Zospeum tholussum is a microscopic terrestrial snail species that was described in 2013 and is known only from the Lukina Jama–Trojama cave system in Croatia. Among terrestrial air-breathing land snails, it is one of the rare examples that live exclusively in cave environments and is classified as troglobiontic. The species was described based on a single living specimen found at a depth of 980 meters. Zospeum tholussum is notable for its transparent and domed shell structure, complete lack of vision, and extremely slow movement. These characteristics make it a highly specialized cave organism both morphologically and ecologically.
Morphological Description and Features
The shell consists of five to six whorls, with the second whorl showing a distinctly domed structure. This dome-like shape also influenced the naming of the species; “tholus” means “dome” in Latin. The shell surface is thin, fragile, and transparent; this transparency is visible in living or recently deceased specimens, though it gradually turns milky white as it ages. There are no teeth structures within the shell, and the columellar fold is weak but noticeable. Morphologically, the species resembles Zospeum amoenum, but DNA barcode analyses have shown a genetic difference of 11.7–12.1% between the two species.
Shell Structure and Size
The shell of Zospeum tholussum is semi-transparent, thin-walled, and colorless. The shell length is approximately 1.41 mm, and the average width is about 0.93 mm. It comprises five to six weakly defined whorls and has a symmetrical shape. The shell aperture is oval with smooth edges. The delicate shell structure has likely developed due to low calcium levels and the influence of the dark cave environment.
Eye Structure and Pigmentation
Like other troglobiontic organisms, Zospeum tholussum lacks pigmentation. The absence of color is associated with adaptation to an environment devoid of light. Additionally, the species is eyeless. The loss of eyes is presumed to be an evolutionary result of visual perception becoming obsolete in dark habitats.
Locomotion Ability
This snail has extremely limited mobility. Observations have shown that it may cover only a few millimeters over the course of several days. This suggests that the species has adapted to a lifestyle compatible with low energy consumption and minimal metabolic activity.
Shell Structure (Created by Artificial Intelligence)
Habitat, Distribution, and Ecological Characteristics
Lukina Jama–Trojama Cave
Zospeum tholussum has been identified exclusively within the Lukina Jama–Trojama cave system located in northern Croatia. This cave is situated in the karst zone of the Dinaric Alps and, with a depth of approximately 1,431 meters, is one of the deepest caves in Europe. The species was discovered as a single individual at a depth of 980 meters, in a moist and muddy environment. This finding indicates that Zospeum tholussum is a microendemic species.
Ecosystem Conditions
The cave environment is entirely dark, and the temperature typically ranges between 4 and 6 °C. These conditions are favorable for organisms with low metabolism and limited energy consumption. Furthermore, the amount of dissolved oxygen in the cave is limited, suggesting that the species is also adapted to low-oxygen environments.
Feeding and Trophic Relationships
Although the feeding behavior of Zospeum tholussum has not been directly observed, it is presumed to be detritivorous, based on the behavior of other species within the same genus and the characteristics of its habitat. It likely feeds on organic matter residues, fungal hyphae, and bacterial biofilms. With this feeding strategy, it plays a decomposer (saprotrophic) role in the cave ecosystem.
Nutrition and Trophic Relationships (Created by Artificial Intelligence)
Discovery and Scientific Significance
Zospeum tholussum was discovered in 2010 by speleologist Jure Zagmajster during cave exploration activities. The single specimen found was later examined in detail by Alexander M. Weigand and colleagues and was formally described as a new species in 2013. The species name “tholussum” is derived from the Latin word tholos, meaning “dome,” referring to the dome-shaped structure on the upper part of the shell.
The discovery holds significance for cave biology, life in extreme habitats, and the conservation of microendemic species. Additionally, Zospeum tholussum was included in Arizona State University’s “Top 10 New Species” list of 2013.
