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Didacna subpyramidata Pravoslavlev, 1939 (Fig. 5: 2). *1939 Didacna subpyramidata Pravoslavlev: 212, pl. 1, figs. 7���8. 1953 Didacna subpyramidata Pravoslavlev, 1939 ��� Fedorov: 122, pl. 9, figs. 4, 8, pl. 10, figs. 7���10. 1969 Didacna subpyramidata Pravoslavlev, 1939 ��� Vekilov: 83, pl. 14, figs. 1���4. 1983 Didacna subpyramidata Pravoslavlev, 1939 ��� Popov: 198, pl. 7, figs. 1���6. 2007 Didacna subpyramidata Pravoslavlev, 1939 ��� Nevesskaja: 908, pl. 13, figs. 13���18. Dimensions ��� max. L 43.8 mm, H 39.5 mm. Characterization ��� Large, high, trigonate-convex species with protruding beak; posterodorsal margin very steep and clearly delimited; the posterior ridge is sharp and massive. The species show 17���21 posterior ribs that are typically 2���3 times as wide as the interspaces. The ribs have a subangular to flat-square crosssection. Shells are thick and robust as is the hinge. Outline of juvenile shells is trigonate and very similar as Didacna emendata but with fewer ribs. Ecology ��� Unknown (extinct)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on pages 1234-1235, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200

Dreissena grimmi Andrusov, 1890 (Fig. 8: 6���7). *1890 Dr.[eissena] Grimmi Andrusov: 233. 2013 Dreissena rostriformis (Deshayes, 1838) ��� Bogutskaya et al.: 330, fig. 117b. Dimensions ��� Max. L 9.9 mm, H 22.5 mm. Characterization ��� Relatively flat comma- to tear-shaped Dreissena lacking a keel; the ventral margin is somewhat concave in its upper half; often a byssate depression is developed; the line of semidiameter runs in the middle of the shell; the hinge platform is flat, wide and robust. Taxonomic notes ��� This species is commonly known as Dreissena rostriformis (Deshayes, 1838), a species originally described from Pliocene deposits of Crimea in the Black Sea Basin. Its lectotype (as illustrated in Rosenberg and Ludyanskiy, 1994) shows a different shell shape with almost parallel sides that result in a subquadrangular overall outline. This shape is mirrored by the pallial line on the interior and has not been seen in the dropshaped outlines of late Quaternary Caspian Dreissena. Hence, the name D. rostriformis should be abandoned for late Quaternary Caspian material; the next available name is D. grimmi (Wesselingh et al., 2019). In addition, the four subspecies described from the CS (Bogutskaya et al., 2013) are likely synonyms as indicated by molecular analyses (Stepien et al., 2013). The latter authors however argue for synonymy with the Black Sea D. bugensis Andrusov, 1897. That species has a slight but consistent genetic differentiation but incompatible ecological preferences in the Black Sea Basin compared to the Caspian mussels (e.g. salinity preferences of 0���2 psu versus 7���12 psu; Stepien et al., 2013). The introduction of D. bugensis in the Volga catchment in the 1980s has not yet led to reconnection of Caspian population of D. grimmi (pers. obs.), hence we consider these species as separate species. Ecology ��� Dreissena grimmi lives in the Middle and South Caspian Basin at depths between 15 and 300 m water depth and salinities between 7 and 12 psu (Bogutskaya et al., 2013)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on page 1237, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Rosenberg, G., Ludyanskiy, M. L., 1994. A nomenclatural review of Dreissena (Bivalvia: Dreissenidae), with identification of the quagga mussel as Dreissena bugensis. Can. J. Fish. Aquat. Sci. 51, 1474 - 1484. https: // doi. org / 10.1139 / f 94 - 147.","Wesselingh, F. P., Neubauer, T. A., Anistratenko, V. V., Vinarski, M. V., Yanina, T. A., ter Poorten, J. J., Kijashko, P. V., Albrecht, C., Anistratenko, O., D'Hont, A., Frolov, P., Martinez Gandara, A., Gittenberger, A., Gogaladze, A., Karpinsky, M. G., Lattua, M., Popa, L., Sands, A. F., van de Velde, S., Vandendorpe, J., Wilke, T., 2019. Mollusc species from the Pontocaspian region - an expert opinion list. Zookeys 827, 31 - 124. https: // doi. org / 10.3897 / zookeys. 