Zospium thollusum - The tale of Unique Snail from the deepest cave of the World 

 DATE :- 08.03.2022                              Koustav Bhattacharjee

Zospeum tholussum or the Domed Land Snail, is a cave-dwelling species of air-breathing land snails in the family Ellobiidae. It is a very small species, with a shell height of less than 2 mm (0.08 in) and a shell width of around 1 mm (0.04 in). Z. tholussum individuals are completely blind and possess translucent shells with five to six whorls. The second whorl of their shells has a characteristic dome-like shape. They are also extremely slow-moving and may depend on passive transportation through running water or larger animals for dispersal.

Zospeum tholussum was discovered at depths of 743 to 1,392 m (2,438 to 4,567 ft) in the Lukina jama–Trojama cave system in Croatia in 2012, during a caving expedition. It was formally described as a new species in 2013 by the taxonomist Dr. Alexander M. Weigand.

On 22 May 2014 International Institute for Species Exploration declared the snail as one of the "Top 10 New Species of 2014" among species discovered in 2013. The reason for its selection is its habitat in complete darkness of a cave some 900 meters below the surface. It lacks eyes and its shell is transparent giving it a ghost-like appearance. 

Like all members of the genus Zospeum, Zospeum tholussum are completely blind. Because of this and their lack of pigmentation, they are considered to be true cave-dwelling organisms (eutroglobionts). Very little is known about their biology, but members of the genus Zospeum are known to prefer muddy to permanently wet subterranean microhabitats. They are generally found along the drainage systems of caves.

 Zospeum tholussum are currently only known from the Lukina jama–Trojama cave system of Croatia. Its distribution range is within the larger distribution range of the morphologically similar Zospeum amoenum (which is found in caves in the Western Balkans in northern Slovenia, western Croatia, Bosnia and Herzegovina, and Montenegro). Since Zospeum species with large inferred distribution ranges are known to actually also contain morphologically similar and unrecognised cryptic species, Weigand postulated that some early records of Zospeum amoenum may have actually been specimens of Zospeum tholussum.