Bythinella serborientalis, Radoman, 1978, Radoman, 1978

Gojšina, Vukašin, Božanić, Milenka, Stojanović, Katarina, Božić, Aleksandar & Živić, Ivana, 2025, New Findings of the Little Known Crenobiotic Snail Bythinella serborientalis Radoman, 1978 (Gastropoda: Truncatelloidea, Bythinellidae) with Notes on its Morphological Variability, Acta Zoologica Bulgarica 77 (2), pp. 173-178 : 175-176

Habitat of B. serborientalis

The type locality of B. serborientalis (Vrelo Spring) is located in the Stara Planina Mountains (south-eastern Serbia), ca. 3 km north from the village of Visočka Ržana. Snails do not actually live in the spring itself but in a smaller spring right next to Vrelo , which forms a small stream connected to the Jelovica River . Around the spring, there was a lot of vegetation and smaller rocks, even a large pipe that ran next to the Jelovica River , just a few meters away from the spring head. Since this locality is also known as a tourist spot, we consider it to be highly endangered. Snails were not numerous at the type locality, but also somehow difficult to search for because of the numerous plants growing around the spring. In contrast, the second known locality in Serbia reported herein (spring of Orlova River) was dominated by these snails and we collected hundreds of specimens on several occasions and in a short time. The spring is located in a forest and is overgrown with macrophytes. A small stream flows from the spring (ca. 0.35–0.4 m deep and 0.4–0.53 m wide). The spring is only ca. 2 meters away from the road and the nearby hotel which indicates higher anthropogenic pressure.

Discussion

After morphological examination, we found that the majority of the specimens examined shared traits such as the penis being longer than the appendix and sometimes quite flat initial whorls. The latter feature can also be observed in specimens found in the Vodni Pech Cave in Bulgaria (see Osikowski et al. 2015). The tubular organ attached to the penis was also variable, of moderate length or long. It also appeared to be attenuated to a different extent near the point where it joins the penis. However, since we have observed considerable variability in both penial and shell morphology, we can conclude that these morphological traits are not sufficient for reliable species identification. This is not surprising, as Bythinella are generally characterised by a lack of distinct morphological features and high variability even within a single population; this also applies to penial morphology variation, as seen in B. hansboetersi ( Falniowski et al. 2009).

Falniowski et al. (2023) reported B. serborientalis from the village of Potpeće near Sevojno in western Serbia (43°47’51.70” N, 19°56’8.90” E). This locality is ca. 240 km west of the type locality of B. serborientalis and these two regions of Serbia do not normally share the same crenobiotic fauna ( Marković et al. 2021). It is unclear why Falniowski et al. (2023) identified these specimens as B. serborientalis , as specimens from the type locality were apparently not analysed and compared. We compared our sequence of the specimen from the type locality with the one for “ B. serborientalis ” provided by Falniowski et al. (2023) and found significant differences: well above the suggested threshold of 1.5% for Bythinella by Bichain et al. (2007), which is why we consider this record erroneous.

Several closely related species have been found in western, south-western Serbia and the adjacent regions of Montenegro ( Falniowski et al. 2012), which usually share the following morphological features: colourless shell, blackish body (which provokes a blue hue through the translucent shell), well thickened peristome at the columellar side of the aperture and a wide black stripe which spreads across the head and tentacles ( Glöer 2008; Glöer & Pešić 2014). These species could be grouped in the „ Bythinella dispersa group“ and include: B. dispersa , B. istoka, B. pesterica and B. taraensis. The molecular distinctness of B. dispersa , B. pesterica and B. taraensis has already been proven ( Falniowski et al. 2012) but nothing is known on B. istoka. The above-mentioned characters are all shared among these taxa and they can be considered as not clearly morphologically distinct among each other, assuming that the male genitalia are also variable. However, this group is well delimited from B. serborientalis and related species, since the morphology of the shell and body colouration is largely different (i.e. B. serborientalis is more brownish, lacks the black stripe across the head and the columellar side of the aperture is usually less thickened). This is an example of two different „species groups“ that could be morphologically separated based on some „secondary“ characters (body and shell colouration, as well as some morphological characters of the shell), while the penial morphology is much more similar among them and showing a clear continuum (and overlap) of characters. The two groups are also well delimited geographically since they occur in different karst areas in Serbia (eastern vs. western parts), which usually do not share the same fauna.