827.31365.","Bogutskaya, N. G., Kijashko, P. V., Naseka, A. M., Orlova, M. I., 2013. Identification keys for fish and invertebrates. Volume 1: Fish and molluscs. KMK Scientific Press Ltd., St. Petersburg - Moscow.","Stepien, C. A., Grigorovich, I. A., Gray, M. A., Sullivan, T. J., Yerga-Woolwine, S., Kalayci, G., 2013. Evolutionary, biogeographic, and population genetic relationships of dreissenid mussels, with revision of component taxa. In: Nalepa, T. F., Schloesser, D. W. (Eds.), Quagga and Zebra Mussels: Biology, Impacts, and Control, Second Edition. CRC Press, Boca Raton, pp. 403 - 444. https: // doi. org / 10.1201 / b 15437.","Andrusov, N., 1897. Fossile und lebende Dreissenidae Eurasiens. Fossil and living Dreissenidae of Eurasia. Tipografiya M. Merkusheva, St. Petersburg."]}

Didacna ebersini Fedorov, 1953 (Fig. 5: 5). *1953 Didacna ebersini Fedorov: 125, pl. 14, fig. 2, fig. 5, and fig. 6 (? non figs. 1, 3, 4). 1999 Didacna ebersini Fedorov ��� Fedorov: pl. 12, fig. 1. 1983 Didacna ebersini Fedorov ��� Popov: 203, pl. 16, figs. 1���16. 2007 Didacna ebersini Fedorov, 1953 ��� Nevesskaja: 931, pl. 20, figs. 6���16. Dimensions ��� max. L 19.0 mm, H 14.9 mm. Characterization ��� Intermediate-sized, flat and thin shelled broad trigonate Didacna with flat ribs; posterodorsal margin is low and in adult stages not well delimited. The posterior ridge is low and subangular and fading during growth. The species have 16���22 posterior ribs. Rib width is somewhat irregular but ribs are about as wide as interspaces. In cross-section they are markedly low. Juvenile outline is trigonate with a distinct often double keel forming the posterior ridge. The base of the juveniles is flat to somewhat concave just before the posterior ridge. Ecology ��� Unknown (extinct)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on pages 1232-1233, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Fedorov, P. V., 1953. Caspian Quaternary mollusks of the genus Didacna Eichwald and their stratigraphical importance. In: Nikolaev, V. I. (Ed.), Stratigraphy of Quaternary Deposits and the Recent Tectonics of the Pre-Caspian Depression. Akademiya Nauk SSSR, Moscow, pp. 112 - 130 (in Russian)."]}

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)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on page 1235, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Eichwald, E., 1829. Zoologia specialis quam expositis animalibus tum vivis, tum fossilibus potissimum Rossiae in universum et Poloniae in specie, in usum lectionum publicarum in Universitate Caesarea Vilnensi habendarum. Pars prior propaedeuticam zoologiae atque specialem heterozoorum expositionem continens. Joseph Zawadzki, Vilnius.","Bogutskaya, N. G., Kijashko, P. V., Naseka, A. M., Orlova, M. I., 2013. Identification keys for fish and invertebrates. Volume 1: Fish and molluscs. KMK Scientific Press Ltd., St. Petersburg - Moscow.","Logvinenko, B. M., Starobogatov, Y. I., 1967. K izucheniyu vidovogo sostava fauny dvustvorchatykh mollyuskov tanatotsenozov podvodnogo sklona Azerbaydzhanskogo poberezh'ya Kaspiya. [Studying the species composition of the fauna of bivalve mollusks of the tanatocenoses of the underwater slope of the Azerbaijan coast of the Caspian Sea]. In: Kudritskiy, D. M. (Ed.), Opyt Geologo-Geomorfologicheskikh i Gidrobiologicheskikh Issledovaniy Beregovoy Zony Morya. Nauka, Leningrad, pp. 225 - 235.","Wesselingh, F. P., Neubauer, T. A., Anistratenko, V. V., Vinarski, M. V., Yanina, T. A., ter Poorten, J. J., Kijashko, P. V., Albrecht, C., Anistratenko, O., D'Hont, A., Frolov, P., Martinez Gandara, A., Gittenberger, A., Gogaladze, A., Karpinsky, M. G., Lattua, M., Popa, L., Sands, A. F., van de Velde, S., Vandendorpe, J., Wilke, T., 2019. Mollusc species from the Pontocaspian region - an expert opinion list. Zookeys 827, 31 - 124. https: // doi. org / 10.3897 / zookeys. 827.31365."]}

