Unio pictorum Linnaeus 1758

Unio pictorum (Linnaeus, 1758)

Synonyms (in the study area)

Unio rostratus Lamarck, 1819. Unio limosus Nilsson, 1822.

Unio longirostris Rossmässler, 1836. Unio protractus Lindholm, 1922.

Description. Shells are relatively thin (up to 7.9 mm in the anterior shell section), more or less inflated, ratio of shell width to its height is 0.61–0.90. Shell shape elongate elliptical, ratio of shell height to its length is 0.37–0.53, dorsal and ventral margins nearly straight and parallel. Periostracum ranges from yellow and greenish to dark brown. Nacre is bluish-white. Umbo is wide, elevated above dorsal shell margin, umbo sculpture is either ‘nodulous’ consisting of two divergent rows of knobs or a complex of knobs and ribs. Pseudocardinal and lateral teeth are relatively thin (0.20–0.26 mm) and lamellar. The outer and inner pseudocardinal teeth of the left valve may be similar or different in length and height, and project inward at the same or different levels or they may occur merged as a single tooth. Dark wedge-like strip on the foot is well visible. Papillae of the incurrent aperture are elongated, spindle-shaped, and arranged in three to four rows; the pigmentation of the excurrent aperture is well expressed.

Comparisons. This species differs from U. tumidus and U. crassus by having an elongate elliptical shell shape. The intraspecific shell convexity and periostracum color variations of specimens collected in different regions of Russia and the Ukraine are probably more related to environmental factors.

Distribution. In Russia and Ukraine, it is restricted to the White, Baltic, Black and Caspian Sea drainage basins, also found in a restricted range in Transbaikalia, Asian Russia (Fig. 1).