Analysis of demographic and morphological parameters of adventitious Stenomphalia ravergiensis (Mollusca, Gastropoda, Pulmonata) populations in the south of the Central Russian Upland

We present demographic and morphological parameters of seven populations of invasive terrestrial mollusk Stenomphalia ravergiensis (Ferussac, 1835) from the south of the Central Russian Upland (Belgorod). A sample of snails from the native area (Armenia) was taken for comparison. The total number of measured shells was 800. We determined the absolute density of organisms at the site (using test plots 1 m each) and their age structure. For сonchometric analysis at each point, we took randomly samples of 100 mature individuals. We measured morphometric parameters and calculated the standard shell index (see Tables). Our study indicates that anthropogenically-altered areas are prevalent for non-indigenous St. ravergiery population, due to the ways of invasions in combination with a high vulnerability of urban ecosystems to invasion. The analysis of variance (ANOVA) demonstrated significant divergence of the groups in biometric shell parameters that is, probably, caused by the difference in biotopic conditions in the urban landscape. We found the largest shells in arid habitats with chalk outcrops. Moreover, we registered the highest index of shell volume/aperture square here. Snails with smaller shell sizes were more common in humid habitats. This fact supports the hypothesis about the selective advantage of forms which have large shells with small aperture among xerophilic and xero-mesophilic species of terrestrial mollusks. In arid habitats these conchological parameters allow collecting and storing water in the body. However, this rule is not absolute for alien population of St. ravergiensis: despite moisture gradient of some plots, samples had the same morphological parameters. This phenomenon may be caused by differences in the age of snail colonies, so in groups, formed recently, we observed the formation of phenotypes which are response to microclimatic conditions of the habitat. We registered the smallest interpopulation dispersion for the index of shell height to shell width, which probably suggests conservative species-specific shell proportions. The results obtained from field research demonstrated that the population was composed of different age groups and had a high level of density (5-58 of individuals per square meter), which showed positive trends of the invasion process and the successful St. ravergiensis expansion across the studied area. The sample of the population from the native species area (surroundings of Sevan Lake, Armenia) is characterized by intermediate values of shell parameters. Comparing alien populations with the native Caucasian groups, we established both significant similarities and significant differences between them. A high level of population density and prevalence of juveniles in the age structure allows predicting further expansion of this alien species across the given territory. Variability of morphometric shell parameters from different habitats may indicate a wide ecological valence of the invasive species, which may influence successful introduction of St. ravergiensis into new territories.

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