Adacna minima Ostroumov, 1907 (Fig. 4: 2). *1907 Adacna minima Ostroumov: 23, pl. 4, figs. 1���5. 1969 Adacna minima ostroumovi Logvinenko and Starobogatov: 338, fig. 354, 3. 2013 Adacna minima ostroumovi Logvinenko and Starobogatov, 1969 ��� Bogutskaya et al.: 378, fig. 146. Dimensions ��� max. L 16.5 mm, H 12.7 mm. Characterization ��� Relatively small subtriangular to subrounded Adacna; shell thin, slightly more convex than other Adacna species and in outline resembling some Monodacna species, but hinge plate very thin, edentulous with the exception of juvenile stages where a small cardinal tooth in LV can be present that disappears in adult stages; dorsal margins usually more or less straight; ribs low, flat regularly spaced and fading towards anterior and posterior margins; in our material specimens with relatively well developed low ribs to almost smooth specimens and intermediates occur; well-developed growth bands in our material. Ecology ��� Species occurs mainly in the middle and south-east of the CS, less often in the north CS (Logvinenko and Starobogatov, 1969). Endemic species living in water with a salinity of at least 5���8 psu, preferably muddy, sandy-muddy and, rarely, sandy bottoms, mainly in the middle and south CS at depths of, Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on page 1231, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Ostroumov, A. A., 1907. On mollusks of the Aral Sea. Izv. Turkest. Otd. Imp. Rus. Geogr. Obs. 4, 20 - 26.","Logvinenko, B. M., Starobogatov, Y. I., 1969. Mollusca, in: Birshtein, Y. A., Vinogradov, L. G., Kondakov, N. N., Kuhn, M. S., Astakhova, T. V., Romanova, N. N. (Ed.), Atlas Bespozvonochnykh Kaspiyskogo Morya [Atlas of Invertebrates of the Caspian Sea]. Pishchevaya Promyshlennost (Vsesoyuznyi Nauchno-issledovatel'skii Institut Morskogo Rybnogo Khozyaistva i Okeanografii), Moscow, pp. 308 - 385.","Bogutskaya, N. G., Kijashko, P. V., Naseka, A. M., Orlova, M. I., 2013. Identification keys for fish and invertebrates. Volume 1: Fish and molluscs. KMK Scientific Press Ltd., St. Petersburg - Moscow."]}

Monodacna semipellucida (Logvinenko and Starobogatov, 1967) (Fig. 4: 3). *1967 Hypanis semipellucida Logvinenko and Starobogatov: 232 ���233. 1973 Hypanis semipellucida Logvinenko and Starobogatov, 1967 ��� Grossu: 144, fig. 27. 2013 Adacna (Monodacna) semipellucida Logvinenko and Starobogatov, 1967 ��� Kijashko in Bogutskaya et al.: 384, fig. 161, photo 55. Dimensions ��� max. L 23 mm, H 16 mm. Characterization ��� This species is relatively thin-walled and low, often has an elliptical outline in adult stages with low, broad and smooth ribs and narrow interspaces. The ribs and interspaces are well visible on the shell���s interior. The most distinct character is the cardinal tooth that is a very narrow but well delimited knob that is located well towards the base of the hinge platform. Ecology ��� Monodacna semipellucida lives in the Middle Caspian Sea (Bogutskaya et al., 2013; Logvinenko and Starobogatov, 1969; Wesselingh et al., 2019), down to 40 m water depth and minimum salinities of 5���8 psu (Bogutskaya et al., 2013; Logvinenko and Starobogatov, 1969)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on pages 1235-1236, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Logvinenko, B. M., Starobogatov, Y. I., 1967. K izucheniyu vidovogo sostava fauny dvustvorchatykh mollyuskov tanatotsenozov podvodnogo sklona Azerbaydzhanskogo poberezh'ya Kaspiya. [Studying the species composition of the fauna of bivalve mollusks of the tanatocenoses of the underwater slope of the Azerbaijan coast of the Caspian Sea]. In: Kudritskiy, D. M. (Ed.), Opyt Geologo-Geomorfologicheskikh i Gidrobiologicheskikh Issledovaniy Beregovoy Zony Morya. Nauka, Leningrad, pp. 225 - 235.","Bogutskaya, N. G., Kijashko, P. V., Naseka, A. M., Orlova, M. I., 2013. Identification keys for fish and invertebrates. Volume 1: Fish and molluscs. KMK Scientific Press Ltd., St. Petersburg - Moscow.","Logvinenko, B. M., Starobogatov, Y. I., 1969. Mollusca, in: Birshtein, Y. A., Vinogradov, L. G., Kondakov, N. N., Kuhn, M. S., Astakhova, T. V., Romanova, N. N. (Ed.), Atlas Bespozvonochnykh Kaspiyskogo Morya [Atlas of Invertebrates of the Caspian Sea]. Pishchevaya Promyshlennost (Vsesoyuznyi Nauchno-issledovatel'skii Institut Morskogo Rybnogo Khozyaistva i Okeanografii), Moscow, pp. 308 - 385.","Wesselingh, F. P., Neubauer, T. A., Anistratenko, V. V., Vinarski, M. V., Yanina, T. A., ter Poorten, J. J., Kijashko, P. V., Albrecht, C., Anistratenko, O., D'Hont, A., Frolov, P., Martinez Gandara, A., Gittenberger, A., Gogaladze, A., Karpinsky, M. G., Lattua, M., Popa, L., Sands, A. F., van de Velde, S., Vandendorpe, J., Wilke, T., 2019. Mollusc species from the Pontocaspian region - an expert opinion list. Zookeys 827, 31 - 124. https: // doi. org / 10.3897 / zookeys. 827.31365."]}

