Theodoxus velox V. Anistratenko in O. Anistratenko et al., 1999 Figure 22A-M

Theodoxus velox V. Anistratenko in O. Anistratenko et al. 1999: 17-18, fig. 4.7; Wesselingh et al. 2019: 66.

Type locality.

Dnieper Delta, Zburievkiy Liman, Kherson Region, Ukraine.

Type material.

Holotype and five paratypes are stored in IZAN (without coll. no.) .

Remarks.

This species was recently discussed by Wesselingh et al. (2019). While it was previously considered a junior synonym of T. fluviatilis (Vinarski and Kantor 2016), molecular data shows strong monophyletic support for the independence of T. velox (Sands et al. 2019a; Fig. 2). Nevertheless, T. velox and T. fluviatilis still hold a sister relationship and likely diverged from one another during the early Pleistocene (Sands et al. 2019a; Fig. 2). Theodoxus velox is challenging to distinguish from T. fluviatilis given the overlap in geographic range and similarity of conchological features (Figs 3, 9, 10, 22). Theodoxus velox has less phenotypic variability compared to T. fluviatilis (Figs 9, 10, 22). Moreover, T. velox display more expansive spiral whorls and, in some instances, a more transparent operculum where the conchioline lamella extends deeper into the calcareous base and the callus is less pronounced (Figs 9, 10, 22).

The geographic distribution range of T. velox overlaps with that of T. sarmaticus (Lindholm, 1901), which has been considered a junior synonym of T. fluviatilis (e.g. Vinarski and Kantor 2016). While T. sarmaticus and T. fluviatilis may on occasion share phenotypic similarity, the morphotypes of T. sarmaticus closely resemble T. velox (Fig. 22A-M versus Fig. 22N-U). The similarity with T. velox could suggest T. sarmaticus is rather conspecific with that species as suggested by Glöer (2019). Unfortunately, no opercula or soft tissues were preserved among the syntypes of T. sarmaticus to corroborate this hypothesis. Molecular analyses of topotypic material is required to address this uncertainty. However, should this be confirmed at a later point, the name T. sarmaticus would have priority.

Distribution.

This species was indicated to be restricted to drainage systems of the northern Black Sea coast (Anistratenko et al. 1999; Kantor and Sysoev 2006). Recent molecular data suggest it is distributed as far North as the eastern part of the Baltic Sea and as far South as Anatolia (Sands et al. 2019a): the only record there derives from Lake Sapanca (Sands et al. 2019a; Fig. 3C).