Contribution to the knowledge of the Bulgarian species of the genus Vitrea (Gastropoda, Pristilomatidae) with the description of a new species

Abstract A new species of genus Vitrea is described: Vitrea ulrichi sp. n. is the eleventh species within the genus found in Bulgaria, and the largest representative of the Bulgarian Vitrea. Some critical notes on the taxonomy of the species V. bulgarica and V. sturanyi are presented. A key is provided for the determination of the species of Vitrea found in the country.


Introduction
The European species belonging to the genus Vitrea Fitzinger, 1833 (Gastropoda: Pristilomatidae) number 56 to date, many with local and scattered distribution patterns on the continent (Welter-Schultes 2012). The shell morphology in this snail group is very important for differentiation of species as they are very diverse (Pintér 1972). In addition, the internal penis structures can be investigated but the external features of the genital organs are not of much taxonomic importance (Riedel 1992). Schileyko (2003) notes that from approximately 50 taxa in the genus known at his time, the anatomy of nearly 30 species is unknown; he also notes that the inner structure of the penis supplies the main differences between the subgenera.
All species of Vitrea living in Bulgaria can well be distinguished by their shell characters Likharev 1975, Irikov et al. 2004), but many aspects of their autecology are still poorly known. Some more new species in the genus can be expected. In the neighboring country of Greece, for comparison, many more species have been described, most of them representing local endemic species. Interestingly, this is not only caused by the isolation of the Greek Island, many of them are described from the continental parts of the country (Riedel 1992).
In this paper we describe a new species, Vitrea ulrichi sp. n. from the Stara Planina Mountain, Bulgaria, which can be distinguished from the most similar species Vitrea kutschigi (Walderdorff, 1864) and V. sturanyi by its larger size, its angled shell, and very prominent shell sculpture.

Material and methods
The specimens of the new species (and other representatives of the local malacofauna) were collected by hand and with a double sieve system (1×1 and 2×2 mm).

Vitrea
Etymology. The species is named after our colleague and good friend Ulrich Schneppat (Natural History Museum, Chur, Switzerland) with gratitude for his great contribution to the knowledge of Bulgarian gastropods and for providing many literature sources, as well as for long and useful discussions with us on snails and slugs by email or around camp fires during our expeditions throughout Bulgaria.
Diagnosis. Of all the Vitrea species reported for Bulgaria, the new species differs by its larger size, large number of whorls, and the intensely radially striated and angular shell. Considering the other European species and those distributed in the neighboring area of Asia Minor, the new species is most similar to V. kutschigi known from Dalmatia, Serbia, Kosovo, and Macedonia, from which it differs by its coarsely striated and larger shell, higher spire, and position of the end of aperture edge on the last whorl, located at 1/3 of the last whorl in the V. ulrichi sp. n. when compared to V. kutschigi, where it is found on the upper side of the last whorl. The shape of the shell somewhat resembles that of Vitrea saboorii Neubert & Bössneck, 2013, but V. ulrichi is bigger and has wider umbilicus.
Description. The shell is translucent, yellowish-white, with 6.25-7 whorls which are densely and coarsely radially striated . The spire is low, broadly conical and elevated. The last whorl is angled at its periphery. The aperture is straight, moderately wide. In funnel perspective, the upper whorls are visible inside. The umbilicus is wide with a diameter of 0.75-1.05 mm, which represents approx. 1/5 of the shell's diameter. The diameter of the last whorl width is less than 2 × the diameter of the penultimate whorl (Dlw 0.65-0.8 mm; Dpw 0.5-0.6 mm). The height of the shell is 2-2.35 mm. According to Welter-Schultes (2012), the shell of V. kutschigi resembles the shell of the freshwater snail Bathyomphalus contortus (Linnaeus, 1758), while the shell of V. ulrichi sp. n. is lens-like (Fig. 2).
Notes on the ecology. The type locality represents the surrounding area of a limestone water cave, with a small spring flowing below the cave near its entrance, providing constant air and soil moisture. The locality, where the new species was found, is a steep carbonate rock on the right side of the cave, densely covered with broad leaf  detritus, mainly from Fagus sylvatica. The area is occupied by F. sylvatica and Carpinus betulus trees and bushes, as well as mosses and ferns (mostly Asplenium scolopendrium) covering the rocks (Fig. 1).
The terrestrial malacofauna diversity at the type locality was very rich. There were more than 20 species of land gastropods registered, within only on a few square meters of area: Carychium tridentatum (Risso, 1826), Agardhiella cf.

