Taking into account their morphological features stygobitic and stygoxenic oligochaete species are indistinguishable. Therefore for this taxonomical group the attribution to stygobionts is based on species distribution. Species found exclusively in subterranean waters (including shallow groundwater sensu Schmidt and Hahn (2012)) are treated as stygobionts. Sometimes they could be found in benthic samples also, mainly in springs and occasionally in running waters during low water level periods when rivers are fed directly by underground waters (Allan 1995). In such a situation other stygobionts, for example Niphargus tatrensis can be also found (Dumnicka 2005). Among oligochaetes there are many stygophilic species that live and reproduce successfully in surface and subterranean waters whereas stygoxenic species could survive for some time in underground waters but do not reproduce effectively in this environment.

The studies on oligochaetous annelids in subterranean waters of Poland as well as other countries of Central Europe: Czechoslovakia (now Czech Republik and Slovakia), Germany and Hungary started many years ago (Vejdovsky 1882, Lengersdorf 1929, Mohr 1929, Dudich 1932, Stammer 1936, Hrabě 1937, 1942), but generally they were done sporadically and in restricted areas. Nevertheless successive studies allowed to add subsequent species to the list of stygobitic oligochaetes or to find new localities of their occurrence. The state of knowledge about stygobitic oligochaetes was first summarized by Juget and Dumnicka (1986) and recently by Des Châtelliers et al. (2009). Moreover information that accumulated in the last years and some data not considered by Des Châtelliers et al. (2009) was taken into account in the present study: for Austria – paper by Danielopol and Pospisil (2001), for Czech Republik - by Hrabě (1981) and Schenkova et al. (2010), for Germany – by Stein et al. (2012), for Slovakia – by Šporka (1992) and Bitušik et al. (2010), for Poland – by Dumnicka (1998, 2000, 2005, 2009a, 2009b) and Dumnicka and Koszałka (2005).

The current list of stygobitic oligochaete species from Poland and Central Europe is presented and the origin of species known exclusively from Central European countries is discussed. The probable ways of recolonisation after the last glaciation period by some species known from southern Europe is also discussed.

The state of knowledge on stygobitic oligochaete fauna in each of the studied countries of Central Europe varies. Aquatic oligochaete fauna was not studied in Hungary whereas there is plenty of information concerning distribution of such fauna in subterranean waters of Germany, Czech Republic, Slovakia and southern part of Poland. Representatives of Enchytraeidae family were studied only in Poland, Hungary and recently in Germany.

Although the diversity of aquatic subterranean oligochaete fauna in Central Europe is distinctly smaller than in Mediterranean countries (Giani et al. 2001, Des Châtelliers et al. 2009, Giani et al. 2011, Achurra and Rodriguez 2008) there are some stygobitic species, known exclusively from this region. Perhaps some of them are true endemics or they have not been found yet in other geographic regions. The widening of the distribution area as a result of intensive studies can be seen in the case of Guestphalinus wiardi, known for many years exclusively from Germany and then found in Slovenia by Hrabě (1973). Stygobitic oligochaete species from family Dorydrilidae (Dorydrilus tetrathecus) and Lumbriculidae such as Trichodrilus moravicus, T. medius, T. tatrensis, T. spelaeus and Stylodrilus absoloni, known only from subterranean waters of Central Europe probably diversified in this region before the Tertiary or during the warm Pleistocene periods, when that area was not glaciated, and they survived in singular or several refugia.

Within the genus Trichodrilus some of the species have high similarity of various anatomical structures (including genital organs) demonstrating their close relationship to other species living both in epigean and hypogean waters of small, separated areas. Molecular studies should allow to establish the degree of their separation (autonomous or sister species, or same species with some morphological and anatomical differences).

The rarity of stygobitic oligochaete species occurrence makes all studies on their distribution, speciation and ecology very difficult. They are usually found in an extremely small number of samples taken from subterranean waters and even the confirmation of species occurrence and findings of new localities are very difficult. During the studies of 261 localities in Slovenia (Giani et al. 2011) among 19 stygobitic oligochaete species known earlier from this country, the occurrence of only three of them was confirmed. Moreover, ten other stygobionts were found on one to eight sampling stations and Trichodrilus pragensis - the most frequently found species - was observed at ten sampling stations (Giani et al. 2011). During large-scale studies on subterranean invertebrate fauna from various geographical regions in Germany, only Dorydrilus michaelseni was found (Stein et al. 2012) beside epigean oligochaete species.

The majority of stygobitic species found in Central Europe (except for Enchytraeidae) is known from southern Europe. They could have recolonized subterranean waters of Central Europe after glacial/interglacial periods along water courses - especially along the Danube River. Its catchment area covers large parts of Czech Republic, Slovakia, Germany and Austria. Such migration towards the north part of Germany along the Rhine River was also possible. Some species could have migrated by underground connections between catchments area of Danube – Oder and Danube –Vistula River in Poland. The presence of such connection is well documented for the Danube – Oder tributaries (Sudetes, Śnieżnik Massive). Kleśnica stream (Poland) is connected by a few kilometers long underground flow with Morava stream (Czech Republic) (Ciężkowski et al. 1996). In the Tatra Mts. such a connection was not confirmed by water-tracing tests, but the existence of such a flow is assumed (Gradziński et al. 2009). The recharge area was located on the southern part of the higher part of the mountains in Slovakia where there is a small sinkhole in the Cicha Valley (catchment of the Danube River). The discharge is probably through resurgences located at the foot of the northern slopes of the mountains in the Bystra Valley in Poland (catchment of the Vistula River). Migration along water courses facilitates wide spreading of species living in hyporheic waters, which usually have wider distribution than those living in phreatic waters. Such a model of species distribution was also mentioned for the genus Niphargus (Karaman and Ruffo 1986). Probably the migration of stygobionts of marine origin, mainly known from southern Europe (Sambugar et al. 1999) took place by latitudinal ice – marginal river valleys. The distribution of Gianius aquaedulcis in northern and central parts of Germany and Poland could be the example of such a migration.

It seems that the occurrence of some oligochaete species in epigean or hypogean habitats depends on water temperature. For example, Rhyacodrilus subterraneus was not found in epigean waters in Poland despite many studies done on their oligochaete fauna, but it lives in epigean streams of northern Europe and mountain streams of France and Spain (see Dumnicka and Koszałka 2005). Trichodrilus tatrensis is also known from subterranean (Hrabě 1960) or cold epigean waters (Šporka 1992, Bitušik et al. 2010). Such a species could be named “local” stygobionts in regions where they occur in subterranean waters only.

All the enchytraeid species described and found up to now in the subterranean environment of Central Europe are listed in Table 2, but due to the taxonomic revision began by Schmelz and Collado (2010) the determination of their exact number is not possible for the moment.

Stygobitic oligochaete fauna of Central Europe is not rich, comprising only 23 species. They were found in various subterranean habitats, but the most frequently in interstitial waters. Some species (14, half of them represents family Enchytraeidae) are known only from Central Europe what suggests their speciation in situ and survival during the glacial/interglacial periods in refugia. The species having a wide distribution could have recolonised subterranean waters of Central Europe along water courses from non-glaciated regions of southern Europe.