Monodacna caspia, s.l.

Monodacna caspia (Eichwald, 1829) s.l.

(Fig. 4: 4, Fig. 7: 1).

*1829 C. [orbula] caspia Eichwald: 281, pl. 5, fig. 6.

1986 Monodacna caspia (Eichwald), 1938 – Yakhimovich et al.: 84, pl. 12, fig. 12.

2013 Adacna (Monodacna) caspia caspia (Eichwald, 1829) – Bogutskaya et al.: 380, fig. 154.

Dimensions – max. L 23.3 mm, H 18.6 mm.

Characterization – The small-sized cardiid shows a variety of shapes and sizes in our Selitrennoye material. The presence of a series of intermediate morphologies (Fig. 7: 1a–d) suggests a single polymorphic species. All specimens have a single cardinal tooth and lack the lateral tooth that typifies Monodacna, yet the thickness and expression of the hinge is extremely variable. Acommon form is thin-shelled, relatively convex with straight posterior margin and relatively well developed, regularly spaced but thin ribs (Fig. 7: 1c). This shape conforms to shells of M. caspia illustrated in Bogutskaya et al. (2013). Another form is flatter, in general slightly thicker shelled and has a distinct wedge-shape (Fig. 7: 1a). Its ribs are lower and slightly more irregular than in the previous form. This wedge-shaped form conforms to several modern Monodacna species as reported in Bogutskaya et al. (2013) such as Monodacna albida (Logvinenko and Starobogatov, 1967) and M. polymorpha (Logvinenko and Starobogatov, 1967) that have been tentatively grouped into one species by Wesselingh et al. (2019).

Taxonomic notes – Our findings of a large morphological variation in Monodacna, with typical forms conforming to modern species but also with all kind of intermediates, either shows that (1) we are dealing with an ancestral species of all or most of the present-day Caspian Monodacna species or (2) that several of the present-day species should be regarded as a single species. Acombined morphological-molecular approach is required to assess the species delimitations within living faunas.

Ecology – Caspian Monodacna species occur in a wide range of habitats today, and given the uncertainty of the identity of these species, it is difficult to report the ecological characteristics. Species live mainly in northern CS on muddy and sandy-muddy substrates at a maximum depth of 40 m (Bogutskaya et al., 2013). Monodacna species are filter feeders whose salinity preferences range mostly from 2 to 8 psu (Bogutskaya et al., 2013).