Shell variation in the genus Discus Fitzinger, 1833 (Gastropoda: pulmonata: endodontidae)

In Europe the genus Discus Fitzinger, 1833 is represented by two subgenera: Gonyodiscus Fitzinger, with D. rotundatus (O. F. Müll.) and D. perspectivus (Mühlf.), and Discus s. str. with D. ruderatus (Fér.). Studies on their shell variation were to ascertain if variation among laboratory-bred individuals differed from such variation in natural populations. The number of whorls, shell height, body whorl height, aperture height, aperture width, shell major and minor diameter, umbilicus major and minor diameter, shell height/major diameter ratio, relative height of body whorl, relative umbilicus diameter and umbilicus major/minor diameter ratio were analysed. Variation ranges of most shell characters in laboratory specimens of D. rotundatus and D. ruderatus were much wider than those found in natural populations. The natural populations of each species differed statistically significantly among themselves in many characters but the differences were much smaller. Laboratory-bred D. perspectivus showed a tendency to produce scalariform shells or descending and partly detached body whorl. Laboratory-bred D. rotundatus and D. ruderatus tended to form a descending body whorl; in all three species the descending/detached part of the body whorl was formed after sexual maturity had been attained.