Intraspecific variation in Nesovitrea hammonis (Ström) (Pulmonata: Zonitidae) in Sweden, with special reference to colour dimorphism

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Swedish Nesovitrea hammonis was investigated with regard to the distribution of a greenish-hyaline colour form (f. viridola); size of the shell (Ømaj ); and development of an internal rib (callus). Material from two localities surveyed every year 1970–82, and material from a great number of localities in SW. Sweden investigated 1920–22, and re-investigated 50 years later, showed a striking constancy In the relative number of f. viridula. This seems to be valid for localities apparently not affected by man, but the proportions were remarkably changed in localities severely influenced by human activities. The distribution of f. viridula seems to indicate a correlation with the maritime climate in SW. Sweden, where the form is common. Eastwards and northwards the frequency decreases rapidly and in northernmost Sweden f. viridula is absent. In the province of Dalsland the distribution of f. viridola was studied in more detail. Here the frequency of the form seems to decrease with decreasing annual precipitation. Shells from N. Sweden differ from those from SW. Sweden in two aspects; 1) the specimens reach a larger maximum size (a greater number of whorls) and 2) for each number of whorls the mean Ømaj is larger. The callus may be differently developed in material from various Swedish districts, often it is absent.