For the Skadar Lake basin, a total of 54 extant gastropod taxa is reported (Glöer and Pešić 2008a, and papers published thereafter, i.e. Glöer and Pešić 2008b, 2009, 2010, Reischütz and Reischütz 2008, 2009, Reischütz et al. 2008) of which we consider at least 50 species to occur in the lake basin (Table 2). In our opinion, four species, i.e., Stagnicola corvus, Gyraulus albus, Planorbis planorbis and Planorbis carinatus were incorrectly reported from the Skadar Lake Basin. Former records of Stagnicola corvus probably refer to Stagnicola montenegrinus, Gyraulus albus records refer to Gyraulus meierbrooki, while the records of Planorbis planorbis and Planorbis carinatus refer to Gyraulus shasi and Planorbis vitojensis, respectively.

At the scale of Skadar Lake, about 31 % of the gastropods (12 out of 39 species sampled in the lake) are endemic. At the scale of the Skadar Lake basin, 38% (19 species) of the total fauna appear to be endemic. Compared with two other famous ancient Balkan lakes (in parentheses % of gastropod endemism, data taken from Albrecht et al. 2009) Ohrid (78%) and Prespa (43%), which gastropod fauna are well studied (Albrecht et al. 2012), Skadar Lake has less taxonomic diversity. Its worth to note, that the number of endemics, however, is likely to change when more faunistical and/or taxonomical data become available. Stagnicola montenegrinus orginally was described from Skadar Lake as an endemic species (Glöer and Pešić 2009). However, recently this species was found in the floodplain of the river Maritza in Bulgaria (Schniebs et al. 2012). Bithynia zeta is known only from Skadar Lake and one spring near Adriatic coast in Montenegro (Glöer and Pešić 2007), but recently this taxon was found in the Drim River in Albania (P.Glöer, unpublished data).

The on-site molluscan species diversity in the investigated area ranged from one to 14 species, with the highest diversity in the sublacustrine springs. Compared to the other investigated habitats of Skadar Lake lacustrine systems, Karuč was species rich. We found a subset of 14 gastropod species, including eight out of 19 proposed endemic taxa.

Taking lake surface areas into account, Albrecht et al. (2009) gave the index of gastropod endemism of 0.304 (log N_{endemic species}/log A_{surface area}) for the Skadar Lake. However they take into account 40 species known for the lake and 7 of them being endemic, as well the average maximum surface area (600 km²). Based on the revised list of Skadar Lake gastropods and endemics (see Table 2) and the mean surface area (472 km²) we get the index of gastropod endemism of 0.478. With this relatively high value, Skadar Lake exceeds such famous lakes as Malawi and Titicaca (see: Rintelen et al. 2007 and, Dejoux and Iltis 1992, respectively).

The faunal relationships of malacofauna among the Balkan’s lakes were analysed by Albrecht et al. (2009). They show that at the species-level, lakes Skadar and Pamvotis (Greece) are clustered as sister group to lakes Trichonis and Lysimachia (both in Greece). Its worth to note that Lake Skadar, inhabited by five Bithynia spp. (Glöer and Pešić 2008, Reischütz et al. 2008) is turned out to be a hot spot of Bithynia evolution. It’s very likely that the absence of major hydrobioid radiations in some ancient lakes like Skadar, Pamvotis or Trichonis could have triggered diversification in bithyniids (Glöer et al. 2007).

Within the Skadar Lake basin, endemism occurs at different spatial scales: (a) species endemic to Skadar Lake and its sublacustrine springs, adjacent pools as well the mouths of the surrounding tributaries and its downstream parts, (b) species endemic to surrounding springs, (c) species endemic to underground waters (interstitial waters of the surrounding tributaries, and surrounding caves). An estimation of the degree of endemism in the late category show that many endemics are characteristic for the subterranean habitat (Pešić and Glöer 2012).

Skadar Lake endemism occurs also at the genus level. Skadar Lake harbors only endemic and monotypic hydrobiid genus Karucia gen. n. Four other Balkan lakes, i.e., Ohrid, Trichonis, Prespa and Mikri Prespa, currently have one endemic genus each (Albrecht et al. 2009).

Despite the still scarce data on the biota of Skadar Lake (e.g. Karanović 2001, Pešić et al. 2010, Pavićević and Pešić 2011, Šundić and Radujković 2012), the currently recognized degree of endemism in different taxa is remarkable, and is not restricted to gastropods but is also evident in some other groups. Talevski et al. (2009) gave total number of 34 native fish species for Skadar Lake and its watershed, with 7 (20.6%) of them being endemic. Karaman (1987) recognized 17 amphipod species for the Skadar lake watershed, 10 of them being endemic (mainly from the subterranean habitat). However, most taxa, remain poorly or even unstudied. Additional field work is highly needed for appropriate evaluation of extant biodiversity of the Skadar Lake.