Adacna laeviuscula (Eichwald, 1829) (Fig. 4: 1). *1829 G. [lycymeris] laeviuscula Eichwald: 279, pl. 5, fig. 1. 1986 Adacna (Adacna) laeviuscula (Eichwald, 1831). ��� Yakhimovich et al.: 85, pl. 11, fig. 8. 2013 Adacna laeviuscula (Eichwald, 1829). ��� Bogutskaya et al.: 377, fig. 154, photo 48. Dimensions ��� max. L 32 mm, H 21 mm. Characterization ��� Large, subquadrate to subtrigonal thinshelled flat cardiid with strong posterior and slight anterior gape; dorsal margins relatively straight; relatively few ribs that are thin in juvenile stages and become very flat and separated by barely perceptible grooves in adult stages; hinge is a mostly edentulous, straight and flat ridge; in juvenile specimens a small cardinal tooth may be present that disappears in adult stages. Ecology ��� This species has been listed from the entire CS from water depths between 35 and 100 m in salinities of at least 4 psu (Bogutskaya et al., 2013; Logvinenko and Starobogatov, 1969). The common occurrence of fresh, paired specimens on exposed beaches of the middle Caspian (Turali, Russia; Sirvan, Azerbaijan: pers. observ. FW) demonstrates that A. laeviuscula has viable populations in foreshore settings above storm wave base indicating shallower habitats than previously reported., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on pages 1230-1231, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Eichwald, E., 1829. Zoologia specialis quam expositis animalibus tum vivis, tum fossilibus potissimum Rossiae in universum et Poloniae in specie, in usum lectionum publicarum in Universitate Caesarea Vilnensi habendarum. Pars prior propaedeuticam zoologiae atque specialem heterozoorum expositionem continens. Joseph Zawadzki, Vilnius.","Bogutskaya, N. G., Kijashko, P. V., Naseka, A. M., Orlova, M. I., 2013. Identification keys for fish and invertebrates. Volume 1: Fish and molluscs. KMK Scientific Press Ltd., St. Petersburg - Moscow.","Logvinenko, B. M., Starobogatov, Y. I., 1969. Mollusca, in: Birshtein, Y. A., Vinogradov, L. G., Kondakov, N. N., Kuhn, M. S., Astakhova, T. V., Romanova, N. N. (Ed.), Atlas Bespozvonochnykh Kaspiyskogo Morya [Atlas of Invertebrates of the Caspian Sea]. Pishchevaya Promyshlennost (Vsesoyuznyi Nauchno-issledovatel'skii Institut Morskogo Rybnogo Khozyaistva i Okeanografii), Moscow, pp. 308 - 385."]}

Didacna emendata Popov, 1983 (Fig. 5: 4). *1983 Didacna delenda emendata Popov: 174, pl. 6, figs. 8���20. 2007 Didacna emendata Popov, 1983 ��� Nevesskaja: 915, pl. 15, figs. 22-23. Dimensions ��� L 21.2 mm, H 18.3 mm. Characterization ��� Medium-sized, subrounded-subtrigonate relatively flat to semiconvex, high and thin-shelled Didacna. Beak clearly defined but low. Posterodorsal margin steep, broad, well delimited. Semidiameter located on posterior ridge that is well developed and subrounded. Only few of the specimens develop two elevated ribs that form the posterior ridge. Posterior margin subangular to angular. Individuals show 22���28 posterior ribs that are thin and flat and about two times as wide as the interspaces. Juveniles have a high subtrigonate outline with a rounded lower margin and a straight acute posterior ridge resembling the outline of Parvicardium species. The hinge plate is narrow, cardinal tooth well developed. Ecology ��� Unknown (extinct)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on page 1233, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Popov, G. I., 1983. Pleystotsen Chernomorsko-Kaspiyskikh prolivov (stratigrafiya, korrelyatsiya, paleofaunistika, geologicheskaya istoriya) [The Pleistocene of the Black Sea-Caspian Straits (Stratigraphy, Correlation, Paleofaunistics, Geological History)]. Nauka, Moscow."]}

