A new species of Bythinella from Strandzha Mountain, SE Bulgaria (Gastropoda: Risooidea)

A new species of the genus Bythinella Moquin-Tandon, 1856 (Gastropoda: Risooidea: Bythinellidae), from a spring in Strandzha Mt, SE Bulgaria was described as new for science. By this there are 22 Bythinella species known from Bulgaria.


Introduction
The genus Bythinella Moquin-Tandon, 1856 (Gastropoda: Risooidea: Bythinellidae) are probably the most diverse of all hydrobioids in Europe (Radoman 1976;Strong et al. 2008). These minute snails are distributed from N-Africa (Boeters 1998) to south-eastern Turkey (Yıldırım et al. 2006). They are considered to have at least two centers of species richness, in France, which has a total of 42 known species (Bichain et al. 2007), and Balkans, especially in Bulgaria -22 known species (Glöer andGeorgiev 2009, 2011;Georgiev and Hubenov 2013). The last area is considered as not completely investigated till now (Georgiev and Hubenov 2013).
In this paper we describe a new species of Bythinella from a spring in Strandzha Mt, SE Bulgaria.

Material and methods
The living snails were collected in Strandzha Mts., spring near river at Nestinarka beach, Bulgaria; 42.1549° N, 27.85583° E; alt. 59 m asl. (Fig. 1) by hand and preserved in 75% ethanol. The measurements were carried out by using a stereo microscope (Zeiss) and an eye-piece micrometer, the photographs were made with a Leica digital camera system. The material is stored in the Zoological Museum Hamburg (ZMH) and in the collections of the authors. For species delimitation in the genus Bythinella we followed the morphological species concept. We selected features which are intraspecifically constant but are interspecifically different (Glöer 2013). The different features used here are: (i) shell size, (ii) morphometry of the tubular gland, and (iii) the proportions of the penis and penial appendix. The main feature is the tubular gland, which can be regularly thick over the whole length, or the proximal part can be thin or thick and the distal part can be regular or bulbed. In

Research Article
addition the number of loops of the tubular gland is a good distinguishing feature, which is counted here as half loops, that means in Figs. 4, 5 counted as four half loops.
Abbreviations used: H -shell height, W -shell width, AH -aperture height. Etymology: Named in memory of Dilian Georgiev's colleague Dr Blagovest Temelkov (1956-2010 who was specialist of fossil and recent Foraminifera, and lecturer at the University of Plovdiv.

Description
Shell. The horn-coloured shell is nearly cylindrical (Figs. 2, 6). The 4.5 whorls are slightly convex with a deep suture. The surface is silky and finely striated. The apex is very small and obtuse, the umbilicus is closed. The aperture is oval, rounded at the top, and from lateral view straight. Shell height 2.1-2.5 mm, width 1.5-1.7 mm, aperture height to shell height ratio 0.48. Animal. The head is dark pigmented and eye spots are visible.
Anatomy. The penis is three times shorter than the penial appendix , the tubular gland is long and regularly thick (Figs. 4,5) with 4 half loops.

Habitat and ecology:
The new species was found in shallow spring among broad leaf bush and tree vegetation, at the bank of a small stream.
Distribution: Known only from the type locality, possibly a local endemic species.