Most authors agree that the Skadar Lake basin is of tectonic origin (e.g., Laska et al. 1981, Radulović 1997) which had been formed due to the complex folding and faulting within north eastern wing of Old Montenegro anticlynorium (High Karst Zone). These movements took place during the Cenozoic period. The Lake basin has been formed as the result of sinking of blocks in the Neogene period or even in Paleogen. In the Miocene and the Pliocene marine conditions prevailed in the Zeta Plain, which was sunk at the beginning of the upper Miocene, and that the sea inundated this plain up to Podgorica during the Pliocene. Radoman (1985) pointed out that sea must have destroyed all the freshwater populations on this plane and in Skadar Lake area. The connection of Skadar Lake with the sea was interrupted during the younger Pliocene (Radulović 1997). The question of the origin of its water is of particular interest for biologists as these waters may have provided its first species and been the basis for its present high degree of endemism (Albrecht and Wilke 2008). Two hypotheses for the limnological origin of extant Lake Skadar can be advance. In the first scenario Skadar Lake have formed “de novo” in a dry plane (polje) from springs (or rivers) on the place of a former marine gulf. As Radoman (1985) pointed in this scenario, Skadar Lake is a relatively new creation as the whole Zeta plain was a marine gulf until recently which dried up by regression and the lake formed in a dry depression. In the second hypothesis proposed by Radoman (1985), Skadar Lake is probably the remnant of previous, much broader interlinked lacustrine system, first brackish and then freshwater but which were gradually fragmented and disappeared. These remnants are the present day springs and rivers in the sea littoral and in the Skadar depression, as well as in lacustrine continuity – Lake Skadar itself. Radoman (1985) applied the both scenario to explain recent distribution of “lacustrine forms” Radomaniola lacustris and Vinodolia scutarica in the lake. In the first scenario according to Radoman (1985) Radomaniola lacustris and Vinodolia scutarica came into existence after the waters of this area become fresh, or after the formation of Skadar Lake, whose vicinity (springs and river) was already inhabited by populations of this genus or ancestral species. In the second scenario, Radomaniola lacustris and Vinodolia scutarica could be primarily (not secondarily) lacustrine forms (‘remnants’ of the populations from various lacustrine systems on these area), while the river and springs forms were evolved from the lacustrine forms. Radoman (1985) reject the de novo hypothesis mainly based on presence of biogeographical data of hydrobioid gastropods and what he called the presence of the ‘lacustrine forms’ Radomaniola lacustris (Fig. 6f) and Vinodolia scutarica (Fig. 6c). However, Vinodolia scutarica recently was found in a spring in Albania (Fehér and Erőss 2009). Further we found this species in relatively high abundance on the rocky shore of the sublacustrine spring Karuč. Radomaniola lacustris was described from sandy bank parts of the lake (Radoman 1983). During our survey from 2005-2012, we could find this taxon at only one sampled locality (sublacustrine spring Karuč). It is very likely that both species are restricted to sublacustrine habitats as the both species inhabit only the southwestern part of the lake, which harbour bays in which the sublacustrine springs are usually to be found. It is worth to note that Karucia sublacustrina sp. n. should be considered as an element of spring (sublacustrine?) fauna, because despite the intensive sampling in the lacustrine habitats around the type locality, we could find this taxon only in the sublacustrine spring.

It should be noted that some other crenobiontic hydrobiid species, inhabitants of neighbouring springs can be also found in lacustrine habitat, at least part of the year as these spring are regularly flooded by the lake water (Fig. 7b). A year study in the spring on the island Vranjina (Fig. 7f), the locus typicus of crenobiontic species Radomaniola elongata(Radoman), shows that connection between eucrenon and the lake and consenquently lake water regime is the main factor influnded changes in the benthic assemblages (Šundić and Pešić 2007). The highest number of snails was observed in October during ‘lacustrine phase’ of the spring (the spring was flooded by the lake water), while the snails were absent during the ‘spring phase’ in September when the connection between spring and lake was completely interrupted. This seems to be correlated with development of dump mosses were snails living and the failing water level in the spring (Šundić and Pešić 2007). Falnowski et al. (2012) studied the morphology of the shell, penis, and female reproductive organs, as well as the mitochondrial COI and ribosomal 18S in 17 populations of Radomaniola from Skadar Lake drainage and shows that the molecular differentiation was not reflected in morphology. They postulated morphostatic evolution, as a result of non-adaptive radiation characterized by the rapid proliferation of species without morphological and ecological differentiation (Gittenberger 1991).

Nonetheless, information on the potential timing of phylogenetic events on the key endemic taxa in Skadar Lake are still lacking, so the exact limnological origin and the origin of faunal or floral elements of Skadar Lake remain uncertain. Despite the limited knowledge about the lake’s evolutionary history Glöer and Pešić (2008a) presumed status of Skadar Lake as an ancient lake. Although the Skadar Lake is the relatively young ancient lake, his importance for evolutionary research should not be underestimated (Glöer and Pešić 2008). Further studies on gastropods (as well other macrozoobenthic taxa) with applying molecular techniques would certainly give new insights into endemism and evolutionary history of Skadar Lake.