Dreissena elata Andrusov, 1897 (Fig. 7: 2, Fig. 8: 4���5). *1897 Dreissensia [sic] polymorpha var. elata Andrusov: 353, pl. 20, fig. 25. 2013 Dreissena elata (Andrusov, 1897) ��� Bogutskaya et al.: 321, fig. 108. Dimensions ��� max. L 9 mm, H 19.4 mm. Logvinenko and Starobogatov (1969) reported Hup to 30 mm. Characterization ��� Medium- to thick-shelled triangular-shaped Dreissena with a wide range of morphologies, ranging from an elongated form (Fig. 8: 4) to a short form (Fig. 8: 5). In general, the valves have a well-developed keel that runs close to the posteroventral margin; the latter is straight or can be concave in its upper half. The initial part of the dorsal margin is flaring and the inflection towards the lower dorsal margin is usually well developed. Adult shells can be somewhat flaring. The overall shell is broad and the lower margin is rounded. Taxonomic notes ��� The systematics and taxonomy of this species are not resolved. Dreissena elata has several characteristics in common with the widespread Eurasian D. polymorpha Pallas, 1771 (whose distribution nowadays also includes North America). The Caspian material resembles D. polymorpha, especially with regard to shape of the keel, but it is broader, thicker and somewhat flatter. However, also very slender forms exist together with intermediates in the studied material (Fig. 7: 2). Molecular studies would be required to assess whether D. elata and D. polymorpha are conspecific. Ecology ��� Before the introduction of Mytilaster minimus, D. elata was abundant in the South and Middle Caspian basins at depths of up to 35 m mainly on firm ground and shelly/stony substrate. By now, it is probably extinct (Logvinenko and Starobogatov, 1969). The species occurred in salinities between 2 and 8 psu (Bogutskaya et al., 2013)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on page 1237, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Andrusov, N., 1897. Fossile und lebende Dreissenidae Eurasiens. Fossil and living Dreissenidae of Eurasia. Tipografiya M. Merkusheva, St. Petersburg.","Logvinenko, B. M., Starobogatov, Y. I., 1969. Mollusca, in: Birshtein, Y. A., Vinogradov, L. G., Kondakov, N. N., Kuhn, M. S., Astakhova, T. V., Romanova, N. N. (Ed.), Atlas Bespozvonochnykh Kaspiyskogo Morya [Atlas of Invertebrates of the Caspian Sea]. Pishchevaya Promyshlennost (Vsesoyuznyi Nauchno-issledovatel'skii Institut Morskogo Rybnogo Khozyaistva i Okeanografii), Moscow, pp. 308 - 385.","Bogutskaya, N. G., Kijashko, P. V., Naseka, A. M., Orlova, M. I., 2013. Identification keys for fish and invertebrates. Volume 1: Fish and molluscs. KMK Scientific Press Ltd., St. Petersburg - Moscow."]}

Didacna cristata Bogachev, 1932 (Fig. 5: 6). *1932 Didacna praetrigonoides var. cristata Bogachev: 44, pl. 4, figs. 9���14. 1948 Didacna cristata Bogachev ��� Fedorov: 19, pl. 1, figs. 4���6, pl. 2, figs. 4���8. 1953 Didacna cristata Bogachev ��� Fedorov: 127, pl. 16, figs. 1��� 11. 1983 Didacna cristata Bogachev, 1932 ��� Popov: 201, pl. 12, figs. 9���22. Dimensions ��� max. L 19.8 mm, H 16.3 mm. Characterization ��� Medium-sized, relatively high, subtrigonate to subovate convex and robust Didacna. Beak clearly defined. Posterodorsal margin steep, thin, well delimited. Semidiameter located on posterior ridge that is well developed and sharp. In about half of the specimens two elevated ribs form the posterior ridge. Posterior margin rounded angular. Between 15 and 21 posterior ribs that are wide and flat and about three times as wide as the interspaces. Juveniles have a wide trapezoid outline with a rounded lower margin and a subtruncate posterior margin. The hinge plate and cardinal teeth are robust. Taxonomic notes ��� Bogachev (1932) distinguished the new variety D. praetrigonoides var. cristata from typical D. praetrigonoides by its smaller size, more triangular outline and sharper keel. Fedorov (1953) showed that the variety not only differs in shape and size from D. praetrigonoides but also in its stratigraphic position. While D. praetrigonoides is characteristic for the upper Khvalynian deposits, D. praetrigonoides var. cristata is an index fossil of the lower Khvalynian of the east coast of the Caspian Sea. Fedorov (1953) mentioned that the juvenile D. praetrigonoides forms are similar to D. cristata. Nevesskaja (1958) disagreed with Fedorov while studying a collection of mollusks from the Khvalynian deposits of West Turkmenistan. She stated that the Khvalynian beds of