A single living specimen was collected on 31.07.2010 (leg. Jana Bedek), which is designated as the holotype specimen (BOLD-ID BARCA210-13) (Fig. 1, life image; Fig. 2, upper row). Eight shells comprise paratype specimens, of which one was partially broken and used to investigate the form of the columellar fold. Shells were collected on several days during the caving expedition from 29.07. - 03.08.2010 (Fig. 2). All investigated specimens are deposited in the Forschungsinstitut und Naturmuseum Senckenberg, Frankfurt am Main, Germany (museum voucher SMF 341633).

The general shape of Zospeum tholussum sp. n. resembles larger specimens of Zospeum amoenum (Frauenfeld, 1856) from which it can be best delineated by the dome-like structured 2^nd whorl (Fig. 2), the weak but present columellar fold (Fig. 3) and by means of their DNA barcodes (11.7 %–12.1 % genetic p-distance between both species).

The eight conchologically investigated specimens (the holotype and seven paratypes) demonstrate moderate shell variability (Table 1). Shells are generally smooth, very thin and fragile. Fresh shells or those of living specimens are translucent, older ones adopt a milky white color. The number of whorls are 5–6, SH ranges from 1.41–1.81 mm, SW from 0.93–1.12 mm, AH from 0.44–0.54 mm and AW from 0.38–0.46 mm. The ratio of SH/SW is between 1.34–1.62 and AH/AW between 1.05–1.30. The 2^nd whorl is remarkably enlarged possessing a height of 2/3–5/6 the height of the 3^rd + 4^th whorl. The columellar is only weakly developed (Fig. 3) and no teeth (i.e. parietalis, palatalis or columellaris) are present. The DNA barcode of the holotype, when compared with other DNA barcodes of Zospeum species deposited in BOLD, shows its lowest interspecific genetic distance to Zospeum pretneri Bole, 1960 (5.6 %; BOLD-ID: BARCA120-10). This result is well above the barcoding gap of 3.2 % for Carychiidae, which is suitable to separate between-species (> 3.2 %, interspecific) and within-species genetic diversity (< 3.2 %, intraspecific) in Zospeum (Weigand et al. 2011, 2013).

The single living specimen was found in an unnamed large chamber at 980 m depth (85 m long, 70 m wide) with lots of stones, rocks and sand (Fig. 1, right site). A temporal small stream of running water was present close to the collecting site. Air temperature was between 3.3–3.5 °C (depending on the measurement device), water temperature 5.1 °C and air humidity 100 %. Shells were observed beginning from 800 m depth till the bottom of the cave. Shells were generally found on layers of mud (Fig. 1, left site). The first 200 m of the entrance passage of Lukina Jama are permanently covered by varying levels of snow and ice.

The Latin word tholus means dome or cupola and refers to the remarkable dome-like shape of the 2^nd whorl.

In addition to the newly described Zospeum tholussum, a second Zospeum species is present in the Lukina Jama–Trojama cave system (Fig. 4C). This species can be differentiated from Zospeum tholussum by the presence of a tooth, its general shell shape, more prominent columellar fold and absence of the characteristic dome-like structured 2^nd whorl. Because only shells were found, no DNA barcodes are available for this species.

So far, Zospeum tholussum is only known from the Lukina Jama–Trojama cave system. However, this cave system is situated in the distribution range of the morphologically related species Zospeum amoenum (Frauenfeld, 1856), which inhabits caves of the West Balkan of North Slovenia, West Croatia, Bosnia and Herzegovina and South to Montenegro (Bole 1974). Absolon (1916) named but did not describe another morphologically related species - Zospeum troglobalcanicum. Since Weigand et al. (2013) revealed that Zospeum spp. with an inferred large distribution range are particularly prone to host additional, morphologically unrecognized species, some of the distribution records for Zospeum amoenum may very well refer to Zospeum tholussum. Moreover, the intraspecific genetic and conchological variability of the new species should be investigated in greater detail, thus to judge about the taxonomic information content of the diagnostic characters applied within this description.

The preliminary information on the habitat of Zospeum tholussum is in congruence with previous findings for Zospeum. Interestingly, the grazing-labyrinth-like structure, in which most of the shells were embedded (Fig. 1, left site), has been already observed during caving expeditions in Northern Spain (Jochum et al. 2012). Finally, the presence of a temporal stream at the collecting site of the living specimen further supports a passive dispersal scenario as suggested by Weigand et al. (2013).