Unio crassus Philipsson in Retzius, 1788

Klishko, Olga, Lopes-Lima, Manuel, Froufe, Elsa, Bogan, Arthur, Vasiliev, Lyudmila & Yanovich, Lyudmila, 2017, Taxonomic reassessment of the freshwater mussel genus Unio (Bivalvia: Unionidae) in Russia and Ukraine based on morphological and molecular data, Zootaxa 4286 (1), pp. 93-112 : 109

Unio crassus Philipsson in Retzius, 1788

Synonyms (in the study area)

Unio musivus Spengler, 1793 .

Unio nana Lamarck, 1819 .

Unio fusculus Ziegler in Rossmässler, 1835 . Unio irgizlaica Lindholm, 1904 .

Description. Shell is oval, thick (up to 8.9 mm in the anterior shell area) and moderately inflated. The ratio of shell width to its height is 0.61–0.80, and the ratio of shell height to its length is 0.46–0.62. The umbo is wide projecting considerably above the dorsal margin. Umbo sculpture is wrinkled or consists of double-looped waves and knobs. Periostracum ranges from green to dark brown, sometimes with rays from the umbo to the ventral margin. Nacre is light blue, light violet or pinkish-white. Pseudocardinal teeth are flattened relatively thick (2.5–3.0 mm) and curved. The outer and inner pseudocardinal teeth of the left valve are generally separated with similar lengths and levels of projection, but occasionally the inner is higher than the outer pseudocardinal tooth. A dark strip on the foot may be well pronounced or not. The papillae of the incurrent aperture are short, conic, and located in two rows close to each other.

Comparisons. This species differs from U. pictorum and U. tumidus by having an oval shell shape, often wrinkled umbo sculpture and a higher shell height/length ratio (0.46–0.62). The labial palps in U. crassus are longer and larger than in U. pictorum and U. tumidus . The papillae of the incurrent aperture of U. crassus are shorter and arranged in two rows, whereas there are three to four rows in U. pictorum and U. tumidus .

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

Figures

FIGURE 1. Regions of study and localities of collection of Unio species. A, overview map of Eurasia. B, the Ukraine. C, European Russia. Symbols: U = Ukraine, ER = European Russia, T = Transbaikalian refuge.