Dreissena grimmi

Wesselingh, Frank, Poorten, Jan Johan ter, Kijashko, Pavel, Albrecht, Christian, Anistratenko, Olga Yu, Frolov, Pavel, Gándara, Alberto Martinez, Gittenberger, Arjan, Gogaladze, Aleksandre, Mikhail Karpinsky, Popa, Luis, Sands, Arthur F, Vandendorpe, Justine & Wilke, Thomas, 2019, Mollusc species from the Pontocaspian region - an expert opinion list, ZooKeys 827, pp. 31-124 : 56-58

Dreissena grimmi (Andrusov, 1890)

Fig. 4b

1877 Dreyssena Brardii var. caspia Grimm: 74-75 [non Dreissena caspia Eichwald, 1855].

*1890 Dr. [eissena] Grimmi Andrusov: 233 [nom. nov. pro Dreissena caspia Grimm, 1877, non Eichwald, 1855].

1897 Dreissensia Grimmi Andrus. - Andrusov: 279-282, pl. 16, figs 16-18.

1897 Dreissensia rostriformis var. distincta Andrusov: 273-278, pl. 14, figs 18-24.

1897 Dreissensia Tschaudae var. pontocaspica Andrusov: 294-297, pl. 9, figs 27-32, pl. 15, figs 29, 30.

1966a Dreissena rostriformis compressa Logvinenko and Starobogatov: 15-16, fig. 3.

1969 Dreissena rostriformis grimmi Andr. - Logvinenko and Starobogatov: 318, fig. 341(3).

1969 Dreissena rostriformis pontocaspica (Andr.). - Logvinenko and Starobogatov: 319, fig. 341(6).

1994 Dreissena rostriformis (Deshayes, 1838). - Rosenberg and Ludyanskiy: 1477-1479, figs 1f, 2 a–j [non Mytilus rostriformis Deshayes, 1838].

2013 Dreissena rostriformis (Deshayes, 1838). - Kijashko in Bogutskaya et al.: 330 [non Deshayes, 1838].

2013 D. [reissena] rostriformis compressa Logvinenko & Starobogatov, 1966. - Kijashko in Bogutskaya et al.: 331, fig. 117a, photo 38.

2013 D. [reissena] rostriformis distincta (Andrusov, 1897). - Kijashko in Bogutskaya et al.: 331, fig. 117c.

2013 D. [reissena] rostriformis grimmi (Andrusov, 1890). - Kijashko in Bogutskaya et al.: 331, fig. 117b.

2013 D. [reissena] rostriformis pontocaspica (Andrusov, 1897). - Kijashko in Bogutskaya et al.: 331, fig. 117d.

Status. Caspian Sea endemic.

Type locality. Caspian Sea.

Distribution. Middle to southern Caspian Sea basins. This species was mentioned from depths between 200 and 400 m in the South Caspian Basin of Azerbaijan ( Mirzoev and Alekperov 2017, who reported the species as D. rostriformis compressa) and found living offshore Aktau (Kazakhstan) in 2017 below 20 m water depth.

Taxonomic notes. This Caspian species is very often cited as Dreissena rostriformis . Rosenberg and Ludyanskiy (1994: 1497) discuss the uncertainties of this attribution but state that "D. pontocaspica, D. distincta, D. compressa, and D. grimmi are synonyms of D. rostriformis" even though they find "some justification for maintaining a distinction between an extinct subspecies, D. rostriformis rostriformis and a living one, for which D. rostriformis grimmi is the oldest name". Their figure of the lectotype of D. rostriformis ( Rosenberg and Ludyanskiy 1994: fig. 2a), which derives from Pliocene deposits of the Black Sea Basin, concerns a relative small, thick-shelled, and low Dreissena with a pointed beak and lacking a keel. On interior view, the shell area outside the pallial line is thick. Deshayes’s lectotype has several characters in common with modern Caspian D. rostriformis and the closely related Black Sea Basin D. bugensis . Yet, the Pliocene form has a broader umbonal area that results in a more subquadrangular shape, which is different from the modern Caspian Dreissena that have tear-drop to pear-shaped shells. The subquadrangular shape of Deshayes’s material is even more pronounced in the pallial line on the shell’s interior, a feature not seen in any modern Caspian material. The Pliocene Black Sea D. rostriformis has its general shape in common with Apsheronian (Early Pleistocene) Caspian dreissenids referred to as D. carinatocurvata as illustrated in Kolesnikov (1950, pl. 14, figs 14-16). Hence, we conclude that the recent Caspian species should be treated different from Pliocene D. rostriformis and the name D. grimmi should be applied instead.

Various subspecies have been attributed to Caspian Dreissena rostriformis (see, e.g., Kijashko in Bogutskaya et al. 2013 for a synonymy list). Even though morphological differences appear to be large, intermediates are known between the morphs. Stepien et al. (2013) reviewed molecular evidence for species boundaries within Dreissena . They concluded that (1) all Caspian Sea forms that have been mentioned in literature as (sub-) species of D. rostriformis (= D. grimmi ) are one and the same species and (2) there is not enough molecular evidence and great difficulty in morphology to separate the Caspian species from the Black Sea Basin D. bugensis . We agree with the first point made by Stepien et al. (2013); all forms reported from the middle and southern Caspian Sea basins appear to be mere morphs of a single species, a feature also noted by Rosenberg and Ludyanskiy (1994). However, we disagree with their second proposal. Dreissena bugensis and D. grimmi have non-overlapping ecological tolerances and are separated geographically ( Rosenberg and Ludyanskiy 1994). This fact together with the very limited but consistent genetic differentiation suggests that it may concern very recently evolved sister species. In the early 1980s, D. bugensis was introduced in the Volga ( Zhulidov et al. 2005) and since then spread from there to central and western Europe and North America. So far, Dreissena bugensis has only been reported from the Volga itself and its delta but not from the northern Caspian Sea Basin. If it would be conspecific with the middle-southern Caspian species, which lives at higher salinities and deeper habitats, we would expect that the invasive populations in the north would have been blended with the Caspian population in the south. With no such intermediate populations found so far we consider both taxa as viable species.

Conservation status. Least Concern (for Dreissena rostriformis ; von Rintelen and Van Damme 2011c).