Planispiral Valvatids (Mollusca: Gastropoda: Valvatidae) of Subgenus Sibirovalvata in Upper Ob Basin

In reservoirs and watercourses of the Upper Ob basin (southern Western Siberia), two representatives of the family Valvatidae (Valvatidae) of the subgenus Sibirovalvata , having a planispiral shell, have been noted. It is shown that two species of planispiral mollusks from this subgenus live in the region: Valvata ( Sibirovalvata ) frigida and V. ( S. ) sibirica . On the material from the Upper Ob basin, significant conchological differences between V. ( S. ) frigida and V. ( S. ) sibirica in regards to key morphometric indices have been noted. According to the results of a comparison with the material from the northern Western Siberia, it is found that the shells of the northern (basin of the Taz River) and southern (basin of the Upper Ob) populations of V. ( S. ) frigida and V. ( S. ) sibirica differ very slightly in morphometric indices, which indicates a slight variability of these traits within the range. At the same time, the detected significant differences between the northern and southern populations in the absolute size of the shell can presumably be explained by the conditions of specific habitats. At the same time, a more favorable temperature regime in the south may be offset by a lack of oxygen, including during the summer–autumn low water period.