Enigmatic Bythinella species in Bükk Mountains with the description of Bythinella thermophila n. sp. (Gastropoda: Amnicolidae)

The junior authors collected in the Bükk Mountains in the rivulet in Kács a Bythinella species new to science. From this region only B. pannonica was known which is endemic to the southeastern part of the Northern Carapthian Mountains (Fehér et al . 2013). This rivulet is fed by two springs, a cold one and a hypothermal spring. Only in the hypothermal branch the new Bythinella sp., described here as Bythinella thermophila n. sp., occurs. Both species are in its shell shape very different from other Bythinella spp. known from Europe and Asia Minor so far. In addition to the description the shells of the holotype and paratypes are depicted, as well as the type localities and living specimens. For comparison a photo of B. pannonica is provided.


Introduction
In Northern Hungary to Southern Slovakia occurs Bythinella pannonica (v. Frauenfeld, 1865), a species which has been described originally as Lithoglyphus pannonica, but the author was not sure that this species belongs to Lithoglyphus in fact but the thickened periostome in the region of the columella led him to his decision (Frauenfeld 1865: 7). Later authors assigned this species to the genus Sadleriana because of shell shape (e.g. Richnovsky & Pintér 1979: 54, Lisický 1991: 226, Fehér & Gubányi 2001: 51, Glöer 2002. Clessin (1887: 668) reports on investigations of the radula by Hazay, who found that this species has to be assigned to the genus Bythinella. In 1972 the species was allocated to the genus Bythinella by Bole (cited after Szawrowska & Wilke (2004: 56)). Szawrowska & Wilke (2004) studied Lithoglyphus pannonica by anatomy and DNA sequencing and could point out that it belongs to the genus Bythinella. Since that time the species has been reported as Bythinella pannonica by several authors (e.g. Horsák et al. 2013: 45, Falkner 2015, Bank 2015. The junior authors collected in a thermal spring and rivulet in Kács (Hungary) a Bythinella sp. which is in shell shape very different from Bythinella and only the penis morphology revealed that it belongs to this genus.
This paper is intended to describe the Bythinella thermophila n. sp.

Material and Methods
The snails were recently collected in Hungary ( Fig. 1) by AV (30.01.2013) with a sieve and by AM (19.08.2012) with hand and fixed in 75% ethanol. The dissections and measurements of the genital organs and the shells were carried out using a stereo microscope (ZEISS); the photographs were made with a digital camera system (Leica R8).
The   Etymology: "thermophila" because the species lives in thermal waters.

Description:
The yellowish-brown shell has 4.5 whorls which are slightly convex. The shell is elongated conical. The aperture is ovate, in some specimens slightly angled at the top. The periostome is thickened at the columella. Umbilicus open or closed. Aperture height to shell height: 0.44-0.48, shell height 2.5-2.8 mm, width 1.6-1.8 mm. Juvenile specimens are globular.

Male copulatory organ:
The penial appendix is longer than the penis. The tubular gland is regularly thick over the whole length. Differentiating features: From Bythinella pannonica (Fig. 15) it can be distinguished by the shell shape which is in B. pannonica globular and not conical. In addition the penis is shorter than the penial appendix while in B. pannonica both are of the same length.

Associated species: Theodoxus prevostianus, Fagotia daudebartii, Pisidium casertanum
Ecology: Bythinella thermophila n. sp. inhabits thermal springs and rivulets with stony ground. In the village of Kács the rivulet is trained with stones and concrete walls. The growth of algae on the surfaces of stones provides nutrition to abundant Theodoxus prevostianus and Fagotia daudebartii which browse the surface, while Bythinella thermophila n. sp. is mainly found on stones in shady places under bridges, in crevices and under stones.
Distribution: Only known from the warm branch of the rivulet of Kács and the rivulet itself, Hungary.

Discussion
Usually Bythinella (Fehér et al. 2013). The COI of three B. thermophila n. sp. specimens (haplotypes Bythe-01 to Bythe03, GenBank KT581249-KT581251) was identical with that of the syntopically living B. pannonica (haplotype bypa100 in Fehér et al. 2013, HQ149608). This might be explained by introgression but needs further study. On the other hand it is known that COI alone is not suitable to distinguish between distinct species (Ballard & Whitlock 2004).
The rivulet in Kács is fed by two branches from a cold and a hypothermal spring (22 °C in January 2013). In the cold branch Bythinella pannonica occurs and in the warm branch lives B. thermophila. The populations are highly vulnerable to changes in the habitats due to tapping of the spring, construction works in the rivulet, use of agrochemicals in the vicinity etc., the species is therefore endangered.