TL;DR: The species is morphologically related to Zospeum amoenum (Frauenfeld, 1856), but can be readily distinguished from the latter by the presence of a weak columellar fold and its domelike structured 2 nd whorl.
Abstract: A new species of the eutroglobiont gastropod taxon Zospeum Bourguignat, 1856 is described. Zospeum tholussum sp. n. is characterized based on a population from the Lukina Jama–Trojama cave system (Ve lebit Mts., Croatia). A single living specimen occurred at 980 m depth. The species is morphologically related to Zospeum amoenum (Frauenfeld, 1856), but can be readily distinguished from the latter by the presence of a weak columellar fold and its domelike structured 2 nd whorl. DNA barcoding is capable to clearly delineate Z. tholussum from other Zospeum spp. as well.