Caspiinae B. Dybowski 1913

Subfamily Caspiinae B. Dybowski, 1913

Diagnosis. Small (height up to c. 3 mm), conical to ovoid or occasionally globular shell with weakly to moderately convex whorls; umbilicus closed, chink-like or round and deep; aperture ovoid, with acute adapical tip; protoconch bears distinct, fine wrinkles and spiral threads; transition to teleoconch marked by distinct growth rim; rachidian tooth of radula with two pairs of basal cusps. Bursa copulatrix and seminal receptacle rs-1 absent in female genitals.

Remarks. The updated morphological diagnosis is based on previously collected data (especially concerning anatomy) as well as new information encountered during the present revision. The radula, characterised by two pairs of basal denticles on the rachidian tooth, differs from those found in the hydrobiid subfamily Pyrgulinae, which lacks basal cusps. In combination with anatomical characteristics, this traits was previously used to separate Caspiinae from Pyrgulinae and classify them as a distinct subfamily (Sitnikova & Starobogatov 1998). Hydrobiinae have one or two pairs of basal denticles on the rachidian tooth and have a bursa copulatrix and seminal receptacle, both of which are missing in Caspiinae. Boeters et al. (2015) reported the presence of rs- 2 in Clathrocaspia milae. Whether or not this feature is characteristic for any other Caspiinae is unknown at present and requires further study. Pyrgulinae have a bursa copulatrix; Turricaspiini, presently considered a tribe within Pyrgulinae (Anistratenko et al. 2019), have a seminal receptacle rs- 1 in addition (see Anistratenko 2008, 2013).

Currently, the suprageneric systematic classification of Caspiinae is based on morphology alone. However, a current study in progress using molecular data for C. knipowitschii suggests that Caspiinae form a distinct clade within Hydrobiidae (B. Csányi et al., unpublished data); its subfamily status is thus retained here.

Current systematics recognises three supra-specific taxa in the Caspiinae, i.e. Caspia, Clathrocaspia and Ulskia, which are variably treated as distinct genera (Neubauer et al. 2018; Wesselingh et al. 2019) or subgenera (Anistratenko et al. 2019). Here we follow the former approach and treat them as distinct genera. The three genera are differentiated based on details in the ornamentation of the protoconch and teleoconch. The key feature of Caspia is a superficially smooth shell occasionally with a single (or two) fine spiral keel(s) below the suture (Dybowski 1887 –1888; Logvinenko & Starobogatov 1969; Anistratenko et al. 2019). Clathrocaspia is characterised by a welldeveloped reticulate teleoconch sculpture (e.g. Logvinenko & Starobogatov 1969; Anistratenko 2013; Boeters et al. 2015). Ulskia groups species with a superficially smooth shell, a dome-like, comparatively large protoconch and a microsculpture on protoconch and teleoconch consisting of tiny rows of grains organised in spiral rows.

Minute species of the genus Andrusovia Brusina in Westerlund, 1903 are here attributed to the subfamily Caspiinae as well. Andrusovia has formerly been affiliated with a series of different families and even subclasses, including Valvatidae (Westerlund 1903), Planorbidae (Starobogatov 1970), Horatiidae (Starobogatov 2000) or Hydrobiidae (subfamily Belgrandiinae; Kantor & Sysoev 2006: 83). Although the valvatoid shells of Andrusovia strongly differ from those of Caspia, Ulskia and Clathrocaspia, the protoconch shares the unique type of growth and ornamentation typical among Caspiinae. Also, Andrusovia shares a similar type of spirally arranged teleoconch microsculpture with Ulskia. In addition, with an overall comparable size of the species involved, we consider these features evidence for grouping Andrusovia in Caspiinae.

The systematic list following below is ordered alphabetically after genus and species and includes (i) species presently considered accepted, (ii) species of doubtful identity (taxa inquirenda and nomina dubia), and (iii) species that have previously been affiliated with Caspiinae but do not belong in the subfamily according to the present revision.