Bythinella fabiae n. sp., a new spring-snail species (Gastropoda: Rissooidea) from the BelasitsaMountains, South-West Bulgaria

A new species of the genus Bythinella is described. Its type locality is: South-West Bulgaria, Belasitsa Mountains, Leshnishki Waterfall, northern slope of Kongur Peak. Because of its locality position it was compared and with Greek and Macedonian species.


Introduction
The freshwater spring-snails Bythinella Moquin-Tanton, 1856 (Gastropoda: Rissooidea: Hydrobiidae) consists of around 80 valid taxa known from Europe (Bichain et al., 2007). They are only absent from Scandinavia, in north-eastern Europe (Baltic republics and Belarus), and perhaps Portugal (Wilke et al., 2010). Members of this genus are minute (2-4 mm in shell length) and live mainly in small springs and rarely in running waters of hypogean habitats localised in the mountains (Boeters, 1979). These gastropods are often characterised by small geographical ranges and a lot of species are endemics (Benke et al., 2009). The evolution of this genus went on the way of the nonadaptive radiations, because the Bythinella species have no clear niche differentiations, they usually have low degree of phenotypical variation, and species usually evolve in allopatry (or peripatry) (Wilke et al., 2010).
The papers of  and Glöer et Georgiev (2009 showed that the species diversity of the genus Bythinella in Bulgaria is higher as yet known, and needs detailed investigations. Up to now there are 20 species described as new in Bulgaria (Georgiev, 2011a). However molecular studies of Falniowski et al., 2009a showed low molecular distinctness of some Bulgarian Bythinella populations, and proposed low diversity of this genus in the country studying cytochrome oxidase subunit I (COI). Such projects are now in progress and possibly will throw some light on the phylogeny and radiation of these minute snails. Many questions were raised as for example how the small, egg-shape shelled strictly stygobiotic species from the Rhodope Mountains, Bythinella markovi Glöer & Georgiev, 2009

Material and methods
The living snails were collected by hand and preserved in 75% ethanol. The dissections and measurements of the shells were carried out by means of a CETI stereo microscope and an eye-piece micrometer; some photographs were made with a camera system with a digital adapter. The holotype and 3 paratypes are stored in the Hungarian Natural History Museum, additional paratypes are stored in the Naturhistorisches Museum Bern, Switzerland.
For species differentiation we accept as main characters the ratio of the penis to the penial appendix, the length and the shape of the flagellum, penis and penial appendix, the soft body pigmentation, and the shell shape, and size (Boeters, 1980, Schütt, 1980, Radoman, 1983, Falniowski et al., 2009b, Glöer & Georgiev, 2011, Glöer, 2013. Habitat data was also considered.

Results and discussion
Bythinella fabiae n. sp.
Etymology: The species was named after Fabia Knechtle Glogger who firstly found that the Leshnishki waterfall was inhabited by freshwater snails and has collected a lot of the material.
Description: Shell: The shell is cylindrical, pale greenish-yellow, with 3.5-4.5 whorls. The apex is obtuse to flat and the aperture is ovoid. The operculum is brown-yellowish, soft and translucent. Shell measurements (Table 1) Soft body morphology: The mantle is grey with white border, other soft parts are yellowish-white with small patches of dark pigment, in some specimens concentrated around the eyes (Fig. 1). Thentacles are a little longer than the snout, with rounded apical parts.
Penis: The penis is as long as the penial appendix, regularly broad with rounded apex. The flagellum is long and irregularly broad, having thinner proximal part and ticker distal one with rounded top. The colour of all these structures is yellowish-white.
Diagnosis: The new species differs from the single other species of Bythinella known from this mountain area, B. slaveyae Glöer & Georgiev, 2009, by its longer penis and flagellum. Both structures are quite shorter and also thicker in the B. slaveyae.
In addition B. elenae and B. margritae have a dark spot on the penes which discern these species from all other species discussed.
It has to be noted and that the first loop of the flagellum of B. fabiae n. sp. is very close to the penis, while in B. srednogorica it is far away from it.
Because Belasitsa Mountains are bordering with two more countries as Greece and Macedonia, we com-pared the new species with the known Bythinella species from these countries.

Greece
In Greece the only species described from the main land was Bythinella charpentieri (Roth, 1855) but only on the shell morphology (Schütt 1980, Reischütz 1988, Bank 2006. B. fabiae n. sp. is different from this species by its quite smaller shell (according Schütt (1980). In B. charpentieri the shell is 3 mm in height and 1.8 mm wide. The smaller number of whorls in B. charpentieri is also characteristic -it has 4.5-5 whorls versus 3.5-4.5 in the new species.
Falniowski & Szarowska (2011) recognised 10 molecularly distinct species in continental Greece but those are not described up to now. Some of them have similar characters (penis:penial appendix ratio, flagellum length and shape) as B. fabiae n. sp. and occur far south to its type locality (approximately at the level of the Korfu Island).
Distribution: Up to now known only from the type locality.
Habitat: Small mountain stream on silicate rocks, surrounded by broad-leaf forest of Castanea sativa, Fagus sylvatica, Carpinus betulus, Tilia sp. The species was found to live on wet moss, under dead leaves of Castanea sativa and under stones in the creek.
Associated molluscs: The B. fabiae was found to live together with Ancylus fluviatilis and an unidentified species of the genus Grossuana.
Identification key to Bythinella spp. known in Bulgaria which having penis:penial appendix ratio 1:1