Turkmenistan yielded relatively small shells (up to 50 mm long), with a well-developed plate like a keel and a small number of ribs (23���27). Nevesskaja (1958) concluded that the variety should not be regarded as a distinct taxon because it only shows the plasticity of the species but cannot be well delineated in time or space (see also Nevesskaja, 2007). In contrast, Popov (1983) supported the opinion of Fedorov (1953) based on a comparative statistical analysis of shell dimensions of D. cristata and D. praetrigonoides (Table 41 in Popov, 1983). He demonstrated that the differences between D. cristata and D. praetrigonides were bigger than between many others Quaternary Didacna species. Here, we follow the view of Fedorov (1953) and Popov (1983). Ecology ��� Unknown (extinct). In general trigonoid Didacna species prefer low salinities (Yanina, 2012), but since D. cristata is extinct the paleosalinities can only be inferred., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on pages 1231-1232, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Bogachev, V. V., 1932. Vedushchiye iskopayemyye razreza Apsheronskogo polustrova i prilegayushchikh rayonov. Chast' 1 [Leading fossils of the Apsheron peninsula and adjacent areas. Part 1]. Tr. Azerb. Neft. Inst. 4, 1 - 92.","Andreev, N. I., Andreeva, S. I., 2003. Evolyutsionnye preobrazovaniya dvustvorchatykh mollyuskov Aral'skogo morya v usloviyakh ekologicheskogo krizisa [Evolutionary transformations of bivalves in the Aral Sea under conditions of an ecological crisis]. Omskiy Gosudarstvennyy Pedagogicheskiy Universitet. In: Omsk.","Bogutskaya, N. G., Kijashko, P. V., Naseka, A. M., Orlova, M. I., 2013. Identification keys for fish and invertebrates. Volume 1: Fish and molluscs. KMK Scientific Press Ltd., St. Petersburg - Moscow.","Logvinenko, B. M., Starobogatov, Y. I., 1969. Mollusca, in: Birshtein, Y. A., Vinogradov, L. G., Kondakov, N. N., Kuhn, M. S., Astakhova, T. V., Romanova, N. N. (Ed.), Atlas Bespozvonochnykh Kaspiyskogo Morya [Atlas of Invertebrates of the Caspian Sea]. Pishchevaya Promyshlennost (Vsesoyuznyi Nauchno-issledovatel'skii Institut Morskogo Rybnogo Khozyaistva i Okeanografii), Moscow, pp. 308 - 385.","Mirzoev, G. S., Alekperov, L. H., 2017. Zoobenthos distribution on biotope in the shelf zone of the Azerbaijan sector of the South Caspian. J. Entomol. Zool. Stud. 5, 953 - 959.","Nevesskaja, L. A., 2007. History of the genus Didacna (Bivalvia: Cardiidae). Paleontol. J. 41, 861 - 949. https: // doi. org / 10.1134 / S 0031030107090018.","Neubauer, T. A., van de Velde, S., Yanina, T. A., Wesselingh, F. P., 2018. A late Pleistocene gastropod fauna from the northern Caspian Sea with implications for Pontocaspian gastropod taxonomy. Zookeys 770, 43 - 103. https: // doi. org / 10.3897 / zookeys. 770.25365.","Parr, T. D., Tait, R. D., Maxon, C. L., Newton, F. C., Hardin, J. L., 2007. A descriptive account of benthic macrofauna and sediment from an area of planned petroleum exploration in the southern Caspian Sea. Estuar. Coast. Shelf Sci. 71, 170 - 180. https: // doi. org / 10.1016 / j. ecss. 2006.07.018.","Mordukhay-Boltovskoy, 1974. Atlas bespozvonochnykh Aral'skogo morya [Atlas of Invertebrates of the Aral Sea]. Pishchevaya Promyshlennost', Moscow.","Starobogatov, Y. I., 1994. Freshwater Zebra Mussel, Dreissena polymorpha (Pall.) (Bivalvia, Dreissenidae): Systematics, Ecology, Practical Meaning, Species of the Fauna of Russia and Adjacent Countries in Russian. Nauka, Moscow.","Starobogatov, Y. I., 2000. Caspian endemic genus Andrusovia (Gastropoda Pectinibranchia Horatiidae). Ruthenica 10, 37 - 42.","Wesselingh, F. P., Neubauer, T. A., Anistratenko, V. V., Vinarski, M. V., Yanina, T. A., ter Poorten, J. J., Kijashko, P. V., Albrecht, C., Anistratenko, O., D'Hont, A., Frolov, P., Martinez Gandara, A., Gittenberger, A., Gogaladze, A., Karpinsky, M. G., Lattua, M., Popa, L., Sands, A. F., van de Velde, S., Vandendorpe, J., Wilke, T., 2019. Mollusc species from the Pontocaspian region - an expert opinion list. Zookeys 827, 31 - 124. https: // doi. org / 10.3897 / zookeys. 827.31365.","Chukhchin, 1984. Ecology of the Black Sea gastropods. Naukova dumka. In: Kiev.","Fedorov, P. V., 1953. Caspian Quaternary mollusks of the genus Didacna Eichwald and their stratigraphical importance. In: Nikolaev, V. I. (Ed.), Stratigraphy of Quaternary Deposits and the Recent Tectonics of the Pre-Caspian Depression. Akademiya Nauk SSSR, Moscow, pp. 112 - 130 (in Russian).","Nevesskaja, L. A., 1958. Chetvertichnyye morskiye mollyuski Turkmenii [Quaternary Sea Mollusks of Turkmenistan]. Tr. Paleontol. Inst. 65, 1 - 82.","Popov, G. I., 1983. Pleystotsen Chernomorsko-Kaspiyskikh prolivov (stratigrafiya, korrelyatsiya, paleofaunistika, geologicheskaya istoriya) [The Pleistocene of the Black Sea-Caspian Straits (Stratigraphy, Correlation, Paleofaunistics, Geological History)]. Nauka, Moscow.","Yanina, T. A., 2012. Neopleistotsen Ponto-Kaspiya: biostratigraphiya, paleogeographiya, korrelyatsiya [Neopleistocene of the Ponto-Caspian Region: Biostratigraphy, Paleogeography, Correlation]. Moscow University Press, Moscow."]}

Dreissena caspia Eichwald, 1855 (Fig. 8: 1���3). *1855 Dreissena caspia Eichwald: 311 ���312, pl. 10, figs. 19���21. 2013 Dreissena caspia Eichwald, 1855 ��� Bogutskaya et al.: 321, fig. 109. Dimensions ��� max. L 7.4 mm, H 22.7 mm. Characterization ��� Elongate, somewhat flat Dreissena with rounded margins; keel low or absent resulting in rounded crossprofile. The semidiameter is initially located at the dorsal margin but in sub adult stages crosses to the ventral margin. In adult stages, the semidiameter becomes less pronounced and is located in the middle of the shell. The posterodorsal margin is broadly rounded (and not subangular as in D. elata), the posterior margin is rounded and the anterior margin is straight to slightly round. The hinge platform is flat and solid. Ecology ��� Dreissena caspia is a filter feeder that used to be common all over the CS but is now possibly extinct (Bogutskaya et al., 2013). The species seems to be completely outcompeted by Mytilaster minimus (Logvinenko and Starobogatov, 1969). The species was most abundant at depths up to 40 m (Logvinenko and Starobogatov, 1969), and in a salinity range between 2 and 8 psu (Bogutskaya et al., 2013)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on page 1236, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Eichwald, E., 1855. Zur Naturgeschichte des Kaspischen Meeres [The natural history of the Caspian Sea]. Nouv. Mem. Soc. Imp. Nat. Moscou 10, 283 - 323.","Bogutskaya, N. G., Kijashko, P. V., Naseka, A. M., Orlova, M. I., 2013. Identification keys for fish and invertebrates. Volume 1: Fish and molluscs. KMK Scientific Press Ltd., St. Petersburg - Moscow.","Logvinenko, B. M., Starobogatov, Y. I., 1969. Mollusca, in: Birshtein, Y. A., Vinogradov, L. G., Kondakov, N. N., Kuhn, M. S., Astakhova, T. V., Romanova, N. N. (Ed.), Atlas Bespozvonochnykh Kaspiyskogo Morya [Atlas of Invertebrates of the Caspian Sea]. Pishchevaya Promyshlennost (Vsesoyuznyi Nauchno-issledovatel'skii Institut Morskogo Rybnogo Khozyaistva i Okeanografii), Moscow, pp. 308 - 385."]}

Didacna subcatillus Andrusov, 1910 (Fig. 5: 3, Fig 6: 1-3). *1910 Didacna catillus var. subcatillus Andrusov: 78, pl. 9, figs. 22���23. 1953 Didacna subcatillus Andrussov ��� Fedorov: 127, pl. 14, figs. 16���20. 1958 Didacna subcatillus Andrussov ��� Nevesskaja: 24, pl. 3, figs. 1���6. 1986 Didacna subcatillus Andrussov, 1910 ��� Yakhimovich et al.: 84, pl. 4, figs. 3, 4. 2007 Didacna subcatillus subcatillus Andrussoff, 1910 ��� Nevesskaja: 935, pl. 21, figs. 6���14. Dimensions ��� max. L 43.4 mm, H 33.8 mm. Characterization ��� Didacna subcatillus is a broad trapezoid highly variable species that does harbor several specific forms that we could link through intermediates forms in the studied material (Fig. 6: 1���3). The posterodorsal keel is usually low, the posterior margin is rounded. The posterior ridge is pronounced and mostly angular in juvenile stages and can become more subrounded in adult stages. The posterior ribs range between 18 and 25. Interspaces are narrow (about 40���60% of the width of the ribs). Towards the anterior margin ribs become often less pronounced and the posterior rib width and prominence increases towards the posterodorsal ridge. The rib cross-section is irregular, usually