Discussion
After the description of this new species, the genus Vitrea in Bulgaria encompasses eleven species. In this number, we also include some doubtful species such as V. bulgarica and V. sturanyi. Due to lack of anatomical data, we are not able to confine the new species to one of the existing subgenera.
The problem of V. bulgarica-V. neglecta Damjanov and Pinter (1969) described the two species V. neglecta (locus typicus: Bulgaria, Rhodope Mountains, some kilometers from the Bachkovski Monastery, Chaya river valley) and V. bulgarica (locus typicus: Bulgaria, Rhodope Mountains, tributary of Chaya river between Asenovgrad and Bachkovo) in the same work. Dedov (1998) suggested that the status of both species should be re-evaluated and that internal anatomies should be studied. Irikov (2001), after examination of shell morphology and anatomy of specimens from both type localities, concluded that V. bulgarica and V. neglecta were synonyms. This opinion was accepted by Welter-Schultes (2012).
After studying these specimens, we found some differences existing between V. bulgarica and V. neglecta, which correspond to the original descriptions of both species (Damjanov and L.Pinter 1969). In V. bulgarica, the whorls increase faster than in V. neglecta; the last whorl is approximately two times larger than the penultimate and the umbilicus is in form of a funnel, respectively. Moreover, in V. neglecta the suture looks much deeper if compared to V. bulgarica. Because of the poor quality of the shell of the paratype specimen of V. neglecta, the radial sculpture of the shell is not clearly visible. At the same time, the more convex lower side of the shell of V. bulgarica (Damjanov and L.Pinter 1969;Damjanov and Likharev 1975) is not clearly discernible; in addition, the correlations of the diameters of the umbilicus to diameter of the shells differs from those given by Damjanov and Likharev (1975) (V. bulgarica Du/D = 1/9; V. neglecta Du/D = 1/10-1/11). According our measurements, the umbilici in both species are wider than the information provided by Damjanov and Likharev (1975). The  Irikov's opinion (2001) "between typical V. neglecta and V. bulgarica there are many intermediate forms, forming gradual series" could be interpreted as a confirmation of Riedel (1992), who commented on the difficulties recognizing apparent differences between the two species in some populations. The different forms and difficulties in determination exist also in other species of this genus (Pintér 1972, Damjanov andLikharev 1975;Riedel 1992). To our opinion, some forms considered as "intermediate" probably represented juvenile or sub-adult stages of the shell (for V. bulgarica and V. neglecta it means less than 4.5-5 whorls). Our observations on the shell morphology of adult specimens (4.5 whorls and more) of V. bulgarica from western Bulgaria shows populations of typical V. bulgarica, with variations in the border of species characters. Thus whenever we speak about intermediate forms within the genus Vitrea it is necessary to indicate the size of the studied species, respectively the number of their whorls.
The most important question for a correct determination of the species in genus Vitrea concerns the structures of the sexual system. According to Pintér (1972), the shell morphology in this genus is paramount for differentiation of species, and Riedel (1992) stated that the external features of the genital organs are not of much taxonomic importance. However, the internal structure of the penis provides information that can be used for a sub-generic distinction (Schileyko 2003). Probably this is the reason, despite their comments about the close relationship between V. bulgarica and V. neglecta, why Damjanov and Likharev (1975) and Riedel (1992) accepted both species as separate. The question is "how far can we rely on the structure of the sexual system in this genus when discussing closely related species?" In our opinion, the structure of the sexual system is important, but is not the single character that should form the basis of a taxonomic opinion. In this case, it is important to study the sexual systems of those specimens, who are considered to represent "border" forms. After that, the probably can be determinate more clearly as known species or intermediate forms. Without completely rejecting the conclusion of Irikov (2001) at this stage, we currently consider the problem V. bulgarica-V. neglecta still as open requiring more detailed studies, which are planed by the authors for the near future.

V. sturanyi
The occurrence of V. sturanyi in Bulgaria, and even on the East Balkans, is disputable. Wagner (1907) described V. sturanyi (as Crystallus sturanyi Wagner, 1907) from Bosnia, Krupa spring near Pazarich. Later, Wohlberedt (1911), Hesse (1916) and Jaeckel (1954) reported this species also for Bulgaria. Pintér (1972) challenged these records and referred them to other Bulgarian species like V. bulgarica, V. neglecta, V. diaphana, V. contracta, and even Oxychilus hydatinus (Rossmässler, 1838) from the family Oxychilidae. Damjanov and Likharev (1975) confirmed the species for Bulgaria from two localities in the Western Rhodope Mountains (Velingrad and Trigrad village), while Welter-Schultes (2012) negates the occurrence of this species in Bulgaria. Our shells from southwestern Bulgaria show some differences when compared to the descriptions of Damjanov and Likharev (1975)-larger diameter of the shell, a smaller number of the whorls, and much more depressed spire. It is currently not clear whether this could be intra-specific variation of V. sturanyi, or whether this represents another new species. To clarify this problem it is needed to compare our Bulgarian populations with the type specimens from Bosnia, which is also another activity for the near future.
Summarising the current knowledge on the genus Vitrea in Bulgaria, we propose the following key to identify the species within the country: