Variation in the genitalia of the land snail Heterostoma paupercula (Lowe, 1831) (Helicidae) in Madeira

Abstract

The small helicid snail Heterostoma paupercula occurs in the Azores and Madeira. Previously regarded as a single species, it was split into two genera on the basis of variation in shell morphology and genital anatomy: Heterostoma having a toothed shell and being hemiphallic, while Steenbergia had a toothless shell and was euphallic. This division was first questioned when toothed shells {Heterostoma) in the Azores were found not to be hemiphallic. In this study the genitalia of 361 specimens from islands in the Madeira archipelago were examined. Four components of the genitalia were measured (penis/epiphallus, flagellum, bursa stalk and bursa) for 158 individuals. Hemiphally was found to be rare, occurring in only a proportion of the populations at two localities and it did not correspond with the expected shell morphology–the shells were untoothed (=Steenbergia), which is the opposite of that previously described. Variation in the genitalia was geographically based: Madeira and its nearest neighbour islands forming one group and the Porto Santo islands another. There is no evidence to suggest that the hemiphallic animals are more than an intraspecific form. It is suggested that the variation between the two island groups is reflective of allopatric differentiation. Without fur ther evidence of divergence, H. paupercula should be regarded as the sole species throughout the range and the genus Steenbergia should be suppressed.