flat or subrounded. The location of the semidiameter is on the posterior ridge in juveniles and close to the center of the shell in adults. The ventral half of the shell is typically straight and dipping. Juvenile shells have already a trapezoid to low trigonate outline with a barely rounded lower margin and a well-developed posterodorsal keel. We found strong variations in shell shapes and sizes and investigated the possible presence of sibling (sub-)species. Some individuals grow as large as 43.4 mm (L) �� 33.8 mm (H), while 95% of individuals show maximum dimensions of 15 mm (L) �� 12 mm (H). One particular phenotype is small, has a thick shell that is somewhat convex and has markedly thick hinge plate and teeth and regularly developed ribs that have a more or less rounded cross-section (Fig. 6: 1a, 2a). This form however is connected via intermediates (Fig. 6: 1b, 1c, 2b, 2c) with thin-shelled forms that are less convex and have thinner hinge features (Fig. 6: 1d, 2d). Taxonomic notes ��� The various forms of Didacna subcatillus have been attributed to a number of species and forms in the past, to which Nevesskaja (2007) attributed biostratigraphic value. However, we were unable to delimit these forms in our variable material (Fig. 8) and consider them synonyms. Ecology ��� Unknown (extinct). Didacna subcatillus preferred lower saline waters than most Didacna species (Nevesskaja, 1958; Yanina, 2012)., Published as part of van de Velde, Sabrina, Yanina, Tamara A., Neubauer, Thomas A. & Wesselingh, Frank P., 2020, The Late Pleistocene mollusk fauna of Selitrennoye (Astrakhan province, Russia): A natural baseline for endemic Caspian Sea faunas, pp. 1227-1239 in Journal of Great Lakes Research 46 (5) on pages 1233-1234, DOI: 10.1016/j.jglr.2019.04.001, http://zenodo.org/record/5019200, {"references":["Andrusov, N., 1910. Studien uber die Brackwassercardiden. Didacna. (erste Halfte). Lieferung II. [Studies on the brackish water cardids. Didacna. (first half). Volume II]. Mem. Acad. imp. Sci. St. - Petersbourg. VIIIe serie. Cl. phys. - math. 25, 1 - 84.","Nevesskaja, L. A., 2007. History of the genus Didacna (Bivalvia: Cardiidae). Paleontol. J. 41, 861 - 949. https: // doi. org / 10.1134 / S 0031030107090018.","Nevesskaja, L. A., 1958. Chetvertichnyye morskiye mollyuski Turkmenii [Quaternary Sea Mollusks of Turkmenistan]. Tr. Paleontol. Inst. 65, 1 - 82.","Yanina, T. A., 2012. Neopleistotsen Ponto-Kaspiya: biostratigraphiya, paleogeographiya, korrelyatsiya [Neopleistocene of the Ponto-Caspian Region: Biostratigraphy, Paleogeography, Correlation]. Moscow University Press, Moscow."]}

Defining and recording the loss of species diversity is a daunting task, especially if identities of species under threat are not fully resolved. An example is the Pontocaspian biota. The mostly endemic invertebrate faunas that evolved in the Black Sea - Caspian Sea - Aral Sea region and live under variable salinity conditions are undergoing strong change, yet within several groups species boundaries are not well established. Collection efforts in the past decade have failed to produce living material of various species groups whose taxonomic status is unclear. This lack of data precludes an integrated taxonomic assessment to clarify species identities and estimate species richness of Pontocaspian biota combining morphological, ecological, genetic, and distribution data. In this paper, we present an expert-working list of Pontocaspian and invasive mollusc species associated to Pontocaspian habitats. This list is based on published and unpublished data on morphology, ecology, anatomy, and molecular biology. It allows us to (1) document Pontocaspian mollusc species, (2) make species richness estimates, and (3) identify and discuss taxonomic uncertainties. The endemic Pontocaspian mollusc species richness is estimated between 55 and 99 species, but there are several groups that may harbour cryptic species. Even though the conservation status of most of the species is not assessed or data deficient, our observations point to deterioration for many of the Pontocaspian species. ispartof: ZOOKEYS vol:827 issue:827 pages:31-124 ispartof: location:Bulgaria status: published