling site, the limnic reservoir, which feeds water to the stream via pipework or a river channel is diverted by hydrotechnic constructions ( Figure 2).  As the region considered in this study contains many aquatic habitats isolated from the sea and large waterways and since it is within a range of active animal migration, it is likely that a biotic pathway is involved in the spread of this species. In Northern Europe, it has been shown, that birds mostly transport this snail (Lassen 1978). Significant concurrence of its spread in the Azov-Black Sea Region with a corridor of birds' migrations "Via Pontica" allows us to assume this pathway is the basic one operating here again. The transfer of this parthenogenetic species by birds is usually successful because only one female is required to start a population in an isolated water body.
Taking into account the rapid expansion of the species and its wide ecological tolerance, it is expected that in the near future it will occupy the majority of small rivers and streams in the region. In this regard it is necessary to discuss the possible ecological consequences of its invasion.
Effects of P. antipodarum can include indirect effects on the trophic dynamics of an ecosystem. In experiments investigating the relationship between P. antipodarum and benthic fauna, invader densities were positively correlated with the total number of native taxa, as well as with total densities (Schreiber et al. 2002). On the other hand, local aquatic ecosystems of streams and little rivers usually have not got native species similar to P.antipodarum, so that effects caused by consequent changes in community structure can have unpredictable ecological consequences. In addition, P. antipodarum can play a